Pressure and Temperature Induced Phase Transition of ZnS from First-Principles Calculations

The pressure induced phase transition of ZnS from the wurtzite （WZ） and the zincblende （ZB） structures to the rocksalt （RS） structure and the temperature induced phase transition from the ZB structure to the WZ structure are investigated by ab initio plane-wave pseudopotential density-functional theory （DFT）, together with the quasiharmonic Debye model. It is found that the zero-temperature transition pressures from the WZ-ZnS and the ZB-ZnS to the RS-ZnS are 17.20 and 17.37 GPa, respectively. The zero-pressure transition temperature from the ZB-ZnS to the WZ-ZnS is 1199 K. All these results are consistent with the available experimental data. Moreover, the dependences of the normalized primitive cell volume V/V0 on pressure and thermal expansion coefficient α on temperature are also obtained successfully.     

Structures and Equation of State of ε-Fe under High Pressure

The equation of state （EOS） and the axial ratio c/a of ε-Fe at high pressures are investigated by using the gen- eralized gradient approximation （GGA） within the plane-wave pseudopotential density functional theory （DFT）. The results show that at the lower pressure, the EOS of ferromagnetic ε-Fe is consistent with the experimental result. While at higher pressure, the EOS of the nonmagnetic ε-Fe is in good agreement with the experimental result. Meanwhile, we find an obvious increase of the axial ratio c/a with pressure, and there is only a small increase with increasing temperature at high pressure.     

极端条件下锆的动力学稳定性研究

过渡金属Zr具有优良的物理、化学及力学性能, 具有广泛的应用价值. 主要通过新近发展的自洽晶格动力学方法, 充分考虑声子间的相互作用, 成功获得了β-Zr的高温高压声子色散曲线, 预测了β-Zr在相图中能够稳定存在的区域, 进一步比较α-Zr, ω-Zr和β-Zr的自由能, 获得了α-β 及ω-β 相变的相边界, 构建了Zr的参考相图. 同时, 也获得了β-Zr的高温状态方程及热膨胀系数, 能够为构建Zr的全区物态方程提供有益的参考.     

从头算自洽晶格动力学研究高压下钛结构相的稳定性

借助能够考虑声子间相互作用的从头算自洽晶格动力学方法,计算了钛的高温声子色散曲线,并通过分析其声子频率,计算了高温高压下的声速,并由声速获得了弹性常数的数据．通过分析声子色散 曲线能够直接获得钛各种相的动力学稳定区域,比较各相的自由能,较为准确地获得了钛的相图．     

高温高压下钼的结构稳定性研究

以钼为代表的一系列过渡金属,在高温高压的相变及结构稳定性研究是实验和理论研究的热点.钼在常温常压下是bcc结构,但是在高温高压下可能的相结构一直未能确定.本文首先预测了几种高压下的结构,并计算了其自由能及力学性质.针对可能的hcp结构,我们通过新近发展的自洽晶格动力学方法,充分考虑声子间相互作用,成功获得了hcp结构高温高压声子色散曲线,结果表明hcp相在热力学及动力学上都是能够稳定存在的结构,是一种可能的高压相.     

First-principles calculations for electronic, optical and thermodynamic properties of ZnS

The electronic, optical and thermodynamic properties of ZnS in the zinc-blende （ZB） and wurtzite （WZ） structures are investigated by using the plane-wave pseudopotential density functional theory （DFT）. The results obtained are consistent with other theoretical results and the available experimental data. When the pressures are above 20.5 and 27 GPa, the ZB-ZnS and the WZ-ZnS are converted into indirect gap semiconductors, respectively. The critical point structure of the frequency-dependent complex dielectric function is investigated and analysed to identify the optical transitions. Moreover, the values of heat capacity Cv and Debye temperature θ at different pressures and different temperatures are also obtained successfully.     

高压下Fe3C的磁性和声速性质

采用基于密度泛函理论的包含自旋极化效应的广义梯度近似方法研究了Fe₃C的高压弹性和声学性质. Fe₃C在压力作用下由铁磁基态转变到非磁性的高压态,相变压力约为73 GPa. 计算了Fe₃C 的Hugoniot线,并根据沿Hugoniot线的弹性常数计算了铁磁状态和非磁性状态的声速. 对比Fe单质,非磁性Fe₃C 的压缩波和剪切波声速与地核的地震波数据更为接近,证实了C元素是构成地核的一种成分.     

First-Principles Calculations for Elastic Properties of ZnS under Pressure

The pressure dependence of elastic properties of ZnS in zinc-blende （ZB） and wurtzite （WZ） structures are investigated by the generalized gradient approximation （GGA） within the plane-wave pseudopotential density functional theory （DFT）. Our results are in good agreement with the available experimental data and other theoretical results. From the high-pressure elastic constants obtained, we find that the ZB and WZ structures of ZnS are unstable when the applied pressures are larger than 15.8 GPa and 21.3 GPa, respectively. The sound velocities along different directions for the two structures are also obtained. It is shown that as pressure increases, the sound velocities of the shear wave decrease, and those of all the longitudinal waves increase. An analysis has been made to reveal the anisotropy and highly noneentral forces in ZnS.     

First principle calculation of elastic and thermodynamic properties of stishovite

Using ab initio plane-wave pseudo-potential density functional theory method,the elastic constants and band structures of stishovite were calculated.The calculated elastic constants under ambient conditions agree well with previous experimental and theoretical data.C13,C33,C44,and C66 increase nearly linearly with pressure while C 11 and C 12 show irregularly changes with pressure over 20 GPa.The shear modulus(C11-C12)/2 was observed to decrease drastically between 40 GPa and 50 GPa,indicating acoustic mode softening in consistency with the phase transition to CaCl 2-type structure around 50 GPa.The calculated band structures show no obvious difference at 0 and 80 GPa,being consistent with the high incompressibility of stishovite.With a quasi-harmonic Debye model,thermodynamic properties of stishovite were also calculated and the results are in good agreement with available experimental data.     

第一性原理计算高压下金红石TiO2的结构不稳定性及热学性质

采用第一性原理计算研究了金红石TiO2结构在高压下的不稳定性及热力学性质. 计算的高压下结构参数和零压的声子色散曲线与实验数据十分吻合. 进一步模拟了在不同压力下的声子曲线,在压力下,声子曲线不断软化,？点附近的振动频率不断减小直至虚频,意味着结构的不稳定,根据计算的不同压力下的弹性常数获得了其力学不稳定性,结果表明金红石结构TiO2在压力高于17.7 GPa时变得不稳定. 根据准谐近似,获得了金红石TiO2结构的热力学性质,计算结果与现有的实验数据相一致。