Abstract: The size-dependent phase stability of γ-Al2O3 was studied by large-scale molecular dynamics simulations over a wide temperature range from 300 to 900 K. For the γ-Al2O3 crystal, a bulk transformation to α-Al2O3 by an FCC-to-HCP transition of the O sublattice is still kinetically hindered at 900 K. However, local distortions of the FCC O-sublattice by the formation of quasi-octahedral Al local coordination spheres become thermally activated, as driven by the partial covalency of the Al-O bond. On the contrary, spherical γ-Al2O3 nanoparticles (NPs) (with sizes of 6 and 10 nm) undergo a crystalline-to-amorphous transformation at 900 K, which starts at the reconstructed surface and propagates into the core through collective displacements of anions and cations, resulting in the formation of 7- and 8-fold local coordination spheres of Al. In parallel, the reconstructed Al-enriched surface is separated from the stoichiometric core by a diffuse Al-depleted transition region. This compositional heterogeneity creates an imbalance of charges inside the NP, which induces a net attractive Coulombic force that is strong enough to reverse the initial stress state in the NP core from compressive to tensile. These findings disclose the delicate interplay between lattice distortions, stresses, and space-charge regions in oxide nanosystems. A fundamental explanation for the reported expansion of metal-oxide NPs with decreasing size is provided, which has significant implications for, e.g., heterogeneous catalysis, NP sintering, and additive manufacturing of NP-reinforced metal matrix composites. read less

Abstract: ABSTRACT Silica coating on a gold nanoparticle can improve its thermal application in cancer thermotherapy. In this paper, the interfacial thermal conductance between gold and silica is calculated utilizing classical non-equilibrium molecular dynamics. It is revealed that the results of molecular dynamics are different from what has been predicted by the conventional diffuse mismatch model. Furthermore, the interfacial thermal conductance between amorphous SiO2 and gold is approximately twice that of crystalline silica, which is explained by calculating the vibrational density of state mismatches. The interfacial thermal conductance variations in terms of van der Waals interaction strength between gold and silica are also investigated. It is revealed that the conductance increases by about 30% by increasing the simulation temperature from 300 to 700 K. The results of this paper can be useful in nanofluid systems, in addition to the application of silica-coated gold nanoparticles in cancer thermal therapy. read less

Abstract: Determining the structure and underlying potential from the experiment data is an important task in the study of disordered systems such as liquids and glasses. In this article, a new approach to tackle this problem is proposed. This method can iteratively refine any interaction potential u with the form of a fixed potential ψ added by a dot product between adjustable parameter θ and some functions of atomic coordinates called features f (i.e., potential u = ψ + θ · f). The updating rule for parameters is very simple as it only uses the difference of the ensemble mean of f between the simulation box and experiment. The solution found by this method minimizes the Kullback-Leibler divergence of the atomic distribution under the parameterized potential u and the prior potential ψ, subject to the condition that the ensemble mean of f of the simulation box is equal to its experimental value, ensuring that the potential given will be the least biased one from the prior potential but still consistent with the experiment. It is also shown that this method approximately minimizes the squared difference between the parameterized potential and the unknown true potential. Furthermore, the flexibility of the potential functional form allows the potential to be automatically fitted to some convenient forms or to encode additional known properties of the system under study. The method is tested on Lennard-Jones liquid as well as SiO2 liquid and glass for potential extraction or structure refinement using simulated data and real experiment data. Good results are obtained for both systems. read less

Abstract: Silica glass structure is simulated applying different annealing conditions that intend to imitate a fiber drawing process using molecular dynamic simulation in order to compare structural properties of differently processed glasses. read less

Abstract: In the upcoming process to overcome the limitations of the standard von Neumann architecture, synaptic electronics is gaining a primary role for the development of in-memory computing. In this fiel... read less

Abstract: A multiscale approach to model fiber‐reinforced composites, those that are characterized by an isotropic orientation of fibers, is presented. To this end, a bottom‐up approach is used to formulate a hierarchical model. The primary basis for the mesoscopic description revolves around the assumption that the composite network consists of fibers resting on foundations of the native material matrix. Molecular dynamics (MD) simulations of such fibers on foundations are performed, and crucial material parameters, such as the stiffness of the particle matrix and Young's modulus of the fibers are evaluated. Subsequently, a micro‐mechanical constitutive model is formulated, wherein fiber‐reinforced composites are characterized by a homogeneous distribution and an isotropic orientation of fibers. The fibers are modeled as beams undergoing bending and stretching while resting on Winkler‐type of elastic foundations. The 3D macroscopic network behavior is finally presented. As an example, the particle matrix used is a silica aerogel and the fibers are modeled as double‐walled carbon nanotubes. In the proposed modeling approach, MD simulations are shown to provide a physical estimation of the micro‐mechanical model parameters. read less

Abstract: In the present work, we performed nanoindentation tests using molecular dynamics (MD) simulations on graphene, native silica aerogels, and single- and multi-layered graphene-reinforced silica aerogel nanocomposites. This work mainly focused on the two aspects of nanoindentation simulations: first, the resultant indentation force–depth curves, and second, the associated mechanical deformation behavior. We found that in the single-layer graphene-reinforced silica aerogel nanocomposite, the indentation resistance was four-fold that of native silica aerogels. Moreover, the combined system proved to be higher in stiffness compared to the individual material. Furthermore, the indentation resistance was increased significantly as we proceeded from single- to two-layered graphene-reinforced silica aerogel nanocomposites. The results of the study provide a detailed understanding of the mechanical behavior during the indentation tests of nanocomposites, which helps to design advanced nanoscale multi-layered materials. read less

Abstract: The mechanical properties of Indium Phosphide (InP) nanowire is an emerging issue due to its application as optoelectronic material. In this paper, atomistic simulations are conducted to find thermo-mechanical properties of Indium Phosphide (InP) nanowire under uniaxial tension. Vashishta potential is employed to define the atomic interactions between the atoms. The effect of variation of temperatures (100K-500K) on the tensile response of the InP nanowires is investigated in this study. Also, size effect is investigated for the temperature of 300 K by varying the cross sectional area of the nanowire. Results suggest that increment of temperature results in the failure of InP nanowire at a lower value of stress (from 8.60 GPa at 100K to 6.50 GPa at 500K) along with the decrement of Young’s modulus. Results also suggest that size has little effect on the tensile properties of this nanowire. Finally, failure mechanisms of indium phosphide nanowire are also investigated from the atomic images obtained from the simulation results.The mechanical properties of Indium Phosphide (InP) nanowire is an emerging issue due to its application as optoelectronic material. In this paper, atomistic simulations are conducted to find thermo-mechanical properties of Indium Phosphide (InP) nanowire under uniaxial tension. Vashishta potential is employed to define the atomic interactions between the atoms. The effect of variation of temperatures (100K-500K) on the tensile response of the InP nanowires is investigated in this study. Also, size effect is investigated for the temperature of 300 K by varying the cross sectional area of the nanowire. Results suggest that increment of temperature results in the failure of InP nanowire at a lower value of stress (from 8.60 GPa at 100K to 6.50 GPa at 500K) along with the decrement of Young’s modulus. Results also suggest that size has little effect on the tensile properties of this nanowire. Finally, failure mechanisms of indium phosphide nanowire are also investigated from the atomic images obtained from t... read less

Abstract: The mechanical properties of indium phosphide (InP) nanowires are an emerging issue due to the promising applications of these nanowires in nanoelectromechanical and microelectromechanical devices. In this study, molecular dynamics simulations of zincblende (ZB) and wurtzite (WZ) crystal structured InP nanowires (NWs) are presented under uniaxial tension at varying sizes and temperatures. It is observed that the tensile strengths of both types of NWs show inverse relationships with temperature, but are independent of the size of the nanowires. Moreover, applied load causes brittle fracture by nucleating cleavage on ZB and WZ NWs. When the tensile load is applied along the [001] direction, the direction of the cleavage planes of ZB NWs changes with temperature. It is found that the {111} planes are the cleavage planes at lower temperatures; on the other hand, the {110} cleavage planes are activated at elevated temperatures. In the case of WZ NWs, fracture of the material is observed to occur by cleaving along the (0001) plane irrespective of temperature when the tensile load is applied along the [0001] direction. Furthermore, the WZ NWs of InP show considerably higher strength than their ZB counterparts. Finally, the impact of strain rate on the failure behavior of InP NWs is also studied, and higher fracture strengths and strains at higher strain rates are found. With increasing strain rate, the number of cleavages also increases in the NWs. This paper also provides in-depth understanding of the failure behavior of InP NWs, which will aid the design of efficient InP NWs-based devices. read less

Abstract: A study of atomic structures and localized symmetrized vibrations in α-quartz with silicon vacancies in different charge states is presented. This study is performed by computer modeling on the basis of the first-principle type potentials. The equilibrium structures are optimized by minimizing the lattice energy. The phonon symmetrized local densities of states are calculated by means of a recursion method. Moreover, frequencies of localized vibrations of A- and B-symmetries induced by silicon vacancies are determined. The results are discussed in comparative manner for the silicon vacancy in three charge states. read less

Abstract: Molecular dynamics (MD) calculations using two effective pair potentials BKS and CHIK have been carried out to represent the structures of the amorphous dehydroxylated silica surface in liquid (3400 and 2500 K) and glassy (1000 and 300 K) states. Previous studies have shown that CHIK performs better to represent the properties of bulk silica and this may result from the different values of the Si–O and O–O parameters, as well as from an additional Si⋯Si short range interaction term. The two potentials show similar trends in the change of structures upon going from the internal to the surface parts of the samples. However, the additional flexibility, likely due to the presence of the Si⋯Si short range interaction, relative to BKS, results in a surface which has overall more defects like in particular small 2-membered rings (SiO2)2 and dangling Si–O bonds. This cumulated density of defects corresponds qualitatively to the density of functionalized Si–OH obtained experimentally. This study shows that CHIK gives a good representation of the silica surface. read less

Abstract: Описан многомасштабный подход для предсказательного моделирования иерархических наноструктурированных материалов для оптических хемосенсоров. Стратегия этого подхода базируется на иерархической структуре материала, в которой ключевым элементом является супрамолекулярный рецепторный центр (СРЦ), представляющий собой молекулу органического красителя вместе с ее ближайшим окружением. Структура, стабильность и спектральный отклик конкретного СРЦ определяются с использованием методов молекулярного моделирования. Структура компонентов СРЦ рассчитывается методами квантовой химии. Возможные конфигурации СРЦ рассчитываются с использованием классического силового поля для описания межмолекулярных взаимодействий компонент СРЦ. Для расчета уточненной геометрии, относительной стабильности, требуемых спектральных свойств СРЦ и его спектрального отклика (изменений в спектрах поглощения или флуоресценции) на взаимодействие с молекулами аналитов. Свободная энергия образования супрамолекулярных комплексов (СРЦ + аналит) может быть определена методом молекулярной динамики. read less

Abstract: The effects of differently charged Ge impurities on the local atomic structure and lattice dynamics of α-quartz were studied. We have determined the equilibrium structures and calculated the symmetrized local density of vibrational states for the Ge-doped α-quartz. The frequencies of localized vibrations of A- and B-symmetries induced by Ge impurities were obtained. Besides, we have analyzed what contribution the vibrations of atoms located around the Ge impurities make to the localized symmetrized vibrations. read less

Abstract: Amorphous transition metal oxides exhibit exotic transport and magnetic properties, while the absence of periodic structure has long been a major obstacle for the understanding of their electronic structure and exchange interaction. In this paper, we have formulated a theoretical approach, which combines the melt-quench approach and the spin dynamic Monte-Carlo simulations, and based on it, we explored amorphous Co0.5Zn0.5O1−y ternary transition metal oxides. Our theoretical results reveal that the microstructure, the magnetic properties, and the exchange interactions of Co0.5Zn0.5O1−y are strongly determined by the oxygen stoichiometry. In the oxygen-deficient sample (y > 0), we have observed the long-range ferromagnetic spin ordering which is associated with the non-stoichiometric cobalt-rich region rather than metallic clusters. On the other hand, the microstructure of stoichiometric sample takes the form of continuous random networks, and no long-range ferromagnetism has been observed in it. Magnetiza... read less

Abstract: Organisms of the phylum Porifera, that is, sponges, utilize enzymatic hydrolysis to concatenate bioavailable inorganic silicon to produce lightweight, strong, and often flexible skeletal elements called spicules. In their optical transparency, these remarkable biomaterials resemble fused silica, despite having been formed under ambient marine biological conditions. Although previous studies have elucidated the chemical mechanisms of spicule formation and revealed the extensive hydration of these glasses, their precise composition and local and medium-range structures had not been determined. We have employed a combination of compositional analysis, (1) H and (29) Si solid-state nuclear magnetic resonance spectroscopy, and synchrotron X-ray total scattering to characterize spicule-derived silica produced by the demosponge Tethya aurantia. These studies indicate that the materials are highly hydrated, but in an inhomogeneous manner. The spicule-derived silica is, on average, perfectly dense for the given extent of hydration and regions of fully condensed and unstrained SiO networks persist throughout each monolithic spicule. To accommodate chemical strain and defects, the extensive hydration is concentrated in distinct regions that give rise to mesostructural features. The chemistry responsible for producing spicule silica resembles hydrolytic sol-gel processing, which offers exceptional control over the precise local atomic arrangement of materials. However, the specific processing involved in forming the sponge spicule silica further results in regions of fully condensed silica coexisting with regions of incomplete condensation. This mesostructure suggests a mechanism for atomistic defect tolerance and strain relief that may account for the unusual mechanical properties of the biogenic spicules. read less

Abstract: Silica Surface Features and Their Role in the Adsorption of Biomolecules: Computational Modeling and Experiments / Albert Rimola;Dominique Costa;Mariona Sodupe;Jean-François Lambert;Piero Ugliengo. In: CHEMICAL REVIEWS. ISSN 0009-2665. STAMPA. 113:6(2013), pp. 4216-4313. Original Citation: Silica Surface Features and Their Role in the Adsorption of Biomolecules: Computational Modeling and Experiments read less

Abstract: Extra-long molecular-dynamics simulations are performed to study the interplay between dielectric properties and microstructures of silicon oxynitride. We quantitatively obtained the ionic permittivity and its linear dependence on nitrogen concentration. Analyses on microstructure of the oxynitride show that, as the N concentration increases, N atoms enter Si-O networks creating smaller-sized rings. While overall tendency of dielectric constant is monotonic increase on the N concentration, microscopic characteristics, such as local displacement of atoms, are strongly depend on local bonding environments. read less

Abstract: Nonlinear absorption of femtosecond laser pulses enables the induction of structural changes in the interior of bulk transparent materials without affecting their surface. In the present study, femtosecond laser pulses were tightly focused within the interior of bulk fused silica specimen. Localized plasma was formed, initiating rearrangement of the random network structure. Cross sections of the induced features were examined via decomposition of spatially resolved Raman spectra and a new method for the quantitative characterization of the structure of amorphous fused silica was developed. The proposed method identifies the volume fraction distribution of ring structures within the continuous random network of the probed volume of the target material and changes of the distribution with laser process conditions. Effects of the different process conditions and the material response to different mechanisms of feature generation were discussed as well. read less

Abstract: We report our study of a silica-water interface using reactive molecular dynamics. This first-of-its-kind simulation achieves length and time scales required to investigate the detailed chemistry of the system. Our molecular dynamics approach is based on the ReaxFF force field of van Duin et al. [J. Phys. Chem. A 107, 3803 (2003)]. The specific ReaxFF implementation (SERIALREAX) and force fields are first validated on structural properties of pure silica and water systems. Chemical reactions between reactive water and dangling bonds on a freshly cut silica surface are analyzed by studying changing chemical composition at the interface. In our simulations, reactions involving silanol groups reach chemical equilibrium in approximately 250 ps. It is observed that water molecules penetrate a silica film through a proton-transfer process we call "hydrogen hopping," which is similar to the Grotthuss mechanism. In this process, hydrogen atoms pass through the film by associating and dissociating with oxygen atoms within bulk silica, as opposed to diffusion of intact water molecules. The effective diffusion constant for this process, taken to be that of hydrogen atoms within silica, is calculated to be 1.68 x 10(-6) cm(2)/s. Polarization of water molecules in proximity of the silica surface is also observed. The subsequent alignment of dipoles leads to an electric potential difference of approximately 10.5 V between the silica slab and water. read less

Abstract: Ab initiomolecular dynamics simulations are performed to study the structural and dynamical properties of molten Si oxides. Segregation of SiO y ( y < 2 ) and pure Si network are clearly observed in the Si-rich oxide liquids. The size of Si-aggregate regions increases with increasing Si composition. The dynamical properties of the Si atoms with different local environments are different due to the “oxygen slowing-down” effect. This structural and dynamical heterogeneity is consistent with previous experimental studies, and provides useful insight into the role of the Si oxide shell in the oxide-assisted growth of Si nanowires. read less

Abstract: Amorphous titania $({\text{TiO}}_{2})$ is an important precursor for synthesis of single-phase nanocrystalline anatase. We synthesized amorphous titania by hydrolysis of titanium ethoxide at the ice point. Transmission electron microscopy examination and nitrogen gas adsorption indicated that the particle size of the synthesized titania is $\ensuremath{\sim}2\text{ }\text{nm}$. Synchrotron wide-angle x-ray scattering (WAXS) was used to probe the atomic correlations in this amorphous sample. Atomic pair-distribution function (PDF) derived from Fourier transform of the WAXS data was used for reverse Monte Carlo (RMC) simulations of the atomic structure of the amorphous ${\text{TiO}}_{2}$ nanoparticles. Molecular-dynamics simulations were used to generate input structures for the RMC. X-ray-absorption spectroscopy (XAS) simulations were used to screen candidate structures obtained from the RMC by comparing with experimental XAS data. The structure model that best describes both the WAXS and XAS data shows that amorphous ${\text{TiO}}_{2}$ particles consist of a highly distorted shell and a small strained anataselike crystalline core. The average coordination number of Ti is 5.3 and most Ti-O bonds are populated around $1.940\text{ }\text{\AA{}}$. Relative to bulk ${\text{TiO}}_{2}$, the reduction in the coordination number is primarily due to the truncation of the Ti-O octahedra at the amorphous nanoparticle surface and the shortening of the Ti-O bond length to the bond contraction in the distorted shell. The pre-existence of the anataselike core may be critical to the formation of single-phase nanocrystalline anatase in crystallization of amorphous ${\text{TiO}}_{2}$ upon heating. read less

Abstract: By using first-principle molecular dynamics within density functional theory, we study the structural properties of amorphous GeSe2 at a temperature T of 300 K. For each property, a statistical average is obtained from six independent partial averages taken on temporal trajectories, each one lasting 12 ps. Each trajectory stems from an initial configuration of the liquid phase at T=1100 K and is generated by extensive annealing at T=300 K. Overall, our level of theory provides a picture of this prototypical disordered network-forming glass that is quantitatively consistent with neutron diffraction data. Very satisfactory agreement with experiments is obtained for the pair correlation functions g(GeSe)(r) and g(SeSe)(r) in terms of peak intensities and positions. This holds true also for the amount of Se-Se homopolar bonds and the Ge-Se and Se-Se coordination numbers. Conversely, the g(GeGe)(r) pair correlation function is much less structured around the main peak position and the concentration of Ge-Ge homopolar bonds is lower than in the experiment. The network organizes itself through the predominant presence of GeSe4 tetrahedra. However, other coordinations occur in non-negligible proportions for both Ge and Se. Total and partial structure factors reproduce very well the experimental patterns for wave numbers k larger than 2 A(-1). For smaller k values, the largest difference between theory and experiment is exhibited by the S-GeGe(k) structure factor, showing a FSDP of lower intensity in the simulation. In agreement with experimental results, a sizeable feature is found at the FSDP location in the Bhatia-Thornton concentration-concentration structure factor S-CC(k). read less

Abstract: By using first-principles molecular dynamics within density functional theory, we study the structural properties of amorphous GeSe2 at T = 300 K. The amorphous configurations have been obtained via cooling from the liquid state followed by extensive relaxation (22 ps) at T = 300 K. The agreement with neutron diffraction experiments is very satisfactory, in particular for the pair correlation functions in real space and the partial structure factors in reciprocal space describing the Ge–Se and the Se–Se correlations. Some residual differences between theory and experiment are found for Ge–Ge correlations. The network organizes itself through the predominant presence of GeSe4 tetrahedra. However, other coordinations exist in non-negligible proportions for both Ge and Se. Homopolar bonds are found for Se, and, in a very limited extent, also for Ge. The number of edge-sharing connections reproduces the experimental data. read less

Abstract: Advances in the microelectronic design implicate a reduction of device dimensions requiring a better understanding of the microscopic processes involved. One of these processes concern charging and discharging of defects via tunneling, which is supposed to constitute a grave contribution to various ongoing reliability issues. A deep understanding and a correct modeling of this mechanism are of utmost importance in this context. Conventionally, tunneling levels are believed to remain at fixed positions within the oxide bandgap regardless whether they are occupied or not. From a theoretical point of view, defect energy levels undergo shifts within the silicon dioxide bandgap after charging or discharging. As a result, defect levels for tunneling into and out of traps have to be distinguished. Based on this understanding of trapping, defects can be characterized as fixed charges, switching oxide charges, interface traps, or other types of defects. In this study, we conduct first-principle investigations on the energetics for a series of individual defects encountered in the context of reliability. In order to deduce their tunneling dynamics, a new model, which accounts for the effects of shifting tunneling levels, has been established. On the basis of the E gamma ' center, the main discrepancies between the model relying on trap level shifts and the model with coinciding trap levels have been highlighted. read less

Abstract: Ab initio molecular dynamics simulations of liquid GeSe2 show that the first sharp diffraction peak in the concentration-concentration structure factor S-CC(k) is due to a sequence of connected fourfold rings. These subunits can be viewed as chains of edge-sharing tetrahedra. Ge atoms at the opposite tails of these chains are mostly miscoordinated and separated from each other by intermediate range distances. This result correlates a specific structural subunit to a subtle intermediate range order property, involving both the structural and chemical composition of the disordered network. read less

Abstract: An effective inter-atomic potential is proposed in order to describe structural and dynamical properties of II-VI and III-V semiconductors. The interaction potential consists of twoand three-body interactions. The two-body term takes into account steric repulsion, charge-induce dipole interaction due to the electronic polarizability of ions, Coulomb interaction due to charge transfer between ions, and dipole-dipole (van der Waals) interactions. The three-body term, which has a modified Stillinger-Weber form, describes bond-bending as well as bond-stretching effects. Here we report the fitting and the application of this interaction potential for InP in the crystalline phase and for CdTe in the crystalline and liquid phases. The structural correlations are discussed through pair distribution, coordination number and bond-angle functions. Vibrational density of states for InP and CdTe as well as the static structure factor for liquid CdTe are in very good agreement with experimental data. read less

Abstract: Coordination-dependent interatomic potentials are proposed for silicon oxides and oxynitrides\char22{}also hydrogenated ones\char22{}with a functional form based on the widely used Tersoff silicon potential. They are intended for an accurate sampling of the configurational space of realistic silicon oxynitride systems and their interfaces with silicon, including defects and changes of oxidation states. The parameters, which are given in the text, are obtained by simultaneously mapping forces and energies onto the results of density-functional-theory calculations performed for a set of diverse systems and configurations and a wide composition range. Application to a larger set of systems and configurations shows the transferability of these augmented Tersoff potentials and their validity in predicting bulk lattice parameters, energetics of defect relaxation, and vibrational spectra. read less

Abstract: Structural, dynamical, and thermal properties of germanium dioxide are investigated with classical molecular dynamics simulations from the amorphous to the liquid state. Pair correlation functions and coordination numbers are computed under pressure change and show the progressive conversion of the tetrahedral network into an octahedral network, in agreement with experiments. The thermodynamical behavior of the liquid is investigated by means of an equation of state that allows a precise estimation of the compressibility. At low temperature, the diffusion constant D shows an Arrhenius law that progressively deviates when the temperature is increased. The overall comparison with simulated silica permits finally to outline not only the differences in the physical behavior of these two similar systems but also to stress the limitation of the employed germania potential. read less

Abstract: We present a statistical study of silicon and oxygen neutral defects in a silica glass model. This work is performed following two complementary approaches: first-principles calculations and empirical potential molecular dynamics. We show that the defect formation energies and structures are distributed and that the energy distributions are correlated with the local stress before the defect formation. Combining defect energies calculated from first principles and local stresses from empirical potential calculations in undefected silica, we are able to predict the formation energy distributions in larger systems, the size of which precludes the use of ab initio methods. Using the resulting prediction we will show that the cell size used in our modeling contains all the formation energy fluctuations needed to describe a real glass read less

Abstract: In this work we used an empirical potential model in order to describe the bulk structure of the oxygen-rich Si-O-N amorphous system. From the atomic configuration obtained by Monte Carlo simulations, the local order structure and the pair correlation function are determined. The results show that the basic structure of the amorphous is well described by a random distribution of tetrahedra with a central Si atom bonded to nitrogen and oxygen atoms. The Fourier transform of the extended x-ray-absorption fine structure (EXAFS) signal obtained by ab initio multiple-scattering calculations, using the theoretical atomic structure as input, shows good agreement with the experimental EXAFS analysis performed at the silicon K edge. read less

Abstract: A classical pair potential, augmented by three-body interactions, for the modeling of borosilazane ceramics has been derived on the basis of both experimental and ah initio data. The primary goals were a good description of structural parameters and applicability in molecular dynamics. Furthermore, major challenges were to enable bond breaking and to avoid Coulomb collapse during simulations. This has been achieved by long-range, exponentially damped, analytical forms and the inclusion of short-range Coulomb tapering functions. We report on the fitting procedure, discuss the analytical forms employed, and demonstrate the abilities of the potential by comparing to ah initio calculations and experiments. read less

Abstract: We construct atomistic models of the Si(100)–SiO2 interface in accord with available experimental data. Combining classical and first-principles simulation methods, we generate transition structures from crystalline silicon to disordered SiO2. The generation procedure accounts for the density of coordination defects, the amount and location of partially oxidized Si atoms, and the mass density profile, as measured in electron-spin-resonance, photoemission, and x-ray reflectivity experiments, respectively. A variety of model interfaces are obtained, differing by the degree of order in the transition region. read less

Abstract: Applying a first-principles scheme to liquid GeSe4, we show that the charge-charge structure factor does not present any feature at the location of the first sharp diffraction peak (FSDP), despite a clear FSDP in the concentration-concentration structure factor S-CC(k). The origin of this effect is assigned in part to the finite extent of the electron density and in part to electron rearrangements. Our result provides evidence in favor of the postulate that, in binary network-forming systems, charge-charge correlations are absent on intermediate-range length scales. A FSDP in the S-CC(k) then signals the occurrence of structural defects. read less

Abstract: Large structural modifications in v-SiO2 are found to be induced by “thermal poling,” a treatment which makes the glass act as a frequency doubler of an impinging infrared light. The electron diffraction patterns of poled silica plates reveal the presence of a large amount (of order 10%) of crystallites showing patterns consistent with partial crystallization of the glassy matrix into the cristobalite polymorph of silica. read less

Abstract: The atomistic computer simulation of ionic materials has undergone massive changes over the last three decades. Major developments will be reviewed, and possible future directions explored. Extended ionic models account for an ion'response to the environment (for example, polarization or a change in size and shape) and introduce a many-body character. The exploration of the applicability of such models, which exclude such effects as charge transfer and chemical bond formation, is practical as large-scale simulations remain possible. High-level ab initio electronic structure calculations, which form a source of parameters in which each aspect of the model is parametrized independently , will be highlighted. The unambiguous nature of the resulting parameter sets will be demonstrated by simulating a range of very different, but linked, systems. Possible future directions will be explored. read less

Abstract: This paper presents a general approach for the description of peculiar dynamic properties of glasses observed at low temperatures and low energies. These are the specific heat linear in temperature, the plateau in the thermal conductivity and its subsequent rise above the plateau temperature, as well as the increased inelastic scattering of light and neutrons at around 5 meV, the so-called Boson peak. The approach naturally accounts for in a unified way the characteristic features observed at low temperatures and low energies for a wide variety of glassy materials. read less

Abstract: The microscopic structure of nanoporous silica is investigated using the Molecular Dynamics simulation method. Porous silica structures are produced by simulated atom aggregation and clustering. The structures are then relaxed at atmospheric pressure. We find that on these length scales the simulated porous silicas have a minimum fractal dimension of 2.6 and density 0.7 g/ml. read less

Abstract: We describe results obtained from a new implementation of Hockney's Particle-Particle Particle-Mesh (PPPM) method for evaluation of Coulomb energies and forces in simulations of charged particles. Rather than taking the usual approach, solving Poisson's equation by means of a Fourier transformation, we use an iterative Poisson solver. In a molecular dynamics (MD) simulation the solution from the previous time-step provides a good starting point for the next solution. This reduces the number of iterations per time-step to acceptable values. The iterative scheme has a complexity O(N), and, in contrast with the Fourier transform based approach, it is easily implemented on a parallel architecture with a minimum of communication overhead. We examine the origin of the errors in the algorithm and find that reasonable accuracies in the Coulomb interaction can best be attained by making the charge density profile as smooth as possible. This involves spreading the particle charges over a large number of gr... read less

Abstract: In spite of numerous severe difficulties specific to metal-oxygen systems, computer simulation of oxides has developed rapidly in recent years. In this paper, we discuss its successes and some of its limitations with a particular emphasis on silica and on non stoichiometric oxides. read less

Abstract: We present a theoretical investigation on diffusion of atomic and molecular hydrogen in crystalline as well as amorphous silicon dioxide by means of molecular dynamics simulations. We report a characterization of the migration process of atomic hydrogen in quartz and tridymite and we provide an atomistic description of the diffusion of both atomic and molecular hydrogen in silica. read less

Abstract: An efficient order of N molecular dynamics method for the simulation of two-body and three-body systems is presented. Due to its high speed it enables one to simulate large MD systems on a mainframe and investigate some complex processes in terms which are closer to experimental conditions (e.g. crystal growth or ion implantation on relatively large-by MD standards-substrates). read less

Abstract: The case in point is afforded by quartz crystal oscillators (QCOs). These devices rely upon very simple mechanical principles for their operation. Present QCOs can be manufactured in the 100 micron length scale regime; they involve megahertz frequencies. Next generation oscillators will hit the sub-micron regime where they will afford gigahertz frequencies and much higher sensitivities (e.g., force, mass, etc.). It is this size regime that may be directly accessed by modern atomistic simulation; that is the multi million atom system size. read less

Abstract: A large molecular dynamics model of amorphous silicon is used to show how the first sharp diffraction peak is a direct consequence of fluctuations in the pair distribution function which persist to above 30 A. The fluctuations can be explained in terms of the sum of neighbour-specific pair distribution functions, which reveal enhanced intensity of alternate (even-n) nth-neighbour peaks, or analogously in terms of the chemical ordering of interatomic voids about each atom. read less

Abstract: Glasses can be formed by many routes. In some cases, distinct polyamorphic forms are found. The normal mode of glass formation is cooling of a viscous liquid. Liquid behavior during cooling is classified between "strong" and "fragile," and the three canonical characteristics of relaxing liquids are correlated through the fragility. Strong liquids become fragile liquids on compression. In some cases, such conversions occur during cooling by a weak first-order transition. This behavior can be related to the polymorphism in a glass state through a recent simple modification of the van der Waals model for tetrahedrally bonded liquids. The sudden loss of some liquid degrees of freedom through such first-order transitions is suggestive of the polyamorphic transition between native and denatured hydrated proteins, which can be interpreted as single-chain glass-forming polymers plasticized by water and cross-linked by hydrogen bonds. The onset of a sharp change in ddT( is the Debye-Waller factor and T is temperature) in proteins, which is controversially indentified with the glass transition in liquids, is shown to be general for glass formers and observable in computer simulations of strong and fragile ionic liquids, where it proves to be close to the experimental glass transition temperature. The latter may originate in strong anharmonicity in modes ("bosons"), which permits the system to access multiple minima of its configuration space. These modes, the Kauzmann temperature TK, and the fragility of the liquid, may thus be connected. read less

Abstract: The effect of induced dipoles on the structure and dynamics of melts of stoichiometry MX2 is discussed. Interest centres on the case of M being a small, highly polarizing cation and X a highly polarizable anion. It is shown that 'covalent' effects on the short range structure and the appearance of a pre-peak in the structure factor may be explained within a purely ionic model. A connection between these structural features and the dynamical properties of 'strong' liquids is suggested. read less

Abstract: Bond directionality and network formation from local structural units are the signature of covalent bonding. On melting a 3D network of covalent bonds tends to break into a metallic liquid (e.g. in Si, Ge and GaAs), unless a sufficiently large electronegativity difference between the components stabilizes the electronic structure through chemical short-range order. The melt may then be a semimetal (e.g. Li4Pb and KPb), an ionic semiconductor (e.g. CsAu) or an insulator (e.g. ZnCl2). Bonding appears to be more stable in networks of lower dimensionality (D=2 as in GeSe2 and YCl3, D=1 as in Se and BeCl2, and D=0 as in P, SbCl3 and AlBr3). Melting from D=2 to D=0 occurs in AlCl3. Intermediate-range order may be preserved in the melt through interatomic correlations over distances of order 5-10 AA. The experimental evidence on illustrative examples of these various trends is reviewed, with emphasis on the interconnection between stable local coordination and intermediate-range order. Parallel illustrations are given of results from simulations based on empirical potentials or fully quantal methods, from data analysis based on the Reverse Monte Carlo method and from primitive models amenable to integral-equations techniques. read less

Abstract: The problem of those discernible features of the intermediate range order (IRO) which can be attributed to the first sharp diffraction peak (FSDP) observed in the structure factor of many liquid and glassy materials is approached by treating this peak as a distinct feature. It is found, by considering the measured partial structure factors, Sαβ(k), for molten ZnCl2, GeSe2, MgCl2, NiBr2 and Nil2 and the measured total structure factors, F(k), for glassy SiO2, PS4 and liquid CCl4, that the propensity of the FSDP to have a prominent effect on the underlying features of the IRO depends noticeably on the system type. Specifically, the FSDP confers a marked oscillatory character of periodicity 2π/k1 (where k1 is the FSDP position) on the IRO when the local structural units, which give rise to the density fluctuations on the IRO scale, exist as stable entities for a timescale τ ≫ 5 × 10-12 s. The FSDP therefore accounts for the discernible features of the underlying IRO for the viscous glass forming liquids ZnCl2 and GeSe2, for the glasses SiO2 and PS4, and for the molecular liquid CCl4. The influence of the FSDP on the IRO is less pronounced for molten MgCl2 and is negligible for molten NiBr2 and Nil2, both of which have a high cation mobility which leads to a relative instability of the Ni2+ centred structural units. The effect on the FSDP of temperature and pressure are briefly considered as are the development of the FSDP in molten ZnX2 (when X is changed from Cl to I to Br) and the minimum size of r-space model which is required if the FSDP is to be accurately predicted. read less

Abstract: Algorithms are designed to implement molecular-dynamics simulations on emerging concurrent architectures. For systems with finite-range interactions, a domain decomposition algorithm is used to implement the multiple-time-step (MTS) approach to molecular-dynamics (MD) simulations on distributed-memory multiple instructions multiple data (MIMD) machines. This approach reduces the computation of forces significantly by exploiting the different time scales for short-range and intermediate-range interactions. Parallel algorithms are also designed for MD simulations of bulk Coulombic systems. The performance of these algorithms is tested on the Intel iPSC/860 system. The computational complexity of these algorithms is O(N) and parallel efficiencies close to 0.9. Molecular-dynamics simulations are carried out to investigate the structural and dynamical properties of highly densified and also porous silica glasses. Changes in the short-range and intermediate-range order in amorphous SiO2 are determined at different densities in the range of 4.28-0.1 g/cm3. Results for internal surface area and surface-to-volume ratio in porous SiO2 are also discussed. © 1993 John Wiley & Sons, Inc. read less

Abstract: The structural properties of a model (AgI)x(Ag2O-2B2O3)1-x glass are determined through molecular dynamics (MD) simulation. The total neutron diffraction pattern, reconstructed from the MD partial structure factors, shows a peak at low wavevectors (k approximately=0.9 AA-1) in qualitative agreement with neutron diffraction experimental data for the real glass. The overall analysis of the MD structural information permits one to conclude that the medium-range order observed in the glass is due to correlation of AgI clusters, which coexist with the B2O3 network. read less

Abstract: The microscopic mechanism of oxygen diffusion in yttria stabilized zirconia (YSZ) is investigated by means of a polyhedron analysis. It is shown that the probability of finding O ion in a tetrahedron increases as Y ions at the corners of the tetrahedron increase, and this tendency becomes marked with increasing dopant concentration, which is consistent with the fact that the oxygen coordination number for Y ion is larger than that for Zr ion. By studying the oxygen migrations between tetrahedra in the [100] direction in detail, it is found that the migrations preferentially occur between tetrahedra having a common Zr–Zr edge, and Y ions restrict the diffusion paths of O ions. The restriction by the Y–Y edge causes the decrease of the oxygen diffusion with increasing dopant concentration. It is discussed that the fluorite structure of YSZ is guaranteed by the defective and disordered state of oxygen sublattice. read less

Abstract: It is proposed that the first sharp diffraction peak (FSDP) in the structure factor of network glasses and liquids is a pre-peak in the concentration-concentration structure factor due to the chemical ordering of interstitial voids around cation-centred clusters in the structure. This model can predict quantitatively the positions of the FSDP for a wide range of oxide and chalcogenide glasses and liquids, and can also successfully rationalize the anomalous temperature and pressure behaviour of the FSDP intensity, as well as the effect of the incorporation of network modifiers. read less

Abstract: A systematic analysis of those liquid binary 2:1 systems (denoted MX2), for which experimental partial structure factors are available from the isotopic substitution method in neutron diffraction, is made using the Bhatia-Thornton (BT) formalism.Particular attention is paid to the origin of the first sharp diffraction peak (FSDP ), which occurs in the measured diffraction patterns for some of the MX2 systems, since it appears, from recent studies, that this feature is a signature of directional bonding. It is found that FSDPS can occur in all three BT partial structure factors SxB(k). A FSDP feature in the concentration-concentration partial structure factor Scc(k) is not, however, pronounced except in the case of MgCl2 and the glass forming network melts ZnCl2 and GeSe2. To the extent that these systems can be regarded as ionic melts a FSDP in Scc(k) implies a non-uniformity in the charge distribution on the scale of the intermediate-range order (IRO). The structure of molten GeSe2 is compared with the structures of molten ZnCl2, glassy GeS2 and glassy Si02. Although the GeSe2 and ZnCl2 melts have different short-range order, there are similarities in the observed IRO which can be attributed to the arrangement of the electropositive species M. The essential features of the measured total structure factor for glassy GeS2 can be reproduced by using the molten GeSe2 SzB(k). This result lends support to the notion that the SzB(k) for liquid GeSe2 (and ZnCl2) are characteristic of both the liquid and glassy states of other network glass forming systems. The structures of molten GeSe2 (or ZnCl2) and glassy Si02 are, however, found to be different. The observed discrepancies are largest in the region of the FSDP which signifies pronounced differences in the nature of the IRO for these systems. read less

Abstract: The phonon densities of states (DOS) of insulating ${\mathrm{BaBiO}}_{3}$ and superconducting ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_{3}$ and the variation of the phonon spectrum of the superconducting compound upon oxygen-isotope ${(}^{16}$O, $^{18}\mathrm{O}$) substitution are determined by inelastic neutron scattering (INS) and molecular-dynamics (MD) simulations. The MD simulations are carried out with an effective interaction potential which includes steric effects, Coulomb interactions, and the charge-dipole interactions due to the electronic polarizability of ${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$. The MD results are in good agreement with the INS experiments and electron-tunneling measurements. Partial DOS of Ba, K, Bi, and O in ${\mathrm{BaBiO}}_{3}$ and ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_{3}$ are also determined from MD simulations. In the superconducting material, the phonon spectrum softens and is comprised of broad bands around 15, 30, and 60 meV. The partial DOS reveal that phonons above 20 meV are due to oxygen vibrations, whereas phonons below 20 meV are due to Ba, K, and Bi. The reference oxygen-isotope-effect exponent, ${\mathrm{\ensuremath{\alpha}}}_{\mathrm{O}}$r=-\ensuremath{\partial} ln〈\ensuremath{\omega}〉/\ensuremath{\partial} ln${\mathit{M}}_{\mathrm{O}}$, of ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_{3}$ is determined to be ${\mathrm{\ensuremath{\alpha}}}_{\mathrm{O}}$r=0.42\ifmmode\pm\else\textpm\fi{}0.05 from the mass (${\mathit{M}}_{\mathrm{O}}$) variation of the first moment of the phonon DOS, $_{\mathrm{O}}^{16}\mathrm{〉}$ and $_{\mathrm{O}}^{18}\mathrm{〉}$. read less

Abstract: A scheme is described for the explicit treatment of charge-transfer processes in molecular-dynamics simulations of condensed phases. Technical details of the simulations are discussed, and it is shown that it is possible to implement the method without serious cost in computing time relative to standard molecular-dynamics calculations for charged systems. The scheme used ensures that the charge-transfer forces are dynamically conservative, and no numerical instabilities have been observed in the integration of the equations of motion. The method is applied to pure amorphous silica and to silica to which small numbers of dilithium oxide impurities have been added. The distributions of atomic charges have their expected form in each case. In particular, addition of the cation impurities leads to the spontaneous generation of an almost equal number of oxygen defects, and it is shown that it is possible to distinguish unambiguously between bridging and dangling oxygens on electrostatic rather than ge... read less

Abstract: Theory and computation play an increasingly important role in the field of mineral physics by allowing the scientist to probe environments, such as the deep Earth, that are challenging or impossible to access extensively by experiment. Quantum mechanical methods are often the technique of choice, usually based on Kohn-Sham density functional theory as the computationally most practical approach for solids. Although calculations at this level can already be performed on thousands of atoms (Soler et al. 2002; Cankurtaran et al. 2008), the ability to sample nuclear configuration space is often restricted. While density functional theory is typically considered the defacto standard, it is important to remember that with current functionals the results will typically be quantitatively in error with respect to experiment, with occasional qualitative errors (Bilic and Gale 2009). The strength of the method is that the errors are generally systematic and can be anticipated a priori . Despite the ever-increasing scope of electronic structure theory for condensed phases, there are still many problems that will lie beyond their reach for the foreseeable future. Consequently, there remains a need for more approximate, but efficient, techniques to complement quantum mechanical studies. Semi-empirical Hamiltonians and tight binding represent one possibility, but if even greater speed is required then force-field methods are a valuable option. As will be discussed later, the boundaries between the aforementioned approaches are continually becoming blurred as the sophistication of force-fields increases. Beside the greater speed, force-field methods have the advantage of a clear conceptual connection between the functional form and the underlying physics. Hence much can be learnt about what physical interactions are important to describe the properties of a system. The use of force-field methods is widespread from biological modelling to mineralogy, aided by the availability of many different programs for interatomic potential simulation. By … read less

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Abstract: The authors developed an empirical potential for interactions between Si and N to describe silicon nitride systems using the Tersoff functional form. With this model, they explored the structural properties of amorphous silicon nitride through the Monte Carlo simulations and compared them to available experimental data. The empirical model provided a very good description of such properties for a-SiN{sub x} (0 < x < 1.5). Electronic structure of amorphous and point defects in crystalline silicon nitride were then studied using first-principles calculations. For such calculations, the configurations were created by the empirical model, with the relaxed structures used as input for the first-principles calculations. Atomic relaxation was later allowed in the first-principles calculations. read less

Abstract: The authors present the formalism of anomalous x-ray scattering as applied to partial structure analysis of disordered materials, and give an example of how the technique has been applied, together with that of neutron diffraction, to investigate short-, intermediate- and extended-range order in vitreous germania and rubidium germanate. read less

Abstract: The authors present a molecular dynamics investigation on hydrogen diffusivity in crystalline quartz by computing the diffusion coefficient over a wide range of temperatures (700K < T < 1,500 K) and by characterizing the diffusion path and mechanism. The main findings are: (1) hydrogen diffusion is anisotropically confined along the c-axis in {alpha}- and {beta}-quartz; (2) hydrogen diffuses through a jump-like mechanism; (3) the temperature-dependent diffusivity follows an Arrhenius law with activation energy of 0.56 eV and 0.27 eV for {alpha}- and {beta}-quartz, respectively. read less

Abstract: We describe both the methodologies and recent applications of atomistic computer modelling techniques in materials chemistry. We discuss first, simulation techniques based on interatomic potentials, where we review the scope of static lattice methods (applied to both perfect and defective crystal structures), molecular dynamics and Monte Carlo techniques. The role of electronic structure methods based on both Hartree–Fock and local density functional methods is then reviewed including reference to periodic boundary conditions and embedded cluster calculations. We illustrate the range of contemporary applications of modelling methods by accounts of recent work on the simulation of the structures of crystalline and amorphous solids, on the structures and properties of surfaces and interfaces, on the dynamics of ions in solids and, finally, on the challenging problems posed by the development and understanding of catalytic processes. read less

Abstract: Molecular dynamics (MD) method is used to investigate structural transformation and the loss of intermediate range order in SiO 2 glass at very large positive pressures and the modification of SiO 2 glass network at very large negative pressures. The nature of molecular vibrations in solid C 60 has been studied with tight binding molecular dynamics (TBMD) method. Implementations of simulation algorithms on parallel computers are also discussed. In a-SiO 2 at high pressures, the height of the first sharp diffraction peak in S(q) is considerably diminished and its position shifts to larger wave vectors. At twice the normal density, Si-O bond length increases, Si-O coordination changes from 4 to 6, and O-Si-O band-angle changes from 109° to 90°. This is clearly a tetrahedral to octahedral transformation, which is observed recently by Meade, Hemley, and Mao in their diffraction experiments using synchrotron radiation. MD simulations of porous silica are carried out in the density range 2.2 - 0.1 g/cm 3 Internal surface area, pore surface-to-volume ratio, gyration radius, and fractal dimension are studied as a function of density. Simulations are in good agreement with the experimental results obtained by x-ray scattering. The results reveal a crossover of the structural correlations between short- to intermediate-range ( Dispersion and density of states (DOS) of inter- and intra-molecular phonons are calculated for orientationally ordered and disordered solid C 60 using the TBMD method. Inter-molecular phonon DOS extends up to 7.6 meV and shows libron peaks at 2.4 meV and 3.7 meV in the orientationally ordered phase. Orientational disorder softens libron modes. Intra-molecular phonons below 70 meV also show significant dispersion. Our results are in good agreement with the recent inelastic neutron scattering experiments. MD is a numerical approach which involves the solution of Newton's equations for particles in the system. The multiple-time-step (MTS) approach reduces this computation significantly by exploiting the different time scales for short-range and intermediate-range interactions. Using the linked-list scheme, parallel algorithms are designed to implement on the in-house 8-node iPSC/860, a MIMD (multiple instruction multiple data) machine. Our group has also developed a quantum dynamical simulation scheme for Computational Nanoelectronics based on the quantum molecular dynamics (QMD) method. The QMD algorithm has been implemented on the in-house 8,192-node MasPar, a SIMD (single instruction multiple data) architecture. read less

Abstract: Aluminum oxide (α-Al2O3) is known as one of the major ceramic oxide and is currently used for its advanced mechanical properties. Nowadays, it requires a more in-depth description at small-scales especially for applications in the fields of nanocrystalline ceramic fabrication and nanomechanics. In this study, we investigate the transferability of several types of interatomic potentials including rigid ion, 2/3-body and many-body variable charge models. In particular, a special attention is paid to the material properties that are the most relevant for nanomechanical applications such as lattice properties, surface and stacking fault energies as well as dislocation modeling. Simulation outcomes are compared to reliable DFT simulations and most up-to-date experiments available from the literature. read less

Abstract: Self-learning hybrid Monte Carlo (SLHMC) is a first-principles simulation that allows for exact ensemble generation on potential energy surfaces based on density functional theory. The statistical sampling can be accelerated with the assistance of smart trial moves by machine learning potentials. In the first report [Nagai et al., Phys. Rev. B 102, 041124(R) (2020)], the SLHMC approach was introduced for the simplest case of canonical sampling. We herein extend this idea to isothermal-isobaric ensembles to enable general applications for soft materials and liquids with large volume fluctuation. As a demonstration, the isothermal-isobaric SLHMC method was used to study the vibrational structure of liquid silica at temperatures close to the melting point, whereby the slow diffusive motion is beyond the time scale of first-principles molecular dynamics. It was found that the static structure factor thus computed from first-principles agrees quite well with the high-energy x-ray data. read less

Abstract: This article reviews recent works concerning the damage area size ahead of a crack front in oxide glasses. Literature exemplifies the importance of understanding the crack propagation dynamics in oxide glasses with a number of papers concerning dynamic and stress corrosion cracking in complex (and simplified) oxide glasses. In general, the linear elastic fracture mechanic equation breaks down near the crack front, in a zone frequently called the process zone. The question is how large is this zone near the crack tip? This paper reviews four different techniques to capture the process zone size in pure silica: Atomic force microscopy imaging, neutron diffraction, photon capture, and molecular dynamics simulations. Velocities of the crack front range between stress corrosion cracking dynamics and dynamic fracture, thus providing a better understanding of how the process zone depends on the velocity of the crack front. read less

Abstract: A natural extension of the descriptors used in the Spectral Neighbor Analysis Potential (SNAP) method is derived to treat atomic interactions in chemically complex systems. Atomic environment descriptors within SNAP are obtained from a basis function expansion of the weighted density of neighboring atoms. This new formulation instead partitions the neighbor density into partial densities for each chemical element, thus leading to explicit multi-element descriptors. For Nelem chemical elements, the number of descriptors increases as Ο(Nelem3), while the computational cost of the force calculation as implemented in LAMMPS is limited to Ο(Nelem2) and the favorable linear scaling in the number of atoms is retained. We demonstrate these chemically aware descriptors by producing an interatomic potential for indium phosphide capable of capturing high-energy defects that result from radiation damage cascades. This new explicit multi-element SNAP method reproduces the relaxed defect formation energies with substantially greater accuracy than weighted-density SNAP, while retaining accurate representation of the bulk indium phosphide properties. read less

Abstract: ABSTRACT Silica and nickel are frequently used in the synthesis of carbon nanotubes by the chemical vapour deposition (CVD) process. The molecular simulation of this process requires the knowledge of force fields to model the interactions occurring between those species at the atomic scale. This work proposes a combined force field to model the silica/nickel system when these components are in contact as substrate and catalyst, respectively, in the CVD process. The proposed combined force field includes the Lennard–Jones (n–m) potential for modelling the silicon/nickel pair interactions and the Buckingham potential for the oxygen/nickel pair interactions. The combined force field is completed by the Tersoff potential to model silica (SiO2) and the Sutton–Chen potential for the cohesive forces present in the nickel clusters. Parameters for the Lennard–Jones (n–m) and Buckingham pair potentials were fitted, by the least squares technique, to interaction energies data for the silica/nickel system. The energies were obtained from Ab-initio (DFT) calculations using the VASP code. It was found that the combined force field reproduces adequately, by molecular dynamics simulation, the adherence (adsorption) of nickel clusters on the silica surface. Keeping stable this configuration is crucial in modelling the carbon nanotubes synthesis by the CVD process. read less

Abstract: We combine state-of-the-art Green's-function methods and nonequilibrium molecular dynamics calculations to study phonon transport across the unconventional interfaces that make up crystal-phase and twinning superlattices in nanowires. We focus on two of their most paradigmatic building blocks: cubic (diamond/zinc blende) and hexagonal (lonsdaleite/wurtzite) polytypes of the same group-IV or III-V material. Specifically, we consider InP, GaP and Si, and both the twin boundaries between rotated cubic segments and the crystal-phase boundaries between different phases. We reveal the atomic-scale mechanisms that give rise to phonon scattering in these interfaces, quantify their thermal boundary resistance and illustrate the failure of common phenomenological models in predicting those features. In particular, we show that twin boundaries have a small but finite interface thermal resistance that can only be understood in terms of a fully atomistic picture. read less

Abstract: Aluminum oxide nanoparticles are increasingly sought in numerous technological applications. However, as the nanoparticles grow during the synthesis, two phase transitions occur. At the nanoscale, numerical simulation of the stability of the alumina phases requires the use of empirical potentials that are reliable over a large range of system sizes going from a few atoms to several hundred thousand atoms. In this work, we confronted four different empirical potentials that are currently employed for bulk alumina. We found that only two of them are correct at the molecular level when compared to DFT calculations. Furthermore, the two potentials remain the best at the nanoscale as they reproduce one or two phase transitions that were observed experimentally: from amorphous solid to cubic crystal ({\gamma}) and from cubic to hexagonal ({\alpha}, i.e. corundum) crystal. read less

Abstract: Ta addition has considerable effects on the microstructures and mechanical performances of CoCrFeNi alloy systems. Structure search with the special quasirandom structure method and structure identification with first-principles calculations were carried out to investigate the structural, thermodynamic and mechanical properties of CoCrFeNiTax (x = 0.0–1.0) high-entropy alloys in the fcc and bcc lattice frameworks. The predicted lattice parameters of identified structures are in agreement with available experiments. Phase transition between the fcc and bcc lattices was predicted for the lowest-energy structures with increasing Ta content. The predicted temperature dependence of specific heat capacity for the identified structures matches well with the Dulong–Petit, Kepp and Debye Models. Both vibration and configuration entropy contribute to the stabilization of alloy systems, while the latter is about 2–3 times greater than the former. The elastic constants and moduli vary with composition and phase structure. Ta atoms have preference to some atoms like Ni, and form relatively strong bonds with adjacent atoms. The introduction of Ta promotes the electron localization and favors the formation of mixed structures. read less

Abstract: In this paper, we present visco‐plastic effects of vitreous silica. By introducing a stepwise loading‐relaxation procedure, we evaluate a quasi‐static hysteresis and observe permanent plastic deformation. The investigation of the medium‐range order of the sample at various deformation states provides new insights into the changes of the atomistic arrangements caused by the plastification. read less

Abstract: Using molecular dynamics simulations, we prepare vitreous silica by quenching molten silica to ambient temperature. Varying the quenching rate, we show that the latter barely influences the short‐range order, whereas the medium‐range configuration is significantly affected. We subject the prepared silica specimens to tension until fracture occurs and demonstrate the effect on the material behaviour. We evaluate the medium‐range configuration in terms of the ring statics of the glass network structure. Linking the ring statistics with the deformation and fracture behaviour of vitreous silica, we provide a topologically motivated explanation of the material behaviour. read less

Abstract: We apply a recently developed optimization scheme to obtain effective potentials for alkali and alkaline-earth aluminosilicate glasses that contain lithium, sodium, potassium, or calcium as modifiers. As input data for the optimization, we used the radial distribution functions of the liquid at high temperature generated by means of ab initio molecular dynamics simulations and density and elastic modulus of glass at room temperature from experiments. The new interaction potentials are able to reproduce reliably the structure and various mechanical and vibrational properties over a wide range of compositions for binary silicates. We have tested these potentials for various ternary systems and find that they are transferable and can be mixed, thus allowing us to reproduce and predict the structure and properties of multicomponent glasses. read less

Abstract: We propose a new scheme to parameterize effective potentials that can be used to simulate atomic systems such as oxide glasses. As input data for the optimization, we use the radial distribution functions of the liquid and the vibrational density of state of the glass, both obtained from ab initio simulations, as well as experimental data on the pressure dependence of the density of the glass. For the case of silica, we find that this new scheme facilitates finding pair potentials that are significantly more accurate than the previous ones even if the functional form is the same, thus demonstrating that even simple two-body potentials can be superior to more complex three-body potentials. We have tested the new potential by calculating the pressure dependence of the elastic moduli and found a good agreement with the corresponding experimental data. read less

Abstract: Computation on dynamic n-tuples of particles is ubiquitous in scientific computing, with an archetypal application in many-body molecular dynamics (MD) simulations. We propose a tuple-decomposition (TD) approach that reorders computations according to dynamically created lists of n-tuples. We analyze the performance characteristics of the TD approach on general purpose graphics processing units for MD simulations involving pair (n = 2) and triplet (n = 3) interactions. The results show superior performance of the TD approach over the conventional particle-decomposition (PD) approach. Detailed analyses reveal the register footprint as the key factor that dictates the performance. Furthermore, the TD approach is found to outperform PD for more intensive computations of quadruplet (n = 4) interactions in first principles-informed reactive MD simulations based on the reactive force-field (ReaxFF) method. This work thus demonstrates the viable performance portability of the TD approach across a wide range of applications. read less

Abstract: In this study, we evaluate three functionals (B3LYP, PBE, and M06) and 29 basis sets to find a suitable model to calculate silicon oxide clusters using density functional theory. The experimental values of electron affinities and vertical detachment energies of SiO2 and Si2O4 clusters were compared with the calculated ones using combinations of the three functionals and 29 basis sets. The calculated SiO bond lengths and SiOSi angles of Si2O7 cluster were also compared with the experimental values. Our conclusion is that the PBE/DGDZVP model is the best for calculating the structural and electronic properties of silicon oxide clusters. read less

Abstract: Using molecular dynamics simulations, we study collisions between amorphous silica nanoparticles. Our silica model contains uncontaminated surfaces, that is, the effect of surface hydroxylation or of adsorbed water layers is excluded. For central collisions, we characterize the boundary between sticking and bouncing collisions as a function of impact velocity and particle size and quantify the coefficient of restitution. We show that the traditional Johnson-Kendall-Roberts (JKR) model provides a valid description of the ingoing trajectory of two grains up to the moment of maximum compression. The distance of closest approach is slightly underestimated by the JKR model, due to the appearance of plasticity in the grains, which shows up in the form of localized shear transformation zones. The JKR model strongly underestimates the contact radius and the collision duration during the outgoing trajectory, evidencing that the breaking of covalent bonds during grain separation is not well described by this model. The adhesive neck formed between the two grains finally collapses while creating narrow filaments joining the grains, which eventually tear. read less

Abstract: Bulk-type all-solid-state Na/S cells, which are expected to have high capacity, be highly safe, and have low material cost, were fabricated using a Na3PS4 glass-ceramic as a solid electrolyte. The sulfur composite electrodes were prepared by mechanical milling of sulfur active material, a conductive additive (acetylene black), and a Na3PS4 glass-ceramic electrolyte. The all-solid-state Na/S cells used the reaction up to the final discharge product of sulfur active material, Na2S, and achieved a high capacity of ∼1100 mAh (g of S)−1 at room temperature. The rate of utilization of sulfur active material was ∼2 times higher than that of high-temperature-operating NAS batteries (commercially available NAS batteries, Na/sintered β″-alumina/S), where Na2Sx melts with bridging sulfurs contribute to redox in the sulfur electrodes. The open circuit potential curve of the discharge process of the Na/S batteries operating at room temperature was similar to that of the NAS batteries operating at high temperatures; X-... read less

Abstract: Silica aerogels are nanostructured, highly porous solids which have, compared to other soft materials, special mechanical properties, such as extremely low densities. In the present work, the mechanical properties of silica aerogels have been studied with molecular dynamics (MD) simulations. The aerogel model of 192 000 atoms was created with different densities by direct expansion of β-cristobalite and subjected to series of thermal treatments. Because of the high number of atoms and improved modeling procedure, the proposed model was more stable and showed significant improvement in the smoothness of the resulting stress-strain curves in comparison to previous models. Resulting Poisson's ratio values for silica aerogels lie between 0.18 and 0.21. The elasticity moduli display a power law dependence on the density, with the exponent estimated to be 3.25 ± 0.1. These results are in excellent agreement with reported experimental as well as computational values. Two different deformation scenarios have been discussed. Under tension, the low-density aerogels were more ductile while the denser ones behaved rather brittle. In the compression simulations of low-density aerogels, deformation occurred without significant increase in stress. However, for high densities, atoms offer a higher resistance to the deformation, resulting in a more stiff response and an early densification. The relationship between different mechanical parameters has been found in the cyclic loading simulations of silica aerogels with different densities. The residual strain grows linearly with the applied strain (≥0.16) and can be approximated by a phenomenological relation ϵp = 1.09ϵmax - 0.12. The dissipation energy also varies with the compressive strain according to a power law with an exponent of 2.31 ± 0.07. Moreover, the tangent modulus under cyclic loading varies exponentially with the compressive strain. The results of the study pave the way toward multiscale modeling of silica as well as reinforced silica aerogels. read less

Abstract: The study of the structure and dynamics of network-forming materials is reviewed. Experimental techniques used to extract key structural information are briefly considered. Strategies for building simulation models, based on both targeting key (experimentally-accessible) materials and on systematically controlling key model parameters, are discussed. As an example of the first class of materials, a key target system, SiO2, is used to highlight how the changing structure with applied pressure can be effectively modelled (in three dimensions) and used to link to both experimental results and simple structural models. As an example of the second class the topology of networks of tetrahedra in the MX2 stoichiometry are controlled using a single model parameter linked to the M–X–M bond angles. The evolution of ordering on multiple length-scales is observed as are the links between the static structure and key dynamical properties. The isomorphous relationship between the structures of amorphous Si and SiO2 is discussed as are the similarities and differences in the phase diagrams, the latter linked to potential polyamorphic and ‘anomalous’ (e.g. density maxima) behaviour. Links to both two-dimensional structures for C, Si and Ge and near-two-dimensional bilayers of SiO2 are discussed. Emerging low-dimensional structures in low temperature molten carbonates are also uncovered. read less

Abstract: In this paper, we use a molecular dynamics simulation and Raman scattering measurements to study the vibrational and structural characteristics of barium disilicate, BaSi2O5, in vitreous and crystalline states. Our proposed atomic interaction potential describes the structural and dynamic behaviour of this material very well and can also be used for further extended studies. In addition, Raman spectroscopy enabled validation of the predictions of the potential by comparing the simulated vibrational density of states with the spectrum of the material in its vitreous state. Furthermore, we characterized the kinetics of the crystallization process through in situ Raman measurements as a function of temperature. read less

Abstract: Thermal conductivity in amorphous SiO 2 ( 𝑎 -SiO 2 ) has been studied in a wide range of temperatures, by using the nonequilibrium molecular dynamics method and the Beest–Kramer–Santen, Tersoff, and Vashishta empirical potentials. The thermal conductivity of an 𝑎 -SiO 2 -based composite with Si nanocrystals is calculated with the use of the Tersoff potential. The thermal conductivity of the nanocomposite is shown to firstly decrease and then to increase, as the silicon volumetric ratio grows. The obtained results are explained by the enhanced scattering of thermal vibrations at the matrix–Si nanocrystal boundaries. read less

Abstract: Relationships between the structural and dynamical properties of network-forming materials are investigated. A generic model is utilised for systems of stoichiometry MX2 which are linked in the sense that they can all be usefully considered as constructed from linked MX4 tetrahedra. A single model parameter (the anion polarizability) is varied systematically to control the mean MXM bond angles (and hence the network topologies). The networks evolve from those dominated by corner-sharing units to those dominated by edge-sharing structural motifs. These changes are accompanied by changes in the characteristic length-scales, with the emergence of ordering on intermediate length-scales. Key dynamical properties (the liquid relaxation just above the melting point and the liquid fragility) are studied and their relationship to the underlying static structure analysed. read less

Abstract: In an effort to extend the reach of current ab initio calculations to simulations requiring millions of configurations for complex systems such as heterostructures, we have parameterized the third-generation Charge Optimized Many-Body (COMB3) potential using solely ab initio total energies, forces, and stress tensors as input. The quality and the predictive power of the new forcefield are assessed by computing properties including the cohesive energy and density of SiO2 polymorphs, surface energies of alpha-quartz, and phonon densities of states of crystalline and amorphous phases of SiO2. Comparison with data from experiments, ab initio calculations, and molecular dynamics simulations using published forcefields including BKS (van Beest, Kramer, and van Santen), ReaxFF, and COMB2 demonstrates an overall improvement of the new parameterization. The computed temperature dependence of the thermal conductivity of crystalline alpha-quartz and the Kapitza resistance of the interface between crystalline Si(001) and amorphous silica is in excellent agreement with experiment, setting the stage for simulations of complex nanoscale heterostructures. read less

Abstract: The companion paper (Part I; Dragic et al., this issue),1 resolved an anomaly involving the acoustical properties of high alumina content glass optical fibers. In Part II, some attributes of alumina and silica polymorphs are considered and used to infer approximate pertinent properties of amorphous Al2O3 “glass.” An efficient computational method is used, based on a simple, self-consistent version of the Voigt-Reuss-Hill procedure. Applied to SiO2 polymorphs, it provides insights into the short-range ordering in “amorphous” silica. read less

Abstract: Understanding protein–inorganic surface interactions is central to the rational design of new tools in biomaterial sciences, nanobiotechnology and nanomedicine. Although a significant amount of experimental research on protein adsorption onto solid substrates has been reported, many aspects of the recognition and interaction mechanisms of biomolecules and inorganic surfaces are still unclear. Theoretical modeling and simulations provide complementary approaches for experimental studies, and they have been applied for exploring protein–surface binding mechanisms, the determinants of binding specificity towards different surfaces, as well as the thermodynamics and kinetics of adsorption. Although the general computational approaches employed to study the dynamics of proteins and materials are similar, the models and force-fields (FFs) used for describing the physical properties and interactions of material surfaces and biological molecules differ. In particular, FF and water models designed for use in biomolecular simulations are often not directly transferable to surface simulations and vice versa. The adsorption events span a wide range of time- and length-scales that vary from nanoseconds to days, and from nanometers to micrometers, respectively, rendering the use of multi-scale approaches unavoidable. Further, changes in the atomic structure of material surfaces that can lead to surface reconstruction, and in the structure of proteins that can result in complete denaturation of the adsorbed molecules, can create many intermediate structural and energetic states that complicate sampling. In this review, we address the challenges posed to theoretical and computational methods in achieving accurate descriptions of the physical, chemical and mechanical properties of protein-surface systems. In this context, we discuss the applicability of different modeling and simulation techniques ranging from quantum mechanics through all-atom molecular mechanics to coarse-grained approaches. We examine uses of different sampling methods, as well as free energy calculations. Furthermore, we review computational studies of protein–surface interactions and discuss the successes and limitations of current approaches. read less

Abstract: Transient atomic displacements during a resonant thickness-shear vibration of AT-cut α-quartz are revealed by time-resolved X-ray diffraction under an alternating electric field. The lattice strain resonantly amplified by the alternating electric field is ∼104 times larger than that induced by a static electric field. The resonantly amplified lattice strain is achieved by fast displacements of oxygen anions and collateral resilient deformation of Si−O−Si angles bridging rigid SiO4 tetrahedra, which efficiently transduce electric energy into elastic energy. read less

Abstract: A series of molecular dynamics (MD) simulations are carried out, in which the Na+ content at the interlayers of Na2Ni2TeO6 is systematically varied, keeping the overall charge neutrality of the sys... read less

Abstract: An interatomic potential is proposed for the recently discovered family of superionic solids of the formula Na2M2TeO6, where M = Ni, Zn, Co, or Mg. Molecular dynamics simulations demonstrating the quality of the potential in reproducing various structural and transport properties of this promising class of materials is presented. The study provides fresh insights on the microscopic energetics and Na+ migration pathways. Strong ion–ion correlations, resulting in a highly cooperative conduction mechanism, emerge from the study. read less

Abstract: The superior thermal, optical, acoustical, and mechanical properties of a nano-porous and ultra-light material called silica aerogel have been known to science since the 1930's. In this study, we propose a new analytical–numerical model of a silica aerogel to predict its Young's modulus. The molecular dynamics (MD) simulation method was adopted to model and simulate the backbone of silica aerogels with different densities in an improved negative pressure rupturing method, and their mechanical properties were investigated. To generate the model using MD, the traditional method was improved by changing the sequence of its procedures, and it was proven to be a more stable and physically representative method. In the prediction, we proposed a two-level nano-porous structure model according to our simulations and the widely accepted fractal structure of silica aerogels. The Young's modulus of a silica aerogel, which is shown in a power–law relationship with the density of samples, was derived by the two-level hierarchical model and uniaxial tension tests. We envisage that this new model can be applied in more analytical–numerical studies to reveal other interesting mechanical properties of silica aerogels. read less

Abstract: Porous structures of silica aerogels are generated using classical molecular dynamics, with the Tersoff potential, which has been re-parametrized for modeling silicon dioxides. This work demonstrates that this potential is superior to the widely used BKS potential in terms of characterizing the thermal conductivities of amorphous silica, by comparing the vibrational density of states with previous experimental studies. Aerogel samples of increasing densities are obtained through an expanding, heating and quenching process. Reverse non-equilibrium molecular dynamics is applied at each density to determine the thermal conductivity. A power-law fit of the results is found to accurately reflect the power-law variation found in experimental bulk aerogels. The results are also of the same order of magnitude as experimental bulk aerogels, but they are consistently higher. By analyzing the pore size distribution on different simulation length scales, we show that such a disparity is due to finite sizes of pores that can be represented, where increasing simulation length scales lead to an increase in the largest pore size that can be modeled. read less

Abstract: The structure of several network-forming liquids and glasses is considered, where a focus is placed on the detailed information that is made available by using the method of neutron diffraction with isotope substitution (NDIS). In the case of binary network glass-forming materials with the MX2 stoichiometry (e.g. GeO2, GeSe2, ZnCl2), two different length scales at distances greater than the nearest-neighbour distance manifest themselves by peaks in the measured diffraction patterns. The network properties are influenced by a competition between the ordering on these "intermediate" and "extended" length scales, which can be manipulated by changing the chemical identity of the atomic constituents or by varying state parameters such as the temperature and pressure. The extended-range ordering, which describes the decay of the pair-correlation functions at large-r, can be represented by making a pole analysis of the Ornstein-Zernike equations, an approach that can also be used to describe the large-r behaviour of the pair-correlation functions for liquid and amorphous metals where packing constraints are important. The first applications are then described of the NDIS method to measure the detailed structure of aerodynamically-levitated laser-heated droplets of "fragile" glass-forming liquid oxides (CaAl2O4 and CaSiO3) at high-temperatures (~2000 K) and the structure of a "strong" network-forming glass (GeO2) under pressures ranging from ambient to ~8 GPa. The high-temperature experiments show structural changes on multiple length scales when the oxides are vitrified. The high-pressure experiment offers insight into the density-driven mechanisms of network collapse in GeO2 glass, and parallels are drawn with the high-pressure behaviour of silica glass. Finally, the hydrogen-bonded network of water is considered, where the first application of the method of oxygen NDIS is used to measure the structures of light versus heavy water and a difference of approximately equal to 0.5% is found between the O-D and O-H intra-molecular bond lengths. The experimental data are best matched by using path integral molecular dynamics simulations with a flexible anharmonic water model, and the results support a competing quantum effects model for water in which its structural and dynamical properties are governed by an offset between intra-molecular and inter-molecular quantum contributions. read less

Abstract: While nanowires and nanosheets (NSs) grown on lattice-mismatched substrates have a number of promising technological applications such as solar cells, generation of misfit dislocations (MFDs) at their interfaces is a major concern for the efficiency of these devices. Here, combined molecular-dynamics and quantum-mechanical simulations are used to study MFDs at the interface between a GaAs NS and a Si substrate. Simulation results show the existence of a critical NS thickness, below which NSs are grown free of MFDs. The calculated critical thickness value is consistent with available experimental observations. Charge transfer at the MFD core is found to modify the electronic band profile at the GaAs/Si interface significantly. These effects should have profound impacts on the efficiency of lattice-mismatched NS devices. read less

Abstract: Generation of twin defects during the growth of semiconductor nanowires is a major concern, but their effects on electronic properties are not well understood. Here, combined quantum-mechanical and molecular-dynamics simulations reveal that the radiative decay time of an exciton increases due to twin. Furthermore, the twin-scattering contribution to electron mobility is found to be significant, in conformity with photoconductivity measurements. In addition to acting as a carrier-scattering source, twins in nanowires are found to modify the mobility by changing strain and thereby the effective mass. These effects should have profound impacts on the efficiency of nanowire-based devices. read less

Abstract: The structural and dynamic properties of calcium aluminosilicate (CaO-Al2O3)1-x(SiO2)x melts with low silica content, namely, along the concentration ratio R = 1 are studied by classical molecular dynamics. An empirical potential has been developed here on the basis of our previous ab initio molecular dynamics. The new potential gives a description of the structural as well as the dynamics with a good accuracy. The self-intermediate scattering function and associated α-relaxation times are analyzed within the mode-coupling theory. Our results indicate a decrease of the fragility whose structural origin is a reduction of the number of fivefold coordinated Al atoms and non-bridging oxygen. read less

Abstract: Recent advances in the study of network-forming materials are described for systems dominated both by ionic and covalent interatomic interactions. Modelling strategies are described which focus both on describing specific systems of interest and on modelling the systematic evolution of network topology. The effect of network topology on the presence of ordering both on intermediate- and extended-length-scales is discussed. The effect of the topology on the mechanical rigidity is also described and analysed in terms of a mean coordination model. In addition, the isomorphology between amorphous silicon and the silicon sub-lattice in SiO(2) is described. Polyamorphism in Si and ZnCl(2) is analysed and discussed. Finally, the study of reduced (two) dimensional systems is discussed for carbon, silicon and germanium. read less

Abstract: The effects of network topology on the static structural, mechanical and dynamic properties of MX2 network-forming liquids (with tetrahedral short-range order) are discussed. The network topology is controlled via a single model parameter (the anion polarizability) which effectively constrains the inter-tetrahedral linkages in a physically transparent manner. Critically, it is found to control the balance between the stability of corner- and edge-sharing tetrahedra. A potential rigidity transformation is investigated. The vibrational density of states is investigated, using an instantaneous normal model analysis, as a function of both anion polarizability and temperature. A low frequency peak is seen to appear and is shown to be correlated with the fraction of cations which are linked through solely edge-sharing structural motifs. A modified effective mean atom coordination number is proposed which allows the appearance of the low frequency feature to be understood in terms of a mean field rigidity percolation threshold. read less

Abstract: A new short-range pairwise numerical potential for silica is presented. The potential is derived from a single ab initio molecular dynamics (AIMD) simulation of molten silica using the force-matching method with the forces being represented numerically by piecewise functions (splines). The AIMD simulation is performed using the Born-Oppenheimer method with the generalized gradient approximation (BLYP) for the XC energy functional. The new effective potential includes a soft-repulsive shoulder to describe the interactions of oxygen ions at short separations. The new potential, despite being short-ranged and derived from single-phase data, exhibits a good transferability to silica crystalline polymorphs and amorphous silica. The importance of the O-O soft-repulsive shoulder interaction on glass densification under cold and shock compressions is assessed from MD simulations of silica glass under room and shock Hugoniot conditions, respectively. Results from these simulations indicate that the appearance of oxygen complexes (primarily pairs) interacting through soft-repulsive shoulder potential occurs at 8-10 GPa, and under cold compression conditions becomes notable at 40 GPa, essentially coinciding with the transition to a Si sixfold coordination state. An analysis of changes in system structure in compressed and shocked states reveals that the O ions interacting through the soft-repulsive shoulder potential in denser states of silica glass may create a mechanical multi-stability under elevated pressures and thus to contribute to the observed anomalous densification. read less

Abstract: We present a set of Coulomb point charges and van der Waals parameters for molecular dynamics simulations of interfaces between natively deprotonated amorphous SiO2 surfaces and liquid water, to be used in combination with standard biomolecular force fields. We pay particular attention to the extent of negative charge delocalisation in the solid that follows the deprotonation of terminal silanol groups, as revealed by extensive Bader analysis of electronic densities computed by density functional theory (DFT). The absolute charge values in our force field are determined from best‐fitting to the electrostatic potential computed ab initio (ESP charges). Our proposed parameter set is found to reproduce the energy landscape of single water molecules over neutral and deprotonated amorphous SiO2 surfaces and, after a minor adjustment, over thin oxide films on Si. Our analysis reveals a certain degree of arbitrariness in the choice of the DFT scheme used as the reference for the force‐field optimisation procedure, highlighting its intrinsic limits. read less

Abstract: Atomic-scale formability of nanoimprint lithography using an atomically stepped mold is investigated in a molecular dynamics simulation for inorganic SiO2 glass material. Fast Fourier transformation analysis of the surface height of the glass is performed to confirm the periodicity of the atomic-step pattern. From the analysis, the resolution of glass nanoimprint lithography is found to be 0.2 nm for the atomically stepped mold. This theoretical resolution agrees with the experimental resolution. read less

Abstract: We extend the program potfit, which generates effective atomic interaction potentials from ab initio data, to electrostatic interactions and induced dipoles. The potential parametrization algorithm uses the Wolf direct, pairwise summation method with spherical truncation. The polarizability of oxygen atoms is modeled with the Tangney-Scandolo interatomic force field approach. Due to the Wolf summation, the computational effort in simulation scales linearly in the number of particles, despite the presence of electrostatic interactions. Thus, this model allows to perform large-scale molecular dynamics simulations of metal oxides with realistic potentials. Details of the implementation are given, and the generation of potentials for SiO(2) and MgO is demonstrated. The approach is validated by simulations of microstructural, thermodynamic, and vibrational properties of liquid silica and magnesia. read less

Abstract: The interface between nano-scale films is of relevance in many critical applications. Specifically, recent technological advances in semiconductor industry that utilize silicon-on-insulator devices have given importance to the understanding of thermal transport across ${\rm Si}{\hbox{-}}{\rm SiO}_{2}$ interface. Estimates of interfacial (Kapitza) resistance to the thermal transport across ${\rm Si}{\hbox{-}}{\rm SiO}_{2}$ films do not appear to exist at the present time. In this paper, we develop and carryout reverse non-equilibrium molecular dynamics simulations by imposing known heat flux to determine the Kapitza resistance between ${\rm Si}{\hbox{-}}{\rm SiO}_{2}$ thin films. For the ${\rm Si}{\hbox{-}}{\rm SiO}_{2}$ interface, the average Kapitza resistance for a ${\sim}{8}~{\rm\AA}$ thick oxide layer system was 0.503 ${\times}10^{-9}~{\rm m}^{2}{\rm K}/{\rm W}$ and for a ${\sim}{\rm 11.5}~{\rm\AA}$ thick oxide layer system was 0.518$\,\times 10^{-9}~{\rm m}^{2}{\rm K}/{\rm W}$. These values were of the same order of magnitude as the Kapitza resistance values determined from the acoustic mismatch model and the diffuse mismatch model for the ${\rm Si}\hbox{-}{\rm SiO}_{2}$ interface. read less

Abstract: In this paper, fundamental aspects of the Bias Temperature Instability (BTI) in FETs with metal gate/high-k (HKMG) gate stacks are discussed from a single defect point of view. First, Random Telegraph Noise (RTN) measurements are used to show that the capture/emission processes of individual defects in highly scaled HKMG FETs exhibit very similar Poisson statistics and can be fully characterized by a characteristic electron/hole capture, τc, and emission time, τe, in NFET/PFET. In all cases, capture and emission are found to be thermally activated. These observations suggest that NBTI and PBTI share similar microscopic trapping/de-trapping mechanism, for holes and electrons, respectively. Based on these findings, a simple physical model is introduced which describes the behavior of a distribution of identical defects (characterized by τc and τe) but provides deep insights into the BTI dynamics under AC stress in general. The occupancy level of identical defects at equilibrium is found to becomes frequency, ƒ, independent for ƒ ≫ [1/τc, 1/τe], such that the BTI behavior at operation conditions (∼GHz) can be measured at relatively low frequencies (in the kHz range). The single defect model was then expanded to predict the macroscopic BTI behaviors in NMOS devices for arbitrary stress conditions. Excellent agreement between model prediction and experimental data is demonstrated, confirming that PBTI in HKMG gate stacks can be understood as a superposition of trapping/de-trapping events from individual defects in the gate stack. The overall dynamics of PBTI is thus largely governed by the distribution of electron capture and emission times of the defects in the gate stack. The challenges for using a capture and emission time based model for product lifetime predictions are addressed. read less

Abstract: An effective interatomic interaction potential for AlN is proposed. The potential consists of two-body and three-body covalent interactions. The two-body potential includes steric repulsions due to atomic sizes, Coulomb interactions resulting from charge transfer between atoms, charge-induced dipole-interactions due to the electronic polarizability of ions, and induced dipole–dipole (van der Waals) interactions. The covalent characters of the Al–N–Al and N–Al–N bonds are described by the three-body potential. The proposed three-body interaction potential is a modification of the Stillinger–Weber form proposed to describe Si. Using the molecular dynamics method, the interaction potential is used to study structural, elastic, and dynamical properties of crystalline and amorphous states of AlN for several densities and temperatures. The structural energy for wurtzite (2H) structure has the lowest energy, followed zinc-blende and rock-salt (RS) structures. The pressure for the structural transformation from w... read less

Abstract: We present a reformulation of the Tangney-Scandolo interatomic force field for silica [J. Chem. Phys. 117, 8898 (2002)], which removes the requirement to perform an Ewald summation. We use a Yukawa factor to screen electrostatic interactions and a cutoff distance to limit the interatomic potential range to around 10 Å. A reparametrization of the potential is carried out, fitting to data from density functional theory calculations. These calculations were performed within the local density approximation since we find that this choice of functional leads to a better match to the experimental structural and elastic properties of quartz and amorphous silica than the generalized gradient approximation approach used to parametrize the original Tangney-Scandolo force field. The resulting O(N) scheme makes it possible to model hundreds of thousands of atoms with modest computational resources, without compromising the force field accuracy. The new potential is validated by calculating structural, elastic, vibrational, and thermodynamic properties of α-quartz and amorphous silica. read less

Abstract: Molecular dynamics (MD) simulations are conducted to study the dynamic responses of silicate glass shocked at velocities from 1 to 19 km/s. The simulated pressure and density of the glass under shock increase as the cooling rate increases, although the effect of the cooling rate on the shock wave velocity is limited. It appears the simulation results match well with the experimental and EOS analysis data for the glass shocked under particle velocity below 10 km/s. However, the simulations under-estimate the density of the glass sample shocked at particle velocities between 12 km/s and 19 km/s. read less

Abstract: The mechanisms of physical and chemical interactions of low temperature plasmas with surfaces can be fruitfully explored using molecular dynamics (MD) simulations. MD simulations follow the detailed motion of sets of interacting atoms through integration of atomic equations of motion, using inter-atomic potentials that can account for bond breaking and formation that result when energetic species from the plasma impact surfaces. This paper summarizes the current status of the technique for various applications of low temperature plasmas to material processing technologies. The method is reviewed, and commonly used inter-atomic potentials are described. Special attention is paid to the use of MD in understanding various representative applications, including tetrahedral amorphous carbon film deposition from energetic carbon ions, the interactions of radical species with amorphous hydrogenated silicon films, silicon nanoparticles in plasmas, and plasma etching. read less

Abstract: Indium phosphide is investigated using molecular dynamics (MD) simulations and density-functional theory calculations. MD simulations use a proposed effective interaction potential for InP fitted to a selected experimental dataset of properties. The potential consists of two- and three-body terms that represent atomic-size effects, charge–charge, charge–dipole and dipole–dipole interactions as well as covalent bond bending and stretching. Predictions are made for the elastic constants as a function of density and temperature, the generalized stacking fault energy and the low-index surface energies. read less

Abstract: The role of many-body effects in modeling silica was investigated using self-consistent force matching. Both pairwise and polarizable classical force fields were developed systematically from ab initio density functional theory force calculations, allowing for a direct comparison of the role of polarization in silica. It was observed that the pairwise potential performed remarkably well at reproducing the basic silica tetrahedral structure. However, the Si-O-Si angle that links the silica tetrahedra showed small but distinct differences with the polarizable potential, a result of the inability of the pairwise potential to properly account for variations in the polarization of the oxygens. Furthermore, the transferability of the polarizable potential was investigated and suggests that additional forces may be necessary to more completely describe silica annealing. read less

Abstract: The atomistic origin of the intermediate-range order (IRO) is investigated for an archetypal network-forming liquid. A pairwise additive potential model is chosen which is augmented with a description of the (many-body) anion polarization. The anion polarizability and system temperature are both systematically varied in order to control the network topology. The change in the IRO is monitored via the construction of Bhatia-Thornton structure factors which highlight the effect of chemical composition and network topology. The atomistic origin of the first-sharp diffraction peak in the concentration-concentration function, S(CC)(k), is discussed in terms of the connectivity of the polyhedral network. The atomistic origin of the IRO is discussed by reference to previous analyses. read less

Abstract: The interface between nano-scale films is of relevance in many critical applications. Specifically, recent technological advances in semiconductor industry that utilize Silicon-on-Insulator (SOI) devices have given urgency to understanding thermal transport across Si-SiO2 interface. Estimates of interfacial (Kapitza) resistance to thermal transport across Si-SiO2 films do not appear to exist at the present time. In this paper, we develop and carryout reverse non-equilibrium molecular dynamics (NEMD) simulations by imposing known heat flux to determine the Kapitza resistance between Si-SiO2 thin films. For the Si-SiO2 interface, the average Kapitza resistance for a ~8 Aring thick oxide layer system was 0.503 times 10-9 m K/W and for a ~11.5 Aring thick oxide layer system was 0.518 times 10-9 m K/W. These values were of the same order of magnitude as the Kapitza resistance values determined from the acoustic mismatch model (AMM) and the diffuse mismatch model (DMM) for the Si-SiO2 interface. read less

Abstract: Structural and dynamical properties of crystalline alumina α-Al2O3 and amorphous and molten alumina are investigated with molecular dynamics simulation based on an effective interatomic potentials consisting of two- and three-body terms. Structural correlations are examined through pair distribution functions, coordination numbers, static structure factors, bond angle distributions, and shortest-path ring analyses. The calculated results for neutron and x-ray static structure factors are in good agreement with experimental results. Dynamical correlations, such as velocity autocorrelation function, vibrational density of states, current-current correlation function, and frequency-dependent conductivity, are also discussed. read less

Abstract: We studied the structural and dynamical properties of amorphous germanium oxide (GeO2) by means of the molecular dynamics technique. The simulations were done in the microcanonical ensemble, with a system at a density of 3.7 g cm−3, using a pairwise potential. The resulting neutron static structure factor is compared to experimental results. The network topology of our system is analyzed through partial pair correlations, coordination number and angle distributions. A detailed analysis of the interatomic distances reveals that in the amorphous state there is a short range order dominated by a slightly distorted Ge(O1/2)4 tetrahedron. Beyond that, there is an intermediate range order composed of vertex-sharing tetrahedra. The vibrational properties were characterized by means of the density of states, obtained as a Fourier transform of the velocity autocorrelation function. The vibrational density of states has two bands, a low frequency one related to the inter-tetrahedron vibration and a high frequency band related to the intra-tetrahedron vibration. read less

Abstract: We have developed a classical two- and three-body interaction potential to simulate the hydroxylated, natively oxidized Si surface in contact with water solutions, based on the combination and extension of the Stillinger-Weber potential and of a potential originally developed to simulate SiO(2) polymorphs. The potential parameters are chosen to reproduce the structure, charge distribution, tensile surface stress, and interactions with single water molecules of a natively oxidized Si surface model previously obtained by means of accurate density functional theory simulations. We have applied the potential to the case of hydrophilic silicon wafer bonding at room temperature, revealing maximum room temperature work of adhesion values for natively oxidized and amorphous silica surfaces of 97 and 90 mJm(2), respectively, at a water adsorption coverage of approximately 1 ML. The difference arises from the stronger interaction of the natively oxidized surface with liquid water, resulting in a higher heat of immersion (203 vs 166 mJm(2)), and may be explained in terms of the more pronounced water structuring close to the surface in alternating layers of larger and smaller densities with respect to the liquid bulk. The computed force-displacement bonding curves may be a useful input for cohesive zone models where both the topographic details of the surfaces and the dependence of the attractive force on the initial surface separation and wetting can be taken into account. read less

Abstract: We have carried out high-energy x-ray diffraction measurements on mechanically milled silica glass. It has been found that the first sharp diffraction peak (FSDP) in the structure factor S(Q) of silica glass appreciably decreases in intensity as a result of mechanical milling, whereas the observed features of the other peaks in S(Q) almost remain unchanged. The corresponding real-space correlation function of the milled samples shows a marked decrease in intensity at r∼5 Å. This gives an experimental manifestation that the dominant real-space structural correlation pertaining to the FSDP occurs at r∼5 Å. read less

Abstract: Multimillion-to-billion-atom molecular dynamics simulations are performed to investigate the interaction of voids in silica glass under hydrostatic tension. Nanometer size cavities nucleate in intervoid ligaments as a result of the expansion of Si-O rings due to a bond-switching mechanism, which involves bond breaking between Si-O and bond formation between that Si and a nonbridging O. With further increase in strain, nanocracks form on void surfaces and ligaments fracture through the growth and coalescence of ligament nanocavities in a manner similar to that observed in ductile metallic alloys. read less

Abstract: The chemical structure of an amorphous slag with blast furnace composition was investigated by means of molecular dynamics simulation. Our calculation suggested that the slag had a depolymerized network of SiO4 and AlO4 tetrahedra with interstitial cations, Ca2+ and Mg2+. The structural properties such as average coordination number obtained at 300 K were in good agreement with a recent NMR study, supporting the feasibility of the structure prediction by such simulation technique. At 1 873 K, the coordination numbers of the ions almost remained unchanged, while the intertetrahedral angles were found to be narrower, and the Qn distribution of AlO4 tetrahedra was slightly modified. The small amount but significant incorporation of MgO and Al2O3 influences the network connectivity, which should affect macroscopic properties such as viscosity. read less

Abstract: An effective interatomic interaction potential for SiC is proposed. The potential consists of two-body and three-body covalent interactions. The two-body potential includes steric repulsions due to atomic sizes, Coulomb interactions resulting from charge transfer between atoms, charge-induced dipole-interactions due to the electronic polarizability of ions, and induced dipole-dipole (van der Waals) interactions. The covalent characters of the Si–C–Si and C–Si–C bonds are described by the three-body potential. The proposed three-body interaction potential is a modification of the Stillinger-Weber form proposed to describe Si. Using the molecular dynamics method, the interaction potential is used to study structural, elastic, and dynamical properties of crystalline (3C), amorphous, and liquid states of SiC for several densities and temperatures. The structural energy for cubic (3C) structure has the lowest energy, followed by the wurtzite (2H) and rock-salt (RS) structures. The pressure for the structural t... read less

Abstract: We have applied our previously reported model of silica based on low coordination and strong association [J. Chem. Phys. 121, 8415 (2004)], to the calculation of phase stability of zeolite frameworks SOD, LTA, MFI, and FAU as silica polymorphs. We applied the method of Frenkel and Ladd for calculating free energies of these solids. Our model predicts that the MFI framework structure has a regime of thermodynamic stability at low pressures and above approximately 1400 K, relative to dense phases such as quartz. In contrast, our calculations predict that the less dense frameworks SOD, LTA, and FAU exhibit no regime of thermodynamic stability. We have also used our model to investigate whether templating extends the MFI regime of thermodynamic stability to lower temperatures, by considering templates with hard-sphere repulsions and mean-field attractions to silica. Within the assumptions of our model, we find that quartz remains the thermodynamically stable polymorph at zeolite synthesis temperatures (approximately 400 K) unless unphysically large template-silica attractions are assumed. These predictions suggest that some zeolites such as MFI may have regimes of thermodynamic stability even without template stabilization. read less

Abstract: The transition interface sampling (TIS) technique allows large free energy barriers to be overcome within reasonable simulation time, which is impossible for straightforward molecular dynamics. Still, the method does not impose an artificial driving force, but it surmounts the timescale problem by an importance sampling of true dynamical pathways. Recently, it was shown that the efficiency of TIS when calculating reaction rates is less sensitive to the choice of reaction coordinate than those of the standard free energy based techniques. This could be an important advantage in complex systems for which a good reaction coordinate is usually very difficult to find. We explain the principles of this method and discuss some of the promising applications related to zeolite formation. read less

Abstract: The pressure induced structural phase transformations of InP up to 100 GPa are investigated using molecular‐dynamics simulations. The calculated InP equation of state for the zincblende (ZB) phase at room temperature is in excellent agreement with experiments as well as several thermodynamic properties validating the interatomic potential employed in the simulations. Results show a sequence of dynamic structural transformations from ZB → rocksalt (RS) at 10 GPa, from RS → rhombohedral (RH) at 14 GPa, and from RH → CsCl at 70 GPa. The RS → RH transformation is a weakly first order transition while the other two transformations are typical first order transitions which present large volume drop and hysteresis in the reverse transformation. The RH intermediate phase between RS and CsCl was never considered for InP even though it was predicted for other materials. The ZB → RS transition uses a Pmm2 pathway while the RS → RH → CsCl is consistent with the Buerger mechanism which uses the R$ \bar 3 $m pathway. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) read less

Abstract: Dynamical properties of InP in the zincblende (ZB) are investigated using isothermal–isobaric molecular‐dynamics simulations based on a proposed interaction potential for InP consisting of two‐ and three‐body terms. The two‐body term represents steric repulsion, Coulomb interactions due to charge transfer, induced charge–dipole interaction, and van der Waals dipole–dipole interaction. The three‐body term represents covalent bond bending and stretching. The model is fitted to reproduce crystalline lattice constant, cohesive energy, and the structural transition pressure from ZB to rocksalt. The effects of hydro‐ static pressure and temperature on the vibrational density‐of‐states, phonon anharmonicity, dynamic Debye–Waller factor, thermal expansion coefficient are described as well as the pressure induced structural phase transformation. Results are consistent with available experimental data, in particular the calculated equation of state and phonon density‐of‐states have very good agreement. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) read less

Abstract: The general aim of this study is to test the reliability of polarizable model potentials for the prediction of vibrational (infrared and Raman) spectra in highly anharmonic systems such as high temperature crystalline phases. By using an ab initio parametrized interatomic potential for SiO2 and molecular dynamics simulations, we calculate the infrared and Raman spectra for quartz, cristobalite, and stishovite at various thermodynamic conditions. The model is found to perform very well in the prediction of infrared spectra. Raman peak positions are also reproduced very well by the model; however, Raman intensities calculated by explicitly taking the derivative of the polarizability with respect to the atomic displacements are found to be in poorer agreement than intensities calculated using a parametrized "bond polarizability" model. Calculated spectra for the high temperature beta phases, where the role of dynamical disorder and anharmonicities is predominant, are found to be in excellent agreement with experiments. For the octahedral phases, our simulations are able to reproduce changes in the Raman spectra across the rutile-to-CaCl2 transition around 50 GPa, including the observed phonon softening. read less

Abstract: We have modelled the thermal stability of silica nanoparticles commonly observed as precursors in the synthesis of zeolites. We performed canonical Monte Carlo and parallel tempering simulations on a lattice model that describes the self-assembly of nanoparticles under conditions at which they are observed experimentally. The effect of heating on the relative stability of the phases of the model was analysed by running simulations at various temperatures. At low temperature, the model yields a metastable multi-particle phase with a characteristic size distribution, which is separated by an energy barrier from the true equilibrium phase, a dense silica solid. As temperature increases, the system enters a transition region and eventually reaches the bulk phase. This transition is reminiscent of the experimentally observed transition from nanoparticles to zeolite. The transition temperature scales with the inverse of the system volume, approaching an asymptotic value for large system sizes. This indicates the transition temperature is a reproducible macroscopic property of the system. The transition temperature in the model is within the range of temperatures at which nanoparticles form zeolite crystals in experiments. read less

Abstract: The high-pressure phases of InP up to 100GPa are investigated using ab initio calculations and molecular-dynamics simulations. Simulation results show that the sequence of high-pressure phases is zinc blende (ZB)→rocksalt(RS)→rhombohedral(RH)→CsCl. The continuous RH distortion of the RS structure is consistent with the observed Cmcm-like distortion. Ab initio results indicate that a mixture of Cmcm and RH distortions is energetically possible and could explain the experimental spectra misfit. The calculated equation of state is in very good agreement with experiments. read less

Abstract: We report low-temperature measurements of dissipation in megahertz-range, suspended, single-crystal nanomechanical oscillators. At millikelvin temperatures, both dissipation (inverse quality factor) and shift in the resonance frequency display reproducible features, similar to those observed in sound attenuation experiments in disordered glasses and consistent with measurements in larger micromechanical oscillators fabricated from single-crystal silicon. Dissipation in our single-crystal nanomechanical structures is dominated by internal quantum friction due to an estimated number of roughly 50 two-level systems, which represent both dangling bonds on the surface and bulk defects. read less

Abstract: The surface structure of silica glasses has been simulated using molecular dynamics. The surface hydroxyl concentration was estimated to be 4.5/nm 2 , based on surface defect statistics. Hydroxyl-silica potentials were developed and used to study the hydroxylation of silica surface. It is found that the energy of chemisorption of water declines in the sequence: three coordinated silicon (Si 3 ) and non-bridging oxygen (NBO) on separate sites, Si 3 and NBO on combined sites, two- and three-membered rings. Partial hydroxylation of the most reactive sites, which leads to an OH coverage of 2.5/nm 2 , was studied. Structural relaxation after hydroxylation was observed. read less

Abstract: High-resolution inelastic neutron scattering was used to identify major sources of low-frequency vibrations in zeolite crystals. Dispersed and nondispersed modes were found, both of which are prominent in the early stages of compressive amorphization but decline dramatically in strength once a glass of conventional density is created. By identifying the dispersed modes with the characteristic vibrations of the various secondary building units of zeolitic structures, the Boson peak, a characteristic of the glassy state, can be attributed to vibrations within connected rings of many different sizes. The nondispersed phonon features in zeolites, retained in the amorphized glass, were also replicated in silica. These modes are librational in origin and are responsible for destabilizing the microporous crystalline structure, for converting the resulting glass from a low- to a high-density phase, and for the associated changes in network topology that affect the Boson peak. read less

Abstract: In this paper, we present and thoroughly characterize several new models of amorphous binary IV-VI glasses. We apply both a quench from the melt simulation regime and a scheme based on decoration of tetrahedral amorphous networks. We show that for certain binary IV-VI glasses (especially silica), decoration of bond-centered column VI atoms on tetrahedral amorphous networks leads with appropriate re-scaling and relaxation to highly realistic models of the IV-VI glass. In particular, the problem of freezing in too much liquid-like character seems to be significantly ameliorated. We also carry out first-principles molecular dynamics simulations to study the structural, dynamical, and electronic properties of $\mathrm{Ge}{\mathrm{Se}}_{4}$ and $\mathrm{Ge}{\mathrm{Se}}_{9}$. Good agreement with experiment is obtained for the total neutron structure factor over the entire range of $k$-space and for the electronic density of states. read less

Abstract: A model of silica glass consisting of a fully connected corner-sharing network of SiO4 tetrahedra is refined using neutron diffraction data and reverse Monte Carlo modelling. This model is then used to investigate optimal inter-tetrahedral Si–O–Si bond angle distributions. The distribution which is most consistent with the data is found to be centred around θSi−O−Si = 151.0° with a standard deviation of between 9° and 12°. Other recent determinations of the Si–O–Si bond angle distribution are in good agreement with this result. read less

Abstract: The total structure factors, S(Q), obtained from high-energy x-ray and neutron diffraction measurements on vitreous SiO2 (v-SiO2) and vitreous GeO2 (v-GeO2) have been analysed by the reverse Monte Carlo (RMC) modelling technique to generate a three-dimensional structural model. The bond angle distributions and the ring size distributions from the model indicated that the sixfold ring and six- and sevenfold rings are dominant in v-SiO2 and v-GeO2, respectively. However, the fraction of threefold rings of Ge in v-GeO2 is larger than that of Si in v-SiO2 glass. These features are consistent with the published neutron diffraction and Raman scattering studies. read less

Abstract: Consolidation and fracture dynamics in nanophase amorphous Si{sub 3}N{sub 4} are investigated using 10{sup 6}-atom molecular-dynamics simulations. At a pressure of 15 GPa and 2,000 K, the nanophase system is almost fully consolidated within a fraction of a nanosecond. The consolidation process is well-described by the classical theory of sintering. Under an applied strain the consolidation system develops several cracks which propagate parallel to each other, causing failure at multiple sites. The critical strain at which the nanophase system fractures is much larger than that for crystalline Si{sub 3}N{sub 4}. read less

Abstract: We present a simple and computationally efficient classical atomistic model of silica in which the silicon and oxygen are simulated as hard spheres with four and two association sites, respectively. We have performed isobaric-isothermal Monte Carlo simulations to study the mechanical and phase behavior of this model. We have investigated solid phase structures of the model corresponding to quartz, cristobalite, and coesite, as well as some zeolite structures. For the model these phases are mechanically stable and highly incompressible. Ratios of zero-pressure bulk moduli and thermal expansion coefficients for alpha quartz, alpha cristobalite, and coesite are in quite good agreement with experimental values. The pressure-temperature phase diagram was constructed and shows three solid phases corresponding to cristobalite, quartz, and coesite, as well as a fluid or glass phase, behavior qualitatively similar to that seen for silica experimentally. read less

Abstract: Based on density-functional perturbation theory we have computed the photoelastic tensor of a model of sodium silicate glass of composition ${({\mathrm{Na}}_{2}\mathrm{O})}_{0.25}{(\mathrm{Si}{\mathrm{O}}_{2})}_{0.75}$ (NS3). The model (containing 84 atoms) is obtained by quenching from the melt in combined classical and Car-Parrinello molecular dynamics simulations. The calculated photoelastic coefficients are in good agreement with experimental data. In particular, the calculation reproduces quantitatively the decrease of the photoelastic response induced by the insertion of Na, as measured experimentally. The extension to NS3 of a phenomenological model developed in a previous work for pure $a\text{\ensuremath{-}}\mathrm{Si}{\mathrm{O}}_{2}$ indicates that the modulation upon strain of other structural parameters besides the $\mathrm{Si}\mathrm{O}\mathrm{Si}$ angles must be invoked to explain the change in the photoelastic response induced by Na. read less

Abstract: Using an interaction potential scheme, molecular dynamics (MD) simulations are performed to investigate structural, mechanical, and vibrational properties of Ga1−xInxAs alloys in the crystalline and amorphous phases. For the crystalline phase we find that: (i) Ga–As and In–As bond lengths vary only slightly for different compositions; (ii) the nearest-neighbor cation–cation distribution has a broad peak; and (iii) there are two nearest-neighbor As–As distances in the As (anion) sublattice. These MD results are in excellent agreement with extended x-ray absorption fine structure and high-energy x-ray diffraction data and also with ab initio MD simulation results. The calculated lattice constant deviates less than 0.18% from Vegard’s law. The calculated phonon density of states exhibits a two-mode behavior for high-frequency optical phonons with peaks close to those in binary alloys (GaAs and InAs), which agrees well with a recent Raman study. Calculated elastic constants show a significant nonlinear depend... read less

Abstract: The structural, electronic, and adhesive properties of ${\mathrm{C}\mathrm{u}/\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$ interfaces are investigated using first-principles density-functional theory within the local density approximation. Interfaces between fcc Cu (001) and \ensuremath{\alpha}-cristobalite (001) slabs with different surface stoichiometries are considered. Interfacial properties are found to be sensitive to the choice of the termination and the interfacial oxygen density is the most important factor influencing the strength of adhesion. For oxygen-rich interfaces, the O atoms at the interface substantially rearrange after the deposition of Cu layers, suggesting the formation of Cu-O bonds. The large structural rearrangement, site-projected local densities of states, and changes in electron density indicate hybridization between $\mathrm{Cu}\ensuremath{-}d$ and $\mathrm{O}\ensuremath{-}p$ states at the interface. As oxygen is systematically removed from the interface, less rearrangement is observed, reflecting less hybridization and weaker adhesion. Computed adhesion energies for each of the interfaces are consistent with the observed structural and bonding trends, leading to the largest adhesion energy in the oxygen rich cases. The adhesion energy is also calculated between Cu and ${\mathrm{SiO}}_{2}$ substrates terminated with hydroxyl groups, and adhesion of Cu to these substrates is found to be considerably reduced. This work supports the notion that Cu films can adhere well to hydroxyl-free ${\mathrm{SiO}}_{2}$ substrates should oxygen be present in sufficient amounts at the interface. read less

Abstract: The partial structure factors of bulk-quenched glassy GeSe 2 were measured by using the method of isotopic substitution in neutron diffraction to enable the first detailed comparison at the partial pair distribution function level of a covalently bonded network system in both its glassy and liquid phases. The results show that the basic building block of the glass is the Ge(Se 1/2 ) 4 tetrahedron in which 34(5)% of the Ge atoms reside in edge-sharing configurations. The intrinsic chemical order of the glass is, however, broken with a maximum of 25(5 )% Ge and 20(5)% Se being involved in homopolar bonds at distances of 2.42(2) and 2.32(2) A, respectively, which is consistent with the existence of these features in the liquid phase of GeSe 2 . Like for the liquid, concentration fluctuations in the glass are found to extend over distances characteristic of the intermediate-range atomic ordering as manifested by the appearance of a first sharp diffraction peak at 1.00(2) A -1 in the Bhatia-Thornton concentration-concentration partial structure factor. A comparison is made between the measured partial structure factors and recent first principles molecular dynamics simulations for the glassy and liquid phases. It is found that the most significant disagreement between experiment and simulation occurs with respect to the Ge-Ge correlations and that the simulated results for the glass are too liquid-like, reflecting the use of a quench time greatly in excess of that achieved experimentally. read less

Abstract: We present structural and dynamical results of molecular dynamics simulation of vitreous silica undergoing a hydrostatic compression and decompression cycle at room temperature. Structural results as a function of density compare fairly well with experiments as well as with the longitudinal and transverse sound velocity pressure dependence. A shift of the boson peak (BP) toward higher energies and its simultaneous weakening is observed as in experiments. A detailed study of the dispersion of the glass vibration is presented at several densities and for the densified state. Evidence of phonon-like character with two distinct pseudo-periods is shown for longitudinal and transverse dynamics. The relationship between the BP vibrations and the correlation length scale indicated by the first sharp diffraction peak is discussed. read less

Abstract: Structural and dynamic properties of cristobalite silica have been studied using molecular dynamics simulations based on a charge transfer three-body potential model. In this potential model, the directional covalent bonding of SiO 2 is characterized by a charge transfer function of the interatomic distance between Si and O atoms, and in the form of Si‐O‐Si and O‐Si‐Othree-body interactions. The dynamic properties such as infrared spectra and density of states at room and elevated temperatures are in excellent agreement with experiments, and are also consistent with the recently proposed rigid unit modes model. The a- and b-cristobalite crystallographic structures are well reproduced in this model, and the transition between these modifications occurs reversibly and reproducibly in simulations, both as a result of changes in pressure and temperature. The thermally induced transition results in a significantly more disordered b-cristobalite than the pressure-induced b-cristobalite at room temperature. While simulated a-cristobalite exhibits a positive thermal expansion coefficient, it is almost zero forb-cristobalite up to 2000 K and slightly negative at higher temperatures, confirming results from recent x-ray diffraction experiments and other simulations with potential models based on ab initio calculations. © 2003 American Institute of Physics. @DOI: 10.1063/1.1529684# read less

Abstract: We present a classical interatomic force field for liquid SiO2 which has been parametrized using the forces, stresses and energies extracted from ab initio calculations. We show how inclusion of more electronic effects in a phenomenological way and parametrization at the relevant conditions of pressure and temperature allow the creation of more accurate force fields. We compare the results of simulations with this force field both to experiment and to the results of ab initio molecular dynamics simulations and show how our procedure leads to comparisons which are greatly improved with respect to the most widely used force fields for silica. read less

Abstract: ▪ Abstract Atomistic aspects of dynamic fracture in a variety of brittle crystalline, amorphous, nanophase, and nanocomposite materials are reviewed. Molecular dynamics (MD) simulations, ranging from a million to 1.5 billion atoms, are performed on massively parallel computers using highly efficient multiresolution algorithms. These simulations shed new light on (a) branching, deflection, and arrest of cracks; (b) growth of nanoscale pores ahead of the crack and how pores coalesce with the crack to cause fracture; and (c) the influence of these mechanisms on the morphology of fracture surfaces. Recent advances in novel multiscale simulation schemes combining quantum mechanical, molecular dynamics, and finite-element approaches and the use of these hybrid approaches in the study of crack propagation are also discussed. read less

Abstract: An atomic-scale model for silicate solutions is introduced for investigation of the nucleation process during zeolite synthesis in the absence of a structure directing agent. Monte Carlo schemes are developed to determine the equilibrium distribution of silicate cluster sizes within the context of this model. How the nucleation barrier and critical cluster size change with Si monomer concentration is discussed. Distance and angle histograms as well as ring size distributions are calculated and compared with known zeolite structures. The free energies of critical clusters are compared with those for small clusters of α-quartz. read less

Abstract: Measured infrared dielectric function spectra ranging from 300 to 1400 cm−1 for thermally grown silicon dioxide films were studied. The dielectric function spectra were calculated by requiring the calculated spectra to fit to the actual spectra. The peak line shape of the dielectric function can be described by a Gaussian distribution, but not by a Lorentzian line shape. In detail, frequencies of half height for the imaginary part of the dielectric function of high- and low-frequency edges are not actually symmetrical. According to interpretations based on a central and noncentral force network model, the asymmetric distribution of the dielectric function arises from the symmetrical distribution of the bond angles of a random network of the SiO4 tetrahedra. For that reason, we developed a dielectric function model which can describe an asymmetrical Gaussian line shape. This asymmetric Gaussian model minimizes the number of parameters needed, which are the one center position and low- and high-Gaussian dis... read less

Abstract: We use molecular dynamics computer simulations to study the equilibrium properties of the surface of amorphous silica. Two types of geometries are investigated: (i) clusters with different diameters (13.5, 19, and 26.5 A) and (ii) a thin film with thickness 29 A. We find that the shape of the clusters is independent of temperature and that it becomes more spherical with increasing size. The surface energy is in qualitative agreement with the experimental value for the surface tension. The density distribution function shows a small peak just below the surface, the origin of which is traced back to a local chemical ordering at the surface. Close to the surface the partial radial distribution functions as well as the distributions of the bond–bond angles show features which are not observed in the interior of the systems. By calculating the distribution of the length of the Si–O rings we can show that these additional features are related to the presence of two-membered rings at the surface. The surface den... read less

Abstract: Amorphous SiO2 surfaces are generated from bulk–liquid configurations using simulations employing a polarizable-ion model. The surfaces are characterized in terms of the ion environments as a function of depth into the surface. Comparison is made to previous simulation studies and subtle differences are highlighted and attributed to differences in the potential models. The connectivity of the surface sites is established with a view to investigating the hydrolysis of this surface. Dynamical properties are calculated using a simple projected velocity time correlation function and normal mode analysis and compared to the simulated bulk and experimental bulk and surface spectra. read less

Abstract: Neutral oxygen and silicon vacancies’ energies in silica polytypes (α-quartz, β-cristobalite, and stishovite) have been studied using the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange correlation. While the energies of formation of unrelaxed oxygen vacancies are remarkably constant (9.9 ±0.1 eV) in the three studied systems in GGA, the relaxation behavior is quite different: A strong Si–Si bond is formed in α-quartz, a weak one in β-cristobalite, while no bond at all is formed in stishovite. In α-quartz, peroxyl bridges are formed as the consequences of the relaxation of silicon vacancies. Their energy of formation from O2 (gas) is low (about 2 eV). read less

Abstract: A number of computational methods have been developed over the last 40 years to simulate the behavior of solid materials with small dimensions. On the macro-scale, Finite Element analysis calculates mechanical stress on micron-sized cantilevers and motors. On the atomic level, newer ab initio methods compute nuclear and electronic behavior of hundred atom models with unprecedented rigor. By implementing the laws of classic mechanics, empirical Molecular Dynamics (MD) programs help bridge these two computational extremes. MD identifies nonelectronic, particle motion for large 100,000 atom cells with good success. MD derives both equilibrium and nonequilibrium properties for many complex condensed regimes; quantitatively (and qualitatively) reaffirms empirical data; aids discovery of new materials processing techniques, and helps predict unknown physical phenomena often only observable under extreme environmental settings. One material of great technical importance to the semiconductor industry is silicon (... read less

Abstract: We have developed an interatomic potential to investigate structural properties of hydrogenated amorphous silicon nitride. The interatomic potential used the Tersoff functional form to describe the Si–Si, Si–N, Si–H, N–H, and H–H interactions. The fitting parameters for all these interactions were found with a set of ab initio and experimental results of the silicon nitride crystalline phase, and of molecules involving hydrogen. We investigated the structural properties of unhydrogenated and hydrogenated amorphous silicon nitride through Monte Carlo simulations. The results show that depending on the nitrogen content, hydrogen has a different chemical preference to bind to either nitrogen or silicon, which is corroborated by experimental findings. Besides, hydrogen incorporation reduced considerably the concentration of undercoordinated atoms in the material, and consequently the concentration of dangling bonds. read less

Abstract: We investigate the energy landscape of two-dimensional network models for covalent glasses by means of the lid algorithm. For three different particle densities and for a range of network sizes, we exhaustively analyse many configuration space regions enclosing deep-lying energy minima. We extract the local densities of states and of minima, and the number of states and minima accessible below a certain energy barrier, the `lid'. These quantities show on average a close to exponential growth as a function of their respective arguments. We calculate the configurational entropy for these pockets of states and find that the excess specific heat exhibits a peak at a critical temperature associated with the exponential growth in the local density of states, a feature of the specific heat also observed in real glasses at the glass transition. read less

Abstract: The distribution of Ge-O-Ge and Si-O-Si bond angles α in amorphous germania and silica is re-determined on the basis of diffraction experiments. The bond angle α joining adjacent tetrahedra is the central parameter of any continuous random network description (CRN) of these glasses. New high energy photon diffraction experiments on amorphous germania (at photon energies of 97 and 149 keV) are presented, covering the momentum transfer 0.6-33.5 A -1 . In photon diffraction experiments on GeO 2 the contribution of the OO pairs is very small. To obtain a similar information for amorphous SiO 2 , high energy photon diffraction experiments [1] have been combined with neutron diffraction data [2, 3] on amorphous silica in order to eliminate the OO-partial structure factor. With this technique it is shown that the Si-O-Si angle distribution is fairly narrow (σ = 7.5°) and in fact comparable in width to the Ge-O-Ge angle distribution (σ = 8.3°), a result which differs from current opinion. The narrower distribution found in this study are in much better agreement to the determinations based on 29 Si-MAS-NMR. Among the various models relating the chemical shift to the bond angle, best agreement is found with those models based on the secant model. Sharp components in the bond angle distribution can be excluded within the reached real space resolution of 0.09 A read less

Abstract: Extensive molecular-dynamics (MD) simulations are performed for a rigid-ion model of molten metal trichlorides (MCl3), in different regimes of metal radius, density and temperature. The cases of molten YCl3 and AlCl3 are examined in more detail in order to ascertain the presence of medium-range order (MRO) in these materials. For the YCl3 case, the experimental neutron structure factor, signalling the presence of MRO in the system, is fairly well reproduced by MD. The overall structural information available from the simulation indicates that order at intermediate distances basically consists of local 'layers' of corner-sharing (YCl6)3- octahedra. These roughly planar formations may represent a remnant of the real YCl3 crystalline structure, constituted by layers of edge-sharing (YCl6)3- octahedra; the internal topology of the 'planes' is discussed in relation to the assumed model potential and to global charge ordering. The effect of metal size on the MRO features is then investigated by adjusting the model parameters to the case of molten AlCl3, in which the Al3+ effective radius is significantly smaller than that of Y3+. In this case well defined tetrahedral (AlCl4)- units tend to be formed, and these link to each other in a highly connected network responsible for MRO. The presence in the melt of bound pairs of tetrahedra, interpretable as Al2Cl3 'dimers', is also established, in essential agreement with the prediction of recent theoretical work by other authors. read less

Abstract: Structural properties and medium‐range order (MRO) in calcium metasilicate (CaSiO3) glass are investigated at different concentrations of calcium oxide through the molecular dynamics technique. Calculations are based on the Born–Mayer–Huggins potential, and show that the experimental Ca–Ca partial structure factor, which documents the existence in the glass of Ca MRO, can qualitatively be reproduced within such a model. The characteristics of Ca MRO are then examined in the context of the overall structure of the system. At equimolar concentrations of CaO and SiO2, the simulation evidentiates the formation in the glass of clusters of CaO6 octahedra in which Ca ions are roughly lying on a plane, in a configuration that closely resembles the one of crystalline CaSiO3. On the other hand, the ‘network’ structure and MRO of pure SiO2 glass appear sensibly affected by the presence of Ca ions, with considerable loose of connectivity between SiO4 tetrahedral units, and reduction of the first sharp diffraction pea... read less

Abstract: Metal oxides belong to the most important material classes in industrial technology. These high-tech ceramics are irreplacable in lots of modern microelectronic devices due to their excellent insulating properties, high melting points and high degrees of hardness. In the theoretical study of these systems, the atomistic modelling with molecular dynamics simulations and classical effective interaction force fields is a very powerful tool. There, fundamental properties can be uncovered and understood due to the atomic resolution. Both force field generation and simulation of oxide systems are computationally much more demanding than those of metals or covalent materials due to long-range electrostatic interactions. Furthermore, it is often not sufficient to only take Coulomb interactions into account, but to include electrostatic dipole moments. The latter can be integrated with the Tangney Scandolo polarizable force field model, where dipole moments are determined by a self-consistent iterative solution during each simulation time step. Applying the direct, pairwise Wolf summation to interactions between charges and its extension to dipole moments avoids high computational effort due to its linear scaling properties in the number of particles. Three relevant metal oxides have been selected to apply the new high-performance force field generation model. Therewith, a detailed investigation of crack propagation was possible. Both crack propagation insights and the influence of cracks on the dipole field are shown. Finally, the coupling of strain and – even more meaningful – strain gradient with the dipole moments is presented, which gives rise to flexoelectric effects in non-piezoeletric materials. Die vorliegende Arbeit vertieft die breit angelegte Modellierungs-Studie dreier wichtiger Metalloxide. Detaillierte Untersuchungen durch Molekulardynamik- Simulationen mit Kraftfeldern fur Siliziumdioxid, Magnesiumoxid und Alpha-Aluminiumoxid werden dargestellt. Speziell angepasste Visualisierungs-Techniken erweitern die numerischen Einblicke und verhelfen zu neuen Erkenntnissen in den untersuchten ionischen Systemen. Im Wesentlichen leistet die Arbeit einen dreistufigen Beitrag zur numerischen Erforschung ionischer kondensierter Materie: 1. Zunachst wird die neue Art der Kraftfelderstellung dargestellt, welche das Potenzial-Modell von Tangney und Scandolo mit der direkten Wolfsummation verknupft. Gezeigt wird die gewissenhafte Prufung, dass die Vereinigung der Vorteile des TS Modells, welches elektrostatische Dipolmomente mit einbezieht, mit den linearen Skalierungs-Eigenschaften der Wolfsumme gelungen ist. Die Implementierung der Kraftfelderstellung in potfit durch den Autor gibt anderen Simulations-Gruppen die Moglichkeit, diese prazise, effiziente und flexible Methode fur ihre individuellen Studien ionischer Materie einzusetzen. 2. Weiterhin werden die mithilfe der neuen Methodik generierten Kraftfelder fur Siliziumdioxid, Magnesiumoxid und Alpha-Aluminiumoxid vorgestellt. Neben ihrer Erstellung wird in jedem Einzelfall die sorgfaltige Validierung gezeigt. Die neuen Kraftfelder werden nicht nur fur eigene Simulationen eingesetzt, sondern anderen Gruppen offentlich zuganglich gemacht. Dadurch konnen nun vielerorts Simulationen durchgefuhrt werden, die bisher aufgrund von Beschrankungen bezuglich Zeit-, Langenskalen oder Randbedingungen nicht annehmbar realisierbar waren. 3. Unter Einsatz der neuen Kraftfelder wurden verschiedene Simulationen durchgefuhrt, die – auch mithilfe der speziell angepassten Visualisierung der fraktionalen Anisotropie – zu neuen Entdeckungen und grundlegenden Erkenntnissen gefuhrt haben: • Das fur Alpha-Aluminiumoxid erstellte Kraftfeld wurde zur MD Simulation von sich ausbreitenden Rissen verwendet, welche die vor zwei Jahren elektronenmikroskopisch untersuchten Verlaufs-Richtungen von in verschiedenen kristallinen Ebenen existierenden Rissen vollstandig reproduzieren konnte. Daruberhinaus konnte – erstmalig in atomistischen Simulationen von Metalloxiden – der Einfluss von sich ausbreitenden Rissen auf die Orientierung der elektrostatischen Dipolmomente beobachtet und analysiert werden. • Derartige flexoelektrische Phanomene wurden schlieslich sowohl in MD Simulationen als auch mithilfe der Darstellungstheorie untersucht. Die Kombination liefert eine konsistente Vorhersage flexoelektrischen Verhaltens in Alpha-Aluminiumoxid und auch in Periklas, obwohl beide Materialien aufgrund ihrer inversionssymmetrischen Kristallstruktur keine Piezoelektrizitat aufweisen. Die analytisch angenommene und experimentell vorhergesagte lineare Kopplung zwischen Spannungsgradient und flexoelektrischer Polarisation wurde von MD Simulationen bestatigt. Erstmalig wurde flexoelektrische Domanenausbildung modelliert. In Periklas zeigt sich, dass flexoelektrische Domanen exakt von Neel-Wanden abgetrennt werden. read less

Abstract: Non-linear absorption of femtoseco nd laser pulses enables the induction of structural changes in the interior of bulk transparent materials without affecting their surface. This property can be exploited for the transmission welding of transparent dielectrics, three dimensional optical data storages and waveguides. In the present study, femtosecond laser pulses were tightly focused within the interior of bulk fused silica specimen. Localized plasma was formed, initiating rearrangement of the network structure. The change in material properties were studied through employment of spatially resolved Raman spectroscopy, atomic force microscopy and optical microscopy. The nature of the physical mechanisms responsible for the alteration of material properties as a function of process parameters is discussed. read less

Abstract: Amorphous silica and related silica-based materials have been widely developed in optoelectronics and optical telecommunications technology. Despite comprehensive research for over the decades, the subject of amorphous silica continues to excite the interest of researchers in the field of chemistry, physics, and geology. This paper reviews some of the recent experimental and theoretical developments in the field, including medium-range order, structural changes under pressures, vibrational and thermodynamic anomalies, and photoluminescence properties. read less

Abstract: We have applied a 'real-reciprocal space analysis', using the continuous wavelet transform technique, to the experimental neutron and x-ray structure factors of silica glass to elucidate a correlation between the 'first sharp diffraction peak (FSDP)' in reciprocal space and the corresponding length scale in real space. The present analysis allows us to obtain compelling evidence that the dominant interatomic distance linked to the FSDP in silica glass is {approx}5 A, although longer distances are also important, making an exponentially decreasing contribution. Further analysis using molecular-dynamics simulations demonstrates that the interatomic spatial correlations at r{approx}5 A are associated with a couple of local 'pseudo-Bragg' planes having an interlayer separation of {approx}4 A, accounting for the origin of structural ordering on the medium-range length scale in silica glass. read less

Abstract: The paper is devoted to the theoretical study of Coulomb explosion in silicate glasses with low ionization potential under internal alpha-irradiation. The phenomenon was studied in the way of computer simulation, namely by the molecular dynamics (MD) method; parameters of the so-called lavalike fuel-containing materials (LFCM), were chosen as input parameters for the model due to its practical importance. LFCM are high-radioactive glasses, which were formed during an active stage of a well known heavy nuclear accident, occurred on Chornobyl NPP in 1986. Computer simulation revealed that Coulomb explosion really may occur in the LFCMs and leads to additional radiation damages under internal alpha-irradiation. The total quantity of atomic displacements produced in the way of Coulomb explosion from each alpha-particle track is 40000 to 80000, which exceeds radiation damages from alpha-particle and heavy recoil nuclei altogether (about 3500) more than one order. read less

Abstract: curvilinear-coordinate load balancing; iii) hierarchical dynamics via a rigid-body/ implicit-integration/normal-mode approach; iv) variable-charge MD based on electronegativity equalization; and v) multilevel preconditioned conjugate gradient method. Fuzzy clustering is used to facilitate the seamless integration of the multiple levels of read less

Abstract: Properties and processes in silicon nitride and graphite are investigated using molecular-dynamics (MD) simulations. Scalable and portable multiresolution algorithms are developed and implemented on parallel architectures to simulate systems containing 106 atoms interacting via realistic potentials. Structural correlations, mechanical properties, and thermal transport are studied in microporous silicon nitride as a function of density. The formation of pores is observed when the density is reduced to 2.6 g/cc, and the percolation occurs at a density of 2.0 g/cc. The density variation of the thermal conductivity and the Young’s modulus are well described by power laws with scaling exponents of 1.5 and 3.6, respectively. Dynamic fracture in a single graphite sheet is investigated. For certain crystalline orientations, the crack becomes unstable with respect to branching at a critical speed of -60% of the Rayleigh velocity. The origin of the branching instability is investigated by calculating local-stress distributions. The branched fracture profile is characterized by a roughness exponent, a 0.7, above a crossover length of 50A. For smaller length scales and within the same branch, a 0.4. Crack propagation is studied in nanophase silicon nitride prepared by sintering nanoclusters of size 60A. The system consists of crystalline cluster interiors, amorphous intercluster regions, and isolated pores. These microstructures cause crack branching and meandering, and the clusters undergo significant rearrangement due to plastic deformation of interfacial regions. As a result, the system can withstand enormous deformation (30%). In contrast, a crystalline sample in the same geometry cleaves under an applied strain of only 3%. read less

Abstract: Despite a wide range of current and potential applications, one primary concern of brittle materials is their sudden and swift collapse. This failure phenomenon exhibits an inability of the materials to sustain tension stresses in a predictable and reliable manner. However, advances in the field of fracture mechanics, especially at the nanoscale, have contributed to the understanding of the material response and failure nature to predict most of the potential dangers. In the following contribution, a comprehensive review is carried out on molecular dynamics (MD) simulations of brittle fracture, wherein the method provides new data and exciting insights into fracture mechanism that cannot be obtained easily from theories or experiments on other scales. In the present review, an abstract introduction to MD simulations, advantages, current limitations and their applications to a range of brittle fracture problems are presented. Additionally, a brief discussion highlights the theoretical background of the macroscopic techniques, such as Griffith’s criterion, crack tip opening displacement, J-integral and other criteria that can be linked to the fracture mechanical properties at the nanoscale. The main focus of the review is on the recent advances in fracture analysis of highly brittle materials, such as carbon nanotubes, graphene, silicon carbide, amorphous silica, calcium carbonate and silica aerogel at the nanoscale. These materials are presented here due to their extraordinary mechanical properties and a wide scope of applications. The underlying review grants a more extensive unravelling of the fracture behaviour and mechanical properties at the nanoscale of brittle materials. read less

Abstract: This topical review is dedicated to understanding stress corrosion cracking in oxide glasses and specifically the SiO2–B2O3–Na2O (SBN) ternary glass systems. Many review papers already exist on the topic of stress corrosion cracking in complex oxide glasses or overly simplified glasses (pure silica). These papers look at how systematically controlling environmental factors (pH, temperature...) alter stress corrosion cracking, while maintaining the same type of glass sample. Many questions still exist, including: What sets the environmental limit? What sets the velocity versus stress intensity factor in the slow stress corrosion regime (Region I)? Can researchers optimize these two effects to enhance a glass’ resistance to failure? To help answer these questions, this review takes a different approach. It looks at how systemically controlling the glass’ chemical composition alters the structure and physical properties. These changes are then compared and contrasted to the fracture toughness and the stress corrosion cracking properties. By taking this holistic approach, researchers can begin to understand the controlling factors in stress corrosion cracking and how to optimize glasses via the initial chemical composition. read less

Abstract: Anomalous structural characteristics of the so-called first sharp diffraction peak (FSDP) that arises in the total static structure functions of network-forming glasses and liquids at around 1-2 A-1 have been reviewed and discussed in details. Unlike other peaks in the static structure functions, the FSDP has anomalous dependencies on temperature, pressure and composition. Despite the fact that the FSDP is considered as a signature of intermediate range order (IRO) in network-forming glasses and liquids, its structural origin remains unclear and till now, it forms a subject of debate. A brief account for some anomalous characteristics of the FSDP followed by the different controversial interpretations about its structural origin has been reviewed and discussed. Some of the interpretations that seem to be inconsistent with recent experimental results have been ruled out. The most likely structural origins for the occurrence of the FSDP have been highlighted and discussed in details. read less