Molecular dynamics simulation of structural transformation in silicon carbide under pressure

Pressure and temperature stability boundaries of cubic SiC polymorphs: a first-principles investigation

A better understanding of the effects of temperature and pressure on the wide gap SiC semiconductor is necessary for both (i) an improvement of the performance of this compound in a variety of technological applications and (ii) a clarification of controversial issues related to the stability of its cubic polymorphs at high pressure and high temperature. Bearing in mind this double demand, we perform first-principles calculations of the phonon band structures, vibrational density of states, and thermal and mode Grüneisen parameters of the zinc blende (B3) and rock-salt (B1) cubic polymorphs of 3C-SiC covering pressures and temperatures up to 120 GPa and 3000 K, respectively. Under a martensitic description of the B3-B1 transformation, we found that the large hysteresis pressure range observed at room temperature (35-100 GPa) disappears at around 1100 K. The calculated Clapeyron slope of this transformation is slightly negative, with average values of -2.9 MPa K^-1 in the 0-3000 K interval and -3.7 MPa K^-1 at 2000 K. We also study the decomposition reaction of the two cubic polymorphs into their elemental constituents (C and Si), finding a decreasing (increasing) decomposition temperature for the B3 (B1) phase as the pressure is applied. All these results are sustained by good agreement with other recently reported experimental and theoretical thermodynamic data that have also been evaluated under our quasi-harmonic approximation framework.

Temperature-induced phase transitions in the rock-salt type SiC: a first-principles study

The phase transitions in the rock-salt type SiC (B1-SiC) under decompression are studied in the framework of first-principles molecular dynamics simulations up to room temperature. The transformation pathways were determined based on an analysis of the symmetry and phonon spectra of high-symmetry transient structures identified in the simulations. The plausible pathways of the transformation ofB1-SiC into the 3C-, 2H-, 4H-, 12R-SiC polytypes were suggested. The transformation paths were found to depend on both the availability of soft phonon modes in an unreconstructed phase and the initial conditions of the simulation. It is shown that an increase in cell volume at decompression leads to the condensation of a certain phonon mode. As a result, an intermediate state forms due to the atomic displacements and to subsequent strains related to this mode. All the decompressed structures were compressed back under pressure of 120-250 GPa depending on the type of the decompressed phase and simulation temperature that was in the range of 300-1200 K. The suggested scheme of structural identification can be used to determine the transition paths for the structural transformations of other similar structures under pressure.

Molecular-dynamic investigation of the influence of initial temperature on the character of shock-wave processes in silicon carbide nanocluster

In the present study, using the molecular dynamics method, we investigated the impact interaction of a spherical cluster of 3C-SiC silicon carbide with a rigid wall at a wide range of velocities. The influence of cluster initial temperature on the fracture process was analyzed.

The mechanical and thermodynamic properties of β-Si1−xC

We report the enthalpy (a), free energy (b), entropy (c) and heat capacity (d) of β-Si_1–xC.

New transformation mechanism for a zinc-blende to rocksalt phase transformation in MgS

The stability of the zinc-blende structured MgS is studied using a constant pressure ab initio molecular dynamics technique. A phase transition into a rocksalt structure is observed through the simulation. The zinc-blende to rocksalt phase transformation proceeds via two rhombohedral intermediate phases within R3m (No:160) and [Formula: see text] (No:166) symmetries and does not involve any bond breaking. This mechanism is different from the previously observed mechanism in molecular dynamics simulations.

Ab initio molecular dynamics simulation of a pressure induced zinc blende to rocksalt phase transition in SiC

The high-pressure induced phase transformation from the zinc blende to rocksalt structure in SiC has been studied by the ab initio molecular dynamics method. The simulations showed that SiC passes through a tetragonal intermediate state before transforming to a monoclinic phase at 160 GPa. The mechanism for this phase transformation agrees well with recent ab initio MD simulations, in which the applied pressure was as high as ∼600 GPa, but in the present study the transformation occurs at much lower pressure.

The structural phase transition of ZnSe under hydrostatic and nonhydrostatic compressions: an ab initio molecular dynamics study

Ab initio constant pressure molecular dynamics simulations within a generalized gradient approximation (GGA) are carried out to study the structural phase transformation of ZnSe under hydrostatic and nonhydrostatic conditions. ZnSe undergoes a first-order phase transition from the zinc-blende structure to a rocksalt structure having practically identical transformation mechanisms under hydrostatic and nonhydrostatic compressions. This phase transformation is also analyzed using the enthalpy calculations. Our transition parameters and bulk properties are comparable with experimental and theoretical data. Furthermore, the influence of pressure on the electronic structure of ZnSe is investigated. It is found that the band gap energy increases nonlinearly under both hydrostatic and nonhydrostatic conditions and the effect of stress deviations on the band gap energy is small. The computed pressure coefficients and deformation potential of the band gap are in good agreement with experiments.

Interaction potential for indium phosphide: a molecular dynamics and first-principles study of the elastic constants, generalized stacking fault and surface energies

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.

Weight differences of male and female pupae of gypsy moth (Lymantria dispar) and host-sex preference by two parasitoid species Lymantrichneumon disparis and Exorista larvarum

Weight differences of male and female pupae of gypsy moth [Lymantria dispar L. (Lepidoptera: Lymantriidae)] and its two parasitoids Lymantrichneumon disparis (Poda) (Hymenoptera: Ichneumonidae) and Exorista larvarum (L.) (Diptera: Tachinidae) host preference were examined in this study. Lymantria dispar pupae were collected from trunks and branches of 20 Ironwood trees (Parotia persica) in two sampling dates, 10 July 2005 and 24 July 2005. The pest pupae were weighted and then saved at room temperature until adults of gypsy moth or its parasitoids emerged. The most L. dispar pupae collected in the first sampling were male whereas those in the second one were female and both male and female pupae in the second sampling were smaller than those in first sampling. The majority of male pupae (98.29%) were less than 0.6 g and most of female pupae (79.39%) were more than 0.6 g in weight. The most of L. disparis emerged from male pupae of L. dispar, whereas the majority of E. larvarum emerged from female pupae. Implications of the results for biological control strategy of gypsy moth are discussed.

Molecular Dynamics Simulation of Nanoindentation-induced Mechanical Deformation and Phase Transformation in Monocrystalline Silicon

This work presents the molecular dynamics approach toward mechanical deformation and phase transformation mechanisms of monocrystalline Si(100) subjected to nanoindentation. We demonstrate phase distributions during loading and unloading stages of both spherical and Berkovich nanoindentations. By searching the presence of the fifth neighboring atom within a non-bonding length, Si-III and Si-XII have been successfully distinguished from Si-I. Crystallinity of this mixed-phase was further identified by radial distribution functions.

Classical Molecular Dynamics Simulation of Structural and Dynamical Properties of II-VI and III-V Semiconductors

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.

Shock-induced structural phase transition, plasticity, and brittle cracks in aluminum nitride ceramic

Atomistic mechanisms of fracture accompanying structural phase transformation (SPT) in AlN ceramic under hypervelocity impact are investigated using a 209 x 10(6) atom molecular-dynamics simulation. The shock wave generated by the impact splits into an elastic wave and a slower SPT wave that transforms the wurtzite structure into the rocksalt phase. The interaction between the reflected elastic wave and the SPT wave front generates nanovoids and dislocations into the wurtzite phase. Nanovoids coalesce into mode I cracks while dislocations give rise to kink bands and mode II cracking.

Multimillion Atom Simulations of Dynamics of Oxidation of an Aluminum Nanoparticle and Nanoindentation on Ceramics

We have developed a first-principles-based hierarchical simulation framework, which seamlessly integrates (1) a quantum mechanical description based on the density functional theory (DFT), (2) multilevel molecular dynamics (MD) simulations based on a reactive force field (ReaxFF) that describes chemical reactions and polarization, a nonreactive force field that employs dynamic atomic charges, and an effective force field (EFF), and (3) an atomistically informed continuum model to reach macroscopic length scales. For scalable hierarchical simulations, we have developed parallel linear-scaling algorithms for (1) DFT calculation based on a divide-and-conquer algorithm on adaptive multigrids, (2) chemically reactive MD based on a fast ReaxFF (F-ReaxFF) algorithm, and (3) EFF-MD based on a space-time multiresolution MD (MRMD) algorithm. On 1920 Intel Itanium2 processors, we have demonstrated 1.4 million atom (0.12 trillion grid points) DFT, 0.56 billion atom F-ReaxFF, and 18.9 billion atom MRMD calculations, with parallel efficiency as high as 0.953. Through the use of these algorithms, multimillion atom MD simulations have been performed to study the oxidation of an aluminum nanoparticle. Structural and dynamic correlations in the oxide region are calculated as well as the evolution of charges, surface oxide thickness, diffusivities of atoms, and local stresses. In the microcanonical ensemble, the oxidizing reaction becomes explosive in both molecular and atomic oxygen environments, due to the enormous energy release associated with Al-O bonding. In the canonical ensemble, an amorphous oxide layer of a thickness of approximately 40 angstroms is formed after 466 ps, in good agreement with experiments. Simulations have been performed to study nanoindentation on crystalline, amorphous, and nanocrystalline silicon nitride and silicon carbide. Simulation on nanocrystalline silicon carbide reveals unusual deformation mechanisms in brittle nanophase materials, due to coexistence of brittle grains and soft amorphous-like grain boundary phases. Simulations predict a crossover from intergranular continuous deformation to intragrain discrete deformation at a critical indentation depth.

Structural phase transition and dynamical properties of PbTiO3 simulated by molecular dynamics

The temperature- and pressure-induced structural phase transition in PbTiO₃ is studied with the isoenthalpic-isobaric molecular-dynamics method, using an effective two-body interaction potential. The tetragonal to cubic transformation is successfully reproduced with both temperature and pressure. The behaviour of lattice parameters, vibrational density of states, and phonon anharmonicity with temperature and pressure are in very good agreement with experimental data. Two- and three-body correlations were analysed through pair distribution functions, coordination numbers and bond-angle distributions.

Universal transition state for high-pressure zinc blende to rocksalt phase transitions

First-principles density functional calculations show that the high-pressure transitions of different semiconductors from zinc blende to rocksalt go through a transition state, which is universal in the sense that its position along the path and the corresponding geometry is independent of the chemical components of the semiconductor. This is explained using a Landau-like model expansion of the free energy in cosine functions of atomic position.

Multiple grains in nanocrystals: effect of initial shape and size on transformed structures under pressure

Pressure-induced structural transformations in spherical and faceted gallium arsenide nanocrystals of various shapes and sizes are investigated with a parallel molecular-dynamics approach. The results show that the pressure for zinc blende to rocksalt structural transformation depends on the nanocrystal size, and all nanocrystals undergo nonuniform deformation during the transformation. Spherical nanocrystals above a critical diameter >/=44 A transform with grain boundaries. Faceted nanocrystals of comparable size have grain boundaries in 60% of the cases, whereas the other 40% are free of grain boundaries. The structure of transformed nanocrystals shows that domain orientation and strain relative to the initial zinc blende lattice are not equivalent. These observations may have implications in interpreting the experimental x-ray line shapes from transformed nanocrystals.

Microscopic view of structural phase transitions induced by shock waves

Multimillion-atom molecular-dynamics simulations are used to investigate the shock-induced phase transformation of solid iron. Above a critical shock strength, many small close-packed grains nucleate in the shock-compressed body-centered cubic crystal growing on a picosecond time scale to form larger, energetically favored grains. A split two-wave shock structure is observed immediately above this threshold, with an elastic precursor ahead of the lagging transformation wave. For even higher shock strengths, a single, overdriven wave is obtained. The dynamics and orientation of the developing close-packed grains depend on the shock strength and especially on the crystallographic shock direction. Orientational relations between the unshocked and shocked regions are similar to those found for the temperature-driven martensitic transformation in iron and its alloys.

Orthorhombic intermediate state in the zinc blende to rocksalt transformation path of SiC at high pressure

The mechanism of the B3/B1 phase transition of SiC has been investigated by periodic LCAO-DFT least-enthalpy calculations. A new transformation pathway, based on a Pmm2 orthorhombic intermediate state with two SiC units per cell, is found to be energetically favored over the traditional R3m mechanism. The computed activation enthalpy is 0.75 eV/SiC unit at the predicted transition pressure of 92 GPa (B3LYP functional). Activation enthalpy and activation volume vs pressure are analyzed to characterize the kinetic aspects of the transformation.

Grain Boundaries in Gallium Arsenide Nanocrystals Under Pressure: A Parallel Molecular-Dynamics Study

Structural transformation in gallium arsenide nanocrystals under pressure is studied using molecular-dynamics simulations on parallel computers. It is found that the transformation from fourfold to sixfold coordination is nucleated on the nanocrystal surface and proceeds inwards with increasing pressure. Inequivalent nucleation of the high-pressure phase at different sites leads to inhomogeneous deformation of the nanocrystal. This results in the transformed nanocrystal having grains of different orientations separated by grain boundaries. A new method based on microscopic transition paths is introduced to uniquely characterize grains and deformations.