GaAs相变和热力学性质的密度泛函理论研究

利用平面波赝势密度泛函方法,对GaAs从闪锌矿结构到CsCl 结构的相变进行了理论研究.通过Birch-Murnaghan状态方程拟合闪锌矿结构GaAs的能量和体积,得到了GaAs的热力学性质.我们发现对 GaAs来说,闪锌矿结构通常比CsCl结构稳定,由闪锌矿结构到CsCl结构的相变压力在37.019 GPa左右.计算所得到的晶格常数、体弹模量及体弹模量对压强的一阶导数与实验值以及其他作者的计算值相符合.     

ZnS结构相变、电子结构和光学性质的研究

运用第一性原理平面波赝势和广义梯度近似方法, 对闪锌矿结构(ZB)和氯化钠结构(RS) ZnS的状态方程及其在高压下的相变进行计算研究, 分析相变点附近的电子态密度、能带结构和光学性质的变化机理. 结果表明: 通过状态方程得到ZB相到RS相的相变压强值为18.1 GPa, 而利用焓相等原理得到的相变压强值为18.0 GPa; 在结构相变过程中, sp³轨道杂化现象并未消失, RS相ZnS的金属性明显增强; 与ZB相ZnS相比, RS相ZnS的介电常数主峰明显增强, 并向低能方向出现了明显偏移, 使得介电峰向低能方向拓展, 在低能区电子跃迁大大增强. 关键词： 硫化锌 相变 电子结构 光学性质     

氮化铂在高压下的相变及弹性性质的第一原理计算

用PBE形式下的广义梯度近似（GGA）赝势平面波方法研究了氮化铂的结构相变以及弹性性质,计算了氮化铂的氯化钠（B1）、氯化铯（B2）、闪锌矿（B3）、纤维矿（B4）等四种结构并应用高压下的焓与压强的关系,得出在常温常压下B4结构是最稳定的结构,这与Yu 等人得的结果一致,且 B4→B1及B1→B2的相变压强分别发生在36.7 GPa和 185.4 GPa,同时,研究了B4结构在高压的弹性性质,发现弹性常数、体模量、剪切模量、压缩波速、剪切波速以及德拜温度均随着压强的增大而单调增大     

氮化铂在高压下的相变及弹性性质的第一原理计算

用PBE形式下的广义梯度近似（GGA）赝势平面波方法研究了氮化铂的结构相变以及弹性性质，计算了氮化铂的氯化钠（B1）、氯化铯（B2）、闪锌矿（B3）、纤维矿（B4）等四种结构并应用高压下的焓与压强的关系，得出在常温常压下B4结构是最稳定的结构，这与Yu 等人得的结果一致，且 B4→B1及B1→B2的相变压强分别发生在36.7 GPa和 185.4 GPa，同时，研究了B4结构在高压的弹性性质，发现弹性常数、体模量、剪切模量、压缩波速、剪切波速以及德拜温度均随着压强的增大而单调增大     

ZnSe相变、电子结构的第一性原理计算

基于第一性原理平面波赝势(PWP)和广义梯度近似(GGA)方法,对闪锌矿结构(ZB)和岩盐结构(RS)的ZnSe在0—20GPa高压下的几何结构、态密度、能带结构进行了计算研究,分析了闪锌矿结构ZnSe和岩盐结构ZnSe的几何结构.在此基础上,研究了ZnSe的结构相变、弹性常数、成键情况以及相变压强下电子结构的变化机理.结果发现:通过焓相等原理得到的ZB相到RS相的相变压强为15.3GPa,而由弹性常数判据得到的相变压强为11.52GPa,但在9.5GPa左右并没有发现简单立方相的出现;在结构相变过程中,sp3轨道杂化现象并未消除,Zn原子的4s电子在RS相ZnSe的导电性中起主要贡献.     

金红石→萤石TiO2相变的第一性原理计算

利用第一性原理平面波密度泛函理论并结合准谐德拜模型,对TiO₂从金红石结构到萤石结构的相变进行理论研究.依据焓相等原理,得到TiO₂从金红石结构到萤石结构的相变压强为45.32 GPa,并通过吉布斯能的计算发现相变压强随温度的增加而增加,运用拟合的方法估计实验条件下的相变压强(温度为1900-2100K时,P_t=47.604~47.756 GPa).运用等吉布斯能原理,得到零压下从金红石到萤石结构TiO₂的相变温度为2029 K.计算结果与实验及其他理论值相符,并通过准谐德拜模型成功获得相变前后的结构和热力学性质,如相对体积、热膨胀系数、热容和德拜温度等.     

氢化铒高温高压下的弹性及热力学性质

利用第一性原理平面波赝势密度泛函理论, 并结合准谐德拜模型, 计算了立方萤石结构ErH2在不同温度和压强下的体积、热膨胀系数、体弹模量和等体热容等弹性性质及热力学性质。在温度高于1 100 K的条件下,计算出的等体热容趋近于Dulong-Petit极限。得到了绝对零度、零压强下ErH2的该结构的晶格常数为0.523 2 nm,与实验值0.523 0 nm非常接近。由不同的原胞体积得出了该体系的单点能与原胞体积的关系的数据;从计算出的高压下的弹性常数,根据立方晶系的力学稳定性条件,推断出立方萤石结构ErH2的相变压力约为20 GPa。     

AlAs相变及热动力学性质的第一性原理研究

利用基于密度泛函理论的全势能线性糕模轨函法研究了闪锌矿（B3）,NiAs（B8）和岩盐（B1）结构的AlAs的相变、结构性质以及热动力学性质.对B3-B8和B3-B1结构的能量体积曲线做公切线,得到了B3→B8相变压力为5.44 GPa,并预测到B3→B1相变压力为6.46 GPa.同时计算了高压下B8相的结构性质,结果显示当V/V₀≈0.7—1.05时,c/a基本保持恒定（仅有约 0.2%的波动）;当V/V₀≈0.4—0.7,c/a随着V/V₀的减小而近似线性地增大.通过状态方程拟合,得到了AlAs的相对体积V/V₀与压强P的函数关系,B8相的状态方程与实验结果符合很好.最后利用准谐德拜模型得到了AlAs的体弹模量B随压力P的变化关系以及不同压强下热容C_V与温度T的关系. 关键词： 相变 热力学性质 第一性原理     

ZnSe高压相变和声子谱的第一性原理研究

采用基于第一性原理的平面波赝势和广义梯度近似方法,对闪锌矿(ZB)、纤锌矿(WZ)和岩盐(RS)结构的ZnSe在0~40GPa压力下的热力学性质及相变特性进行了研究。通过数值计算得出3种结构ZnSe的能量随体积变化的曲线,在此基础上,研究了不同结构ZnSe的晶格常数、高温高压物态方程、结构相变及弹性性质,解释了高压下结构相变的机理,并通过焓相等原理得到ZB结构到RS结构的相变压力为14.95GPa。最后,利用VASP软件结合Phonopy计算了ZB结构和RS结构ZnSe的声子谱,并将温度效应引入,得到亥姆霍兹自由能、熵和等容热容随温度的变化关系。     

压力效应对CdS电子结构和光学性质的影响

基于密度泛函理论体系下的广义梯度近似(GGA)方法,利用第一性原理计算了不同压强下CdS晶体闪锌矿结构,得到其晶格常数、总能量、电子态密度分布、能带结构、光反射与吸收系数等性质,通过比较能带结构的变化行为,得出CdS在116.8 GPa附近还存在等结构相变,即由直接带隙结构变为间接带隙结构.结合电子结构系统分析了压力效...     

Pressure-induced phase transition in wurtzite ZnS: An ab initio constant pressure study

We study the pressure-induced phase transition of wurtzite ZnS using a constant pressure ab initio technique. A first-order phase transition into a rocksalt state at 30–35 GPa is observed in the constant pressure simulation. We also investigate the stability of wurtzite (WZ) and zinc-blende (ZB) phases from energy–volume calculations and Gibbs free energies at zero temperature and find that both structures show nearly similar equations of state and transform into a rocksalt structure around 14 GPa, in agreement with experiments. Additionally, we examine the influence of pressure on the electronic structure of the wurtzite and zinc-blende ZnS crystals and find that their band gap energies exhibit similar tendency and increase with increasing pressure. The calculated pressure coefficients and deformation potential are found to be comparable with experiments.     

Half-metallic ferromagnetism in rocksalt and zinc-blende MS (M=Li, Na and K): A first-principles study

First-principles full-potential linearized augmented plane-wave method is used to investigate the electronic structure and magnetic properties of hypothetical zinc-blende and rocksalt LiS, NaS and KS. We find that all the compounds except rocksalt LiS exhibit half-metallic ferromagnetism with an integer magnetic moment of 1.00 μ_B per formula unit. The ferromagnetism results from the spin-polarization of p states of anion S. Total energies calculations indicate the rocksalt phase is lower in energy than the zinc-blende one. The total energy differences are about 0.38, 0.36 and 0.32 eV per formula unit for LiS, NaS and KS, respectively. Meanwhile, it is shown that rocksalt NaS and KS have the half-metallic gaps of 0.22 and 0.41 eV, respectively, and the half-metallic gaps are 0.03, 0.46 and 0.65 eV for zinc-blende LiS, NaS and KS, respectively. We also find the half-metallicity is robust against the lattice contraction up to 7% and 13% for rocksalt NaS and KS, respectively. Although rocksalt LiS is nonmagnetic and metallic at the equilibrium lattice constant, it shows half-metallic ferromagnetism when the lattice constant is larger than 5.40 Å.     

Molecular dynamics simulation of structural transformation in silicon carbide under pressure

Pressure-induced structural transformation in cubic silicon carbide is studied with the isothermal-isobaric molecular-dynamics method using a new interatomic potential scheme. The reversible transformation between the fourfold coordinated zinc-blende structure and the sixfold coordinated rocksalt structure is successfully reproduced by the interatomic potentials. The calculated volume change at the transition and hysteresis are in good agreement with experimental data. The atomistic mechanisms of the structural transformation involve a cubic-to-monoclinic unit-cell transformation and a relative shift of Si and C sublattices in the 100 direction.     

Structures and Phase Transition of GaAs under Pressure

A first-principles plane wave method with the ultrasoft pseudopotential scheme in the frame of the density functional theory （DFT） is performed to calculate the lattice parameters a and c, the bulk modulus B0 and its pressure derivative B0 of the zinc-blende GaAs （ZB-GaAs）, rocksalt GaAs （RS-GaAs）, CsCl-GaAs, NiAs- GaAs and wurtzite GaAs （WZ-GaAs）. Our results are consistent with the available experimental data and other theoretical results. We also calculate the phase transition pressures among these different phases. The results are satisfactory.     

First-principles study of zinc-blende to rocksalt phase transition in BN

The structural, electronic and elastic properties of the cubic boron nitride (BN) compound are investigated by a first-principle pseudopotential method. The calculations show that the structural phase transition from the zinc-blende(ZB) structure to the rocksalt (RS) structure occurs at a transition pressure of 1088 GPa and with a volume reduction of 3.1%. Both the ZB and RS structures of BN have indirect gaps, with energy gaps of 4.80 eV and 2.11 eV, respectively. The positive pressure derivative of the indirect band gap (Γ-X) energy for the the ZB phase and the predicted ultrahigh metallization pressure are attributed to the absence of d occupations in the valence bands. The increase of the shear modulus with increasing pressure implies that the lattice stability becomes higher when BN is compressed.     

First-principles calculations of structure and high pressure phase transition in gallium nitride

The phase transitions of semiconductor GaN from the Wurtzite (WZ) structure and the zinc-blende (ZB) structure to the rocksalt (RS) structure are investigated by using the first-principles plane-wave pseudopotential density functional method combined with the ultrasoft pseudopotential scheme in the generalized gradient approximation (GGA) correction. It is found that the phase transitions from the WZ structure and the ZB structure to the RS structure occur at pressures of 46.1 GPa and 45.2 GPa, respectively. The lattice parameters, bulk moduli and their pressure derivatives of these structures of GaN are also calculated. Our results are consistent with available experimental and other theoretical results. The dependence of the normalized formula-unit volume $V/V_{0 }$ on pressure $P$ is also successfully obtained.     

First-principle study of phase stability,electronic structure and thermodynamic properties of cadmium sulfide under high pressure

By employing first principle and a quasi-harmonic Debye model, we study the phase stability, phase transition, electronic structure and thermodynamic properties of cadmium sulfide (CdS). The results indicate that CdS is a typical ionic crystal and that the zinc-blende phase in CdS is thermodynamically unstable. Moreover, the heat capacity of the wurtzite and rocksalt phases of CdS decreases with pressure and increases with temperature, obeying the rule of the Debye T³ law at low temperature and the Dulong–Petit limit at high temperature.     

Elastic,Piezoelectric and Acoustic Properties of Wurtzite MnO from Density Functional Calculation

The wurtzite MnO has been obtained lately and is expected to have large potential in varies applications.Since elastic properties are the bases of various applications, we calculate these properties of wurtzite MnO based on the density-functional theory and compare it with other two phases of MnO (rocksalt and zinc-blende MnO). The Young's modulus of wurtzite and zinc-blende MnO are 65.6 GPa and 73.4 GPa, respectively, which are much lower than those of rocksalt MnO (177.6 GPa). More importantly, both the Poisson ratio and the bulk modulus to shear modulus ratio indicate that wurtzite MnO should have much better ductile properties than rocksalt MnO. The calculated piezoelectric constants of wurtzite MnO are comparable to those of ZnO. This suggests wurtzite MnO is a good piezoelectric material.Furthermore, the slowness surfaces of acoustic waves of them are given from Christoffel equation.     

Electronic properties and elastic constants of wurtzite, zinc-blende and rocksalt AlN

Electronic properties and elastic constants of AlN in the wurtzite, zinc-blende and rocksalt structures are investigated using an ab initio pseudopotential method based on the density-functional theory with both the local-density approximation and the generalized gradient approximation for the exchange-correlation functional. The numerically calculated results compare well with the existing experimental data. For elastic constants of rocksalt AlN our results are predictions.     

Pressure dependence of elastic properties of wurtzite ZnO crystal

We present the pressure dependence of elastic properties of the wurtzite phase of ZnO undergoing wurtzite to the rocksalt phase transition. A simple Landau theory is developed to describe the structural phase transition between wurtzite to rocksalt phases observed in ZnO. We have defined the necessary order parameter of the wurtzite to the rocksalt phase transition. We present a detailed analysis of the pressure dependence of the elastic and shear constants of the wurtzite phase of ZnO. The theoretical predictions are in the line with experimental results.