Phase transition,elastic and electronic properties of topological insulator Sb_2Te_3 under pressure: First principle study

The phase transition, elastic and electronic properties of three phases(phase Ⅰ,Ⅱ, and Ⅲ) of Sb_2Te_3 are investigated by using the generalized gradient approximation(GGA) with the PBESOL exchange–correlation functional in the framework of density-functional theory. Some basic physical parameters, such as lattice constants, bulk modulus, shear modulus,Young's modulus, Poisson's ratio, acoustic velocity, and Debye temperature Θ are calculated. The obtained lattice parameters under various pressures are consistent with experimental data. Phase transition pressures are 9.4 GPa(Ⅰ→Ⅱ) and 14.1 GPa(Ⅱ→Ⅲ), which are in agreement with the experimental results. According to calculated elastic constants, we also discuss the ductile or brittle characters and elastic anisotropies of three phases. Phases Ⅰ and Ⅲ are brittle, while phaseⅡ is ductile. Of the three phases, phaseⅡ has the most serious degree of elastic anisotropy and phase Ⅲ has the slightest one.Finally, we investigate the partial densities of states(PDOSs) of three phases and find that the three phases possess some covalent features.     

Theoretical Calculations for Structural, Elastic and Thermodynamic Properties of MgB2 under High Pressure

We have investigated the structural and elastic properties of MgB₂ under high pressures using the full-potential linearized muffin-tin orbital (FP-LMTO) scheme within the generalized gradient approximation correction (GGA) in the frame of density functional theory. The calculated pressure dependence of the normalized volume is in excellent agreement with the experimental results. At the same time the elastic constants and acoustic anisotropy as a function of applied pressure are presented. Through the quasi-harmonic Debye model, we also investigate the thermodynamic properties of MgB₂.     

First-principles study of the phonon spectrum of ɛ-GaSe

The phonon spectrum and phonon density of states of ?-GaSe layered semiconductor have been studied from the first principles in the linear-response approximation. The elastic constants and acoustic velocities along and across layers have been determined. The study of the equilibrium structure and phonon spectrum of the (0001) surface of ?-GaSe has demonstrated that the volume and surface structural dynamic properties of these crystals differ insignificantly. The calculated frequencies and symmetries of the phonon modes in the center of the Brillouin zone are in good agreement with the experimental data obtained from the Raman scattering and infrared spectra.     

高压下RhB的相变、弹性性质、电子结构及硬度的第一性原理计算

采用密度泛函理论中的赝势平面波方法系统地研究了高压下RhB的结构相变、弹性性质、电子结构和硬度.分析表明,RhB在25.3 GPa时从anti-NiAs结构相变到FeB结构,这两种结构的弹性常数、体弹模量、剪切模量、杨氏模量和弹性各向异性因子的外压力效应明显.电子态密度的计算结果显示,这两种结构是金属性的,且费米能级附近的峰随着压强的增大向两侧移动,赝能隙变宽,轨道杂化增强,共价性增强,非局域化更加明显.此外,硬度计算结果显示,anti-NiAs-RhB的金属性比较弱,有着较高的硬度,属于硬质材料.     

A continuum thermal stress theory for crystals based on interatomic potentials

This paper presents a new continuum thermal stress theory for crystals based on interatomic potentials.The effect of finite temperature is taken into account via a harmonic model.An EAM potential for copper is adopted in this paper and verified by computing the effect of the temperature on the specific heat,coefficient of thermal expansion and lattice constant.Then we calculate the elastic constants of copper at finite temperature.The calculation results are in good agreement with experimental data.The thermal stress theory is applied to an anisotropic crystal graphite,in which the Brenner potential is employed.Temperature dependence of the thermodynamic properties,lattice constants and thermal strains for graphite is calculated.The calculation results are also in good agreement with experimental data.     

钝感高能炸药三氨基三硝基苯高温高压下热力学性质的分子动力学模拟研究

热力学性质是钝感高能炸药1, 3, 5-三氨基-2, 4, 6-三硝基苯(TATB)爆轰性质和安全性评估分析的重要参数. 由于结构的复杂性, TATB炸药尚缺乏系统的实验和理论计算结果. 结合全原子力场和分子动力学的方法, 本文系统研究了不同温度和压力条件下TATB的力学性质和热力学参数, 得到了弹性模量、德拜温度等随温度、压力的变化情况, 并与实验进行了对比分析. 结果表明: 在 0-50 GPa外部压力下, TATB晶体保持力学稳定, 弹性常数和弹性模量随压力升高而增大, 各向异性程度随压力升高而减小, 泊松比和延展性则受压力的影响较小; 随温度的升高, TATB的力学稳定性逐渐下降, 有发生力学失稳的可能, 各弹性常数随温度升高而逐渐减小, 各向异性程度也随之减小; TATB 的声速和德拜温度同样随着压力升高而增大, 平均声速从0 GPa下的1833 m/s, 增加到10 GPa 下的3143 m/s, 德拜温度由0 GPa下的254 K增加到10 GPa的587 K. TATB 热膨胀系数的计算表明, 在200-500 K 温度常压情况下, 其体热膨胀系数为35.9×10^-5 K^-1, 与实验数据符合较好.     

Theoretical Calculations for Structural,Elastic and Thermodynamic Properties of γTiAl Under High Pressure

We investigate the structural and elastic properties of γTiAl under high pressures using the norm-conserving pseudopotentials within the local density approximation (LDA) in the frame of density functional theory. The calculated pressure dependence of the elastic constants is in excellent agreement with the experimental results. The elastic constants and anisotropy as a function of applied pressure are presented. Through the quasi-harmonic Debye model, we also investigate the thermodynamic properties of γTiAl.     

Mechanical and dynamical stability of TiAsTe compound from ab initio calculations

The first-principles calculations are employed to provide a fundamental understanding of the structural features and relative thermodynamical, mechanical and phonon stability of TiAsTe compound. The calculated lattice parameters are in good agreement with available experimental results. We have computed elastic constants, its derived moduli and ratios that characterize mechanical properties for the first time. The calculated elastic constants indicate that these materials are mechanically stable at ambient condition. The minimum thermal conductivities of TiAsTe are calculated using both Clarke’s model and Cahill’s model. Furthermore, the elastic anisotropy has been visualized in detail by plotting the directional dependence of compressibility, Young’s modulus and shear modulus. Our results suggest strong elastic anisotropy for this compound. Additionally, the phonon spectra and phonon density of states are also obtained and discussed. The full phonon dispersion calculations confirm the dynamic stability of TiAsTe.     

Structural and Dynamic Properties of Amorphous Silicon： Tight-Binding Molecular Dynamics Simulation

The tight-binding molecular dynamics simulation has been performed to study structural and dynamical properties of amorphous silicon. It is found that the radial distribution function and static structure factor are in good agreement with the experimental measurements. The bond order parameters Q1 are sensitive to the structure change at different quenching rates. For the dynamical properties, we have calculated the vibration and electronic density of states. The simulation results show that the transverse acoustic is in good agreement with the experimental data, and the high frequency transverse optical (TO) peak shifts to the right of the experimental TO peak.     

The dielectric and dynamical properties of zinc-blende BN, AlN and GaN from first-principle calculation

The ab initio calculations of the electronic structural, dielectric and lattice-dynamical properties of zinc-blende BN, AlN and GaN were presented. The ground-state properties, i.e., the lattice constant, the bulk modulus and band gap, were calculated using a plane-wave-pseudopotential method within the density-function theory. A linear-response approach to the density-function perturb theory was used to derive the Born effective charge, the high-frequency dielectric constants and interatomic force constants for these materials. The interatomic force contants (IFCs) are useful for interpolating the dynamical matrices through the whole Brillouin zone. Phonon frequencies along high-symmetry lines were also obtained by interpolating the dynamical matrices using the interatomic force constants. In this paper, we discussed the difference of dielectric and dynamical properties among zinc-blende BN, AlN and GaN, and meanwhile, also compared these properties with other experimental data available and theoretical values. Generally, the calculations were in good agreement with the other existing experimental data and theoretical values. Supported by the Hunan Provincial Natural Science Foundation of China (Grant No. 05JJ40135) and Xiangnan University Important Science Foundation (Grant No. 2007Z010)     

Lattice dynamics of FCC transition metals: A pseudopotential approach

Simple pseudopotential model for the binding energy of transition metals is proposed. The contribution of thes-like electrons is calculated in the second-order perturbation theory for the local model pseudopotential while that of thed-like electrons is taken into account by introduction of repulsive short-range interatomic potential. Model parameters were determined for ten fcc transitions metals (Cu, Ni, Fe, Co, Ag, Pd, Rh, Au, Pt, and Ir). This model was used for the calculation of the phonon dispersion and the density of states, as well as for the elastic constants and their pressure derivatives. Good agreement with experimental data was achieved for the overall shape of phonon spectra and even for the position of the Kohn anomalies in Pd and Pt. Existence of such anomalies is also stated for predicted phonon spectra of rhodium and iridium.     

Thermal equation of state, and melting and thermoelastic properties of bcc tantalum from molecular dynamics

We performed molecular dynamics simulations with the extended Finnis-Sinclair (EFS) potential to investigate thermal equation of state (EOS), and melting and thermoelastic properties of tantalum. The agreement of the obtained thermal EOS with experiments at ambient conditions is reasonably good. The EFS potential with the two-phase method also reproduced very satisfyingly the high-pressure melting curve, excellently consistent with both the experiments of melting temperature at ambient pressure and shock melting at high pressure. From molecular dynamics simulations, we also obtained the thermoelastic properties of Ta for temperatures up to 3000 K at ambient pressure. Fully including anharmonic effects in molecular dynamics, our calculated elastic constants are in excellent agreement with experimental data. Shear modulus G decreases quickly with increasing temperature.     

Study of phase transformation and elastic properties of ThX (X = N,P, As and Sb) under high-pressure

An improved interaction potential model (IIPM) has been formulated to theoretically predict the pressure induced phase transition, elastic properties and thermophysical properties of thorium monopnictides (ThX; X = N, P, As and Sb). The phase transition pressures and volume drop obtained from this model show a better agreement with the available experimental than theoretical results. We have achieved elastic moduli, anisotropy factor, Poisson's ratio, Kleinman parameter, shear and stiffness constants on the basis of the calculated elastic constants. To know the anharmonic properties, we have also computed the third-order elastic constants, first-order pressure derivatives of second-order elastic constants and thermophysical quantities. Our results are in reasonable agreement with available measured and others reported data which supports the validity of model.     

First-principles study of bonding,elasticity-relevant and acoustic properties of BaAlBO3F2

The band structure, density of states, charge densities, and elasticity-relevant properties of BaAlBO₃F₂ are obtained by first-principles density functional calculations within the generalized gradient approximation. Other elasticity-relevant constants, such as the the Young's modulus, Poisson ratio, velocity of acoustic waves, and the Debye temperature are also deduced from the elastic constants. Bonding analysis demonstrates that BaAlBO₃F₂ has different bonding properties between basal (ab) plane and c axis. The analyses on elasticity-relevant properties indicate BaAlBO₃F₂ is mechanically stable and anisotropic. It is also shown that BABF is an ionic crystal with brittle character. Our these results give a reasonable explanation for the experimental finding that BaAlBO₃F₂ is apt to crack along c axis. Proceeding from Christoffel equation, we discuss the propagation properties of acoustic modes in BaAlBO₃F₂ to give a theoretical guidance for measuring its elastic constants. Research shows that the calculated average velocities of longitudinal and transverse modes from Christoffel equation are in good agreement with those from classic Debye model.     

TiC lattice dynamics from ab initio calculations

Ab initio calculations and a direct method have been applied to derive the phonon dispersion curves and phonon density of states for the TiC crystal. The results are compared and found to be in a good agreement with the experimental neutron scattering data. The force constants have been determined from the Hellmann-Feynman forces induced by atomic displacements in a supercell. The calculated phonon density of states suggests that vibrations of Ti atoms form acoustic branches, whereas the motion of C atoms is confined to optic branches. The elastic constants have been found using the deformation method and compared with the results obtained from acoustic phonon slopes. Received 23 February 1998     

BaTiO3晶体结构及弹性的分子动力学模拟

有效的势函数是分子动力学模拟的关键. 引入了一种势函数,该势函数的特点是运用参数r_eff计算原子间的静电作用. 通过分子动力学方法模拟得到了BaTiO₃晶体立方相、四方相结构的对关联函数和X射线衍射谱,计算得出了它们的晶格常数及弹性常数. 模拟结果与实验结果符合较好.该势函数可以有效地模拟BaTiO₃晶体的热学和力学性能. 关键词： 分子动力学模拟 势函数 3铁电晶体')" href="#">BaTiO₃铁电晶体     

Elasticity and thermodynamic properties of RuB2 under pressure

First-principles calculations of the crystal structure and the elastic properties of RuB₂ have been carried out with the plane-wave pseudopotential density functional theory method. The calculated values are in very good agreement with experimental data as well as with some of the existing model calculations. The elastic constants c_ij, the aggregate elastic moduli (B, G, E), Poisson's ratio, and the elastic anisotropy with pressure have been investigated. Through the quasi-harmonic Debye model considering the phonon effects, the isothermal bulk modulus, the thermal expansions, Grüneisen parameters, and Debye temperatures depending on the temperature and pressure are obtained in the whole pressure range from 0 to 60 GPa and temperature range from 0 to 1100 K as well as compared to available data.     

Microscopic theory of second-order and third-order elastic constants for an NaCl crystal

Relations between the second-order and third-order symmetry-independent elastic constants and the energy of interatomic interactions dependent on the mutual arrangement of pairs and triplets of atoms are obtained for crystals belonging to the crystal class O _h. The derived relations and experimental data on the elastic constants are used to calculate four third-order elastic constants and the temperature dependence of the elastic anisotropy factor a(T) for an NaCl crystal. The calculated dependence a(T) is in qualitative agreement with the experimental dependence a _exp(T).     

Lattice instability and martensitic transformation in LaAg predicted from first-principles theory

The electronic structure, elastic constants and lattice dynamics of the B(2) type intermetallic compound LaAg are studied by means of density functional theory calculations with the generalized gradient approximation for exchange and correlation. The calculated equilibrium properties and elastic constants agree well with available experimental data. From the ratio between the bulk and shear moduli, LaAg is found to be ductile, which is unusual for B(2) type intermetallics. The computed band structure shows a dominant contribution from La 5d states near the Fermi level. The phonon dispersion relations, calculated using density functional perturbation theory, are in good agreement with available inelastic neutron scattering data. Under pressure, the phonon dispersions develop imaginary frequencies, starting at around 2.3 GPa, in good accordance with the martensitic instability observed above 3.4 GPa. By structural optimization the high pressure phase is identified as orthorhombic B(19).     

Temperature dependence of the elastic constants of copper,gold and silver

From an inverse power type interatomic potential energy function and the Boltzmann distribution function for interatomic distances, the second-order elastic constants of copper, gold and silver are calculated as a function of temperature without involving any undetermined parameters. The results are found to be in fair to good agreement with the experimental observations in the temperature range of-196 to 25°C.