Effect of off-stoichiometry on the thermal conductivity of amorphous GeTe

The reversible phase change of Germanium Telluride (GeTe) is essential for developing advanced non-volatile devices. We investigate off-stoichiometric effect on the thermal and structural properties of amorphous Ge$_{1-\delta}$Te (0 $\le$ $\delta$ $\le$ 0.12) via molecular dynamics. The structural optimization due to off-stoichiometry was taken into account with an empirical potential. Our simulated thermal conductivity is in the range of experimental observations. With increasing $\delta$, the thermal conductivity tends to be slightly reduced. Analysis on the coordinate number and the bond angle distribution indicates that the off-stoichiometric Ge$_{1-\delta}$Te still retain its ability of rapid phase transition. These results are helpful in reliable device design and modeling.

Thickness-Dependence of Surface Reconstruction on the (001) Surface of Ultrathin Silicon Nanosheets by Density Functional Tight Binding Simulations

The influences of the thickness of ultrathin Si nanosheets on the (001) surface morphologies and charge distribution were identified by using density functional tight binding (DFTB) simulations. The differences in structure and electronic properties were elucidated on the basis of bond lengths, bond angle distribution, and arrangement patterns in (001) surface atoms of Si nanosheets with their thickness decreasing from 1.5 nm to 0.4 nm. The surface atoms in some nanosheets present perfect zig-zag patterns in their dimers. The amounts of the trimers are far less than those of the dimers in the surface. The formation of the dimers lowers the surface energy of the nanosheets. Analysis of Mülliken gross populations indicates that there is the charge transfer from the inner part of the nanosheet to the surface. The moving distance and direction of the surface atoms can affect the charge distribution.

The influence of pressure on the structural transformation and diffusion mechanism in lithium-silicate melt: Molecular dynamics simulation

The structural transformation and diffusion mechanism of lithium-silicate melt is carried by molecular dynamics method. In order to investigate the nature of the pressure-induced structural transformations, the pair radial distribution function (PRDF), distribution of SiO[Formula: see text], OSi[Formula: see text] and LiO[Formula: see text] coordination units, bond angle distribution (BAD) and bond distance distribution (BDD) are analyzed. The investigation reveals that there is a structural transformation in the structure of lithium-silicate. The addition of alkali oxides results in the formation of nonbridging oxygens (NBOs) by disruption of the Si–O network and it has a slight effect on the topology of SiO[Formula: see text] and OSi[Formula: see text] units. Furthermore, we show that the diffusion of network-former atom in lithium-silicate melt is anomaly and Li atoms have significantly faster diffusion rate than those of oxygen or silicon atoms. Therefore, there is an existence of two diffusion mechanisms in lithium-silicate.

Structural Simulation of Mg2SiO4 under Compression

The microstructure in Mg2SiO4 glass under high compression is studied by molecular dynamic method. This work revealed the correlation between pair radial distribution functions (PRDF) of Si-Si pair and bond angle distribution (BAD) of Si-O-Si and focus on clarifying the split peak of Si-Si PRDF. Moreover, visualizing the bonds of Si-Si at different pressures show changing of Si-Si bonds with pressure. In particularly, as increasing pressure, it forms corner-sharing, edge-sharing and face-sharing bond between SiOx coordination units results in the first peak splitting of Si-Si PRDF at high pressure. The results of Si-Si’s PRDF have also been analyzed and explained in detail.

Study of Structure Transition and Crystallization of Amorphous Silica under Compression

In this work, we use molecular dynamic (MD) simulation to study of the structure transition and crystallization of amorphous silica (SiO2) under compression. The structural evolution of amorphous SiO2 is explained through radial distribution function, coordination number distribution, bond angle distribution and visualization. Simulation result shown that there is a structural transformation from tetrahedral to octahedral network through SiO5 units. In the 5-15 GPa pressure range, structural transformation occurs powerfully and there are three structural phases corresponding to SiO4-, SiO5-, and SiO6- ones. At 15 GPa, octahedral-network (SiO6) is dominant. It is the first time we showed that when pressure is higher than 20 GPa, octahedral-network of amorphous SiO2 has a tendency to transform to stishovite crystalline phase.

Atomistic Simulation for Rejuvenation of Cu-Zr Metallic Glasses by Strain

Molecular dynamics simulations were carried out to investigate the atomic structure of a model Cu64Zr36 bulk metallic glass (BMG). It is found that the amount of icosahedral content of the system is significantly increased in a well relaxed structure. While we considered four connection types of vertex-, edge-, face-, and volume-sharing, the huge cluster in the relaxed samples mainly involve volume-type connection and exhibits a remarkable athermal plasticity that great stiffness and great yield strength compared to the as-quenched samples. In addition, the bond-angle distribution of annealed sample shows sharp peaks at specific bond angles which is an evidence of crystallized Laves-phase formed by icosahedral atoms, however the peaks are to be broaden after loading, which indicates decreasing amount of icosahedral content and their shape distortion. These results suggest that icosahedral content in a bulk metallic glasses plays a key role to determine the mechanical properties such as rigidity and maximum stress carrying capacity.

Structure and Correlation Between the Fraction of Structural Units and Bond Angle Distribution in Liquid B2 O3 Under Compression

Structure of network-forming liquid B2 O3 is investigated by Molecular dynamics simulation (MDS) at 2000K and in the 0-40 GPa pressure range (corresponding to the 1.71-3.04 g/cm3 density range). Results indicate that network structure of liquid B2 O3 comprises of basic structural units BO3 and BO4 . The topology and size of BO3 and BO4 units at different densities are identical. The O-B-O and B-O-B partial bond angle distributions (BADs) can be determined through the fraction of BO3 and BO4 units. Furthermore, the total BADs are directly related to the partial BADs and the fraction of structural units. It means the fraction of units BOX (X = 3,4) and units OBy (y = 2,3) can be determined from the experimental BADs. The spatial distribution of BO3 and BO4 units is not uniform but forming clusters of BO3 and BO4 . This leads to the polyamorphism in liquid B2 O3 . It also shows that the dynamical heterogeneity in liquid B2 O3 due to the lifetimes of BO3 and BO4 units are very different. The structural heterogeneity is origin of spatially heterogeneous dynamics in liquids B2 O3 .

Structure and Properties of the GeAsS Glasses from Ab initio Calculations

The structure and properties of the GexAsxS100-2x have been studied by ab initio molecular dynamics simulation. By calculating the pair distribution functions, bond angle distribution functions, we analyze the structure and properties of the alloys. Calculations show that Ge and As are all well combined with S atoms. When x is smaller than 25.0 the binding increases with x , when x is larger than 25.0 the binding decreases with increasing x . The intervention of As atom does not affect the GeS2 formation in Ge40As40S80

MICROSTRUCTURAL EVOLUTION OF SiC DURING MELTING PROCESS

Microstructural evolution of SiC during melting process is simulated with Tersoff potential by using molecular dynamics. Microstructural characteristics are analyzed by radial distribution function, angle distribution function and Voronoi polyhedron index. The results show that the melting point of SiC with Tersoff potential is 3249 K. Tersoff potential can exactly describe the changes of bond length, bond angle and Voronoi clusters during the process of melting. Before melting, the length of the C – C bond, Si – Si bond and Si – C bond is 3.2, 3.2 and 1.9 Å, respectively. The bond angle distributes near the tetrahedral bond angle 109°, and the Voronoi clusters are all (4 0 0 0) tetrahedron structures. After melting, the C – C bond and Si – Si bond are reduced, while the Si – C bond is almost unchanged. The range of bond angle distribution is wider than before, and most of the (4 0 0 0) structures turn into three-fold coordinated structures, (2 3 0 0), (0 6 0 0) and (2 2 2 0) structures. The simulation results clearly present the microstructural evolution properties of SiC during the melting process.