第一性原理对金的高压相变和零温物态方程的计算

在密度泛函理论下，用缀加平面波加局域轨道方法，分别采用广义梯度近似(GGA)和局域密度近似(LDA)对金的面心立方晶格结构(fcc)、体心立方晶格结构(bcc)和六角密堆积结构(hcp)的结构能量进行了计算.在GGA下，计算得出fcc向hcp和hcp向bcc的相变分别发生在380GPa 和1250GPa；而LDA下相变分别发生在490GPa和790GPa.当计算压强达到2TPa时，bcc在这两种近似下仍然保持稳定的结构.根据不同体积下不同结构的电子态密度的特征，对发生相变的物理原因进行了定性的分析，在此基础上得到了金的零温状态方程. 关键词： 缀加平面波方法 固态相变 电子态密度 物态方程     

高压下ZnO的结构、弹性性质和吸收光谱的第一性原理研究

 运用基于密度泛函理论（DFT）的平面波赝势方法（PWP）,结合局域密度近似（LDA）以及广义梯度近似（GGA）,系统地研究了ZnO的纤锌矿结构（B4结构）,NaCl结构（B1结构）和CsCl结构（B2结构）在不同压强下的几何结构、弹性性质和吸收光谱。详细研究了ZnO发生的两次相变（B4→B1及B1→B2相变）,得到了不同近似下的相变压强,以及两次相变过程中其弹性常数随压强的变化,并分析了这种变化与相变的关系。发现在高压作用下,ZnO的吸收光谱发生蓝移。通过计算结果和实验结果的比较可以看出,LDA近似下的计算结果更加符合实验结果。     

Elastic properties and phonon dispersions of rhenium in hexagonal-close-packed structure under pressure from first principles

The elastic properties of the hexagonal-close-packed (hcp) structure rhenium (Re) and their behavior under pressure are investigated using the local density approximation (LDA) and the generalized gradient approximation (GGA). The obtained high pressure elastic constants are well consistent with previous theoretical date, while large discrepancies are found between theory and the high pressure experiments. The calculated isothermal bulk modulus B₀ (376 GPa for GGA and 389 GPa for LDA) and its initial pressure derivative (4.52 for LDA and 4.58 for GGA) compare favorably with the experimental values. Moreover, it is found that the value of c/a, B/G, Poisson's ratio, and B_c/B_a are virtually independent of pressure. We also performed calculation for phonon dispersions at high pressure. GGA in our calculation exhibits a same trend as the high pressure experimental curve.     

高压下TATB压缩性质的LDA和GGA比较研究

本文采用第一性原理密度泛函理论,结合平面波赝势方法,采用局域密度近似(LDA)和广义梯度近似(GGA)两种方法计算了TATB晶体在高压(0~7 GPa)压缩下的结构和物理性质,并与实验数据进行了比较.详细讨论了TATB晶体的晶体结构和分子构型随压力的变化.     

Structural,elastic, thermodynamic and electronic properties of LuX (X = N,Bi and Sb) compounds: first principles calculations

ABSTRACT

The structural, electronic, elastic and thermodynamic properties of LuX (X = N, Bi and Sb) based on rare earth into phases, Rocksalt (B1) and CsCl (B2) have been investigated using full-potential linearized muffin-tin orbital method (FP-LMTO) within density functional theory. Local density approximation (LDA) for exchange-correlation potential and local spin density approximation (LSDA) are employed. The structural parameters as lattice parameters a₀, bulk modulus B, its pressure derivate B’ and cut-off energy (E_c) within LDA and LSDA are presented. The elastic constants were derived from the stress–strain relation at 0 K. The thermodynamic properties for LuX using the quasi-harmonic Debye model are studied. The temperature and pressure variation of volume, bulk modulus, thermal expansion coefficient, heat capacities, Debye temperature and Gibbs free energy at different pressures (0–50 GPa) and temperatures (0–1600 K) are predicted. The calculated results are in accordance with other data.     

Magnetic moment collapse induced by high-pressure in semi-borides TM2B (TM = Fe,Co). A first-principles study

The high-pressure effects are investigated on the structure, magnetic phase transition, and anisotropic elastic properties of the 3d transition-metal semi-borides TM₂B (TM?=?Fe, Co) by using the generalized gradient approximation (GGA) within the framework of density functional theory (DFT). At equilibrium spin polarization, calculations show that the Fe₂B and Co₂B compounds are ferromagnetic (FM). In the applied pressure range from 0 to 90?GPa, the magnetic moment of Fe₂B and Co₂B slowly decreases and then abruptly drops to zero at 85?GPa, indicating a state transition from the ferromagnetic to the nonmagnetic (NM) state (a first-order quantum phase transition). The collapse of the magnetic moment is accompanied by an abrupt change in the lattice parameters and elastic constants. In addition to this phenomenon, the density of states (DOS), and anisotropic elastic properties are presented at 0?GPa and at the critical transition pressure. Furthermore, I have plotted the three-dimensional (3D) surfaces and planar contours for the Young and bulk moduli of the compounds at several crystallographic planes, ((100) and (001)) to reveal their elastic anisotropy. On the basis of anisotropic elastic properties, I have predicted the easy and hard axes of magnetization for the TM₂B compounds.     

Theoretical study of B2 type technetium AB (A=Tc,B=Ti,V, Nb and Ta) intermetallic compounds

The structural, electronic, elastic and thermal properties of the cubic AB type (A=Tc, B=Ti, V, Nb and Ta) technetium intermetallic compounds have been studied using the full potential linearized augmented plane wave (FP-LAPW) method within the generalized gradient approximation (GGA) and local density approximation (LDA) used for the exchange-correlation potential. The calculated lattice parameters agree well with the experimental results. The calculated electronic properties reveal that these compounds are metallic in nature with partial ionic bonding. The elastic constants obey the stability criteria for cubic system. Ductility for these compounds has been analyzed using the Pugh's rule and Cauchy's pressure revealing ductile in nature of all the compounds. Bonding nature is discussed using Fermi surface, band structure and charge density difference plots.     

Ground state properties of titaniumdiboride

Abstract

The electronics structure, the charge distribution and the total energy of hexagonal titaniumdiboride are calculated using non-local pseudopotentials in both the local density approximation (LDA) and the generalized gradient expansion approximation (GGA). In the LDA we obtain a = 3.023 Å, c = 3.166 Å and B_o = 271. GPa. For these quantities the GGA values are slightly lower and both compare well with experiment. We also determined selected elastic constants by fitting the total energies to a quadratic surface in the lattice parameters. Using strains that do not break the hexagonal symmetry we obtain C₁₁ + C₁₂ = 777.GPa, C₁₃ = 83. GPa and C₃₃ = 568. GPa. Again slightly lower values are obtained using the GGA. These values agree well with a recent experiment.     

Simulation of the phase transition of graphite to the diamond-like <Emphasis Type="Italic">LA</Emphasis>3 phase

The phase transition of graphite to a diamond-like LA3 phase is simulated by the methods of the density functional theory (DFT). The calculations are performed in the local density approximation (LDA) and the generalized gradient approximation (GGA). It is found that the structural transformation must occur at a pressure of 60 or 74 GPa according to calculations based on the DFT–LDA and DFT–GGA, respectively. The height of the potential barrier separating the structural state corresponding to the LA3 phase from the state corresponding to graphite exceeds 0.13 eV/atom. This indicates the possibility of stable existence of the diamond-like LA3 phase under standard conditions.     

Diamond under pressure: Ab-initio calculations of the equation of state and optical phonon frequency revisited

The pressure–volume relationship and the zone-center optical phonon frequency of cubic diamond at pressures up to 600?GPa have been calculated based on density functional theory (DFT). Different methods (ranging from pseudopotential to all-electron approaches) and different approximations [local density approximation (LDA), generalized gradient approximation (GGA)] for the exchange-correlation (XC) energy have been applied. The results are compared to recent precise measurements up to 140?GPa. Possible implications for the experimental pressure determination based on the 1986 calibration of the ruby luminescence method are discussed.     

Investigation of structural stability and electronic properties of group III nitrides: a first principles study

The high-pressure structural phase transition, electronic, superconducting and elastic properties of group III nitrides (ScN, YN and LaN) are investigated by first principles calculation with the density functional theory. The calculated lattice parameters are in good agreement with the experimental and other theoretical values. Electronic structure reveals that these materials are semiconductors with an indirect band gap of 1.4, 0.87 and 0.65?eV for ScN, YN and LaN, respectively. The obtained cubic NaCl structure is energetically the most stable structure at ambient pressure. A pressure-induced structural phase transition from NaCl to CsCl structure is predicted. The structural phase transition of ScN, YN and LaN occurs at a pressure of 158, 132 and 26.5?GPa, respectively. On further increase in the pressure, semiconductor-to-metallic transition and superconductivity is observed in these nitrides. The estimated T _c values as a function of pressure for ScN, YN and LaN are 31.79, 15.50 and 12.84?K, respectively.     

Mechanical,electronic and thermodynamic properties of TE-C36 under high pressure

The structural parameters, mechanical, electronic and thermodynamic properties of TE-C36 under high pressure were calculated via the density functional theory in combination with the quasi-harmonic Debye model. The results show that the pressure has significant effects on the equilibrium structure parameters, mechanical, electronic and thermodynamic properties of TE-C36. The obtained ground state structural parameters are in good agreement with previous theoretical results. The mechanically and dynamically stable under pressure were confirmed by the calculated elastic constants and phonon dispersion spectra. The elastic constants, elastic modulus, B/G ratio, Poisson’s ratio and Vicker’s hardness were determined in the pressure range of 0–100?GPa. The elastic anisotropy of TE-C36 under pressure are also determined in detail. The electronic structure calculations reveal that TE-C36 remains a direct band gap semiconductor when the pressure changes from 0 to 100?GPa, and the band gap decreases with increasing pressure. Furthermore, the pressure and temperature dependence of thermal expansion coefficient, heat capacity and Debye temperature are predicted in a wide pressure (0–90?GPa) and temperature (0–2500?K) ranges. The obtained results are expected to provide helpful guidance for the future synthesis and application of TE-C36.     

从头算方法研究面心立方铝在高温高压下的热力学状态方程

用从头算方法优化计算了面心立方铝的电子结构和总能,得到了它在零温下的状态方程和弹性性质.将得到的总能与晶格体积拟合到Debye模型,获得了非平衡态下的Gibbs自由能与温度、压力之间的关系,在此基础上计算了相应的热状态方程,利用Burakovsky-Preston-Silbar （BPS） 熔化模型计算了铝的熔化曲线.所有的电子结构和总能计算都是基于局域密度近似（LDA）和广义梯度近似（GGA）的平均得到的.计算得到的铝在高温、高压下的状态方程与一些热力学性质和熔化曲线同冲击波和静高压实验数据在225 G 关键词： 铝 热力学状态方程 从头算 熔化曲线     

Elastic constants and anisotropy of RuB2 under pressure

The structural, elastic constants and anisotropy of RuB2 under pressure are investigated by first-principles calcula-tions based on the plane wave pseudopotential density functional theory method within the local density approximation (LDA) as well as the generalized gradient approximation (GGA) for exchange and correlation. The results accord well with the available experimental and other theoretical data. The elastic constants, elastic anisotropy, and Debye temperature Θ as a function of pressure are presented. It is concluded that RuB2 is brittle in nature at low pressure, whereas it becomes ductile at higher pressures. An analysis for the calculated elastic constant has been made to reveal the mechanical stability of RuB2 up to 100 GPa.     

Melting curve of MgO from first-principles simulations

First-principles calculations based on density functional theory, both with the local density approximation (LDA) and with generalized gradient corrections (GGA), have been used to simulate solid and liquid MgO in direct coexistence in the range of pressure 0 < or = p < or = 135 GPa. The calculated LDA zero pressure melting temperature is T(LDA)m = 3110 +/- 50 K, in good agreement with the experimental data. The GGA zero pressure melting temperature T(GGA)m = 2575 +/- 100 K is significantly lower than the LDA one, but the difference between the GGA and the LDA is greatly reduced at high pressure. The LDA zero pressure melting slope is dT/dp approximately 100 K/GPa, which is more than 3 times higher than the currently available experimental one from Zerr and Boehler [Nature (London) 371, 506 (1994)]. At the core mantle boundary pressure of 135 GPa MgO melts at Tm = 8140 +/- 150 K.     

First-principles calculations for elastic properties of OsB2 under pressure

The structure, elastic properties and elastic anisotropy of orthorhombic OsB₂ are investigated by density functional theory method with the ultrasoft pseudopotential scheme in the frame of the generalized gradient approximation (GGA) as well as local density approximation (LDA). The obtained structural parameters, elastic constants, elastic anisotropy and Debye temperature for OsB₂ under pressure are consistent with the available experimental data and other theoretical results. It is found that the elastic constants, bulk modulus and Debye temperature of OsB₂ tend to increase with increasing pressure. It is predicted that OsB₂ is not a superhard material from our calculations.     

Stability of Ferromagnetism in Fe, Co, and Ni Metals under High Pressure with GGA and GGA+U

The stability of the ferromagnetic state in Fe, Co, and Ni metals under high pressure is investigated using generalized gradient approximation (GGA) and GGA+U within the density functional theory (DFT). It is found that the ferromagnetic state under pressure is very different for Fe, Co, and Ni metals, and is closely associated with the crystal structure. In the case of Fe, a ferromagnetic bcc ground state is obtained at ambient pressure and a nonmagnetic hcp ground state is found at pressure around 12 and 115 GPa for GGA and GGA+U, respectively. For Co, the phase transition from a ferromagnetic hcp to a nonmagnetic fcc is found around 107 GPa for GGA. In contrast to Fe and Co, a ferromagnetic fcc state in Ni is maintained even at 200 GPa. The calculated results suggest that the suppression of ferromagnetism in Fe, Co, and Ni is due to pressure-induced decrease of the density of state at the Fermi level.     

YPd3X（X=B,C）晶体结构、弹性性质与电子结构的第一性原理研究

在广义梯度近似（GGA）和GGA+U（在位库仑势）下,采用第一性原理方法系统地研究了三元过渡金属硼碳化合物YPd3X(X＝B,C)的晶体结构、弹性性质、电子结构和成键特性．计算的晶格参数和体弹性模量均与报道的实验结果吻合,而YPd3X(X＝B,C) 的弹性参数计算值则表明YPd3C的硬度大于YPd3B．根据晶体机械稳定标准得到YPd3B和YPd3C的失稳临界压强分别约为16.5GPa和23GPa．由Pugh经验关系可知YPd3X(X=B,C)均属于韧性材料,且YPd3B的韧性略高于YPd3C．电子能带结构分析表明YPd3B和YPd3C均具有金属特性,且导电能力相当．由态密度和电荷密度分析得知,X与Pd之间形成较强的共价键,而Y与Pd3X之间形成离子键,化学键键能的不同是两种材料的弹性参数存在差异的内在原因．上述的研究结果为YPd3X(X=B,C)的力电材料的设计和应用提供了一定的理论依据．     

First-principles study of electronic structure and elasticity of UAlx (x=1,2,3) system

A detailed theoretical study of structural, electronic, and elastic properties of cubic UAl_x (x=1,2,3) is presented employing the pseudopotential plane-wave method based on density-functional theory. The structure parameters of these three compounds have been calculated within generalized gradient approximation (GGA) and local density approximation (LDA). The calculated results were compared with the experimental data and previous research. With the GGA approximation, the elastic constants, shear modulus, Young's modulus, and Poisson's ratio of UAl_x (x=1,2,3) are derived. According to the generalized mechanical stability criteria for cubic crystals, our calculation suggested that C15 UAl₂ and L12 UAl₃ are stable substance under hydrostatic pressures, but B2 UAl might be expected as a metastable compound, which is not reported in previous literature, and future experimental confirmation is needed. Furthermore, the calculated energy band structure and density of state (DOS) are found to be in good agreement with the theoretical values. Additionally, the charge density of these compounds have also been worked out and analyzed.     

First-principles investigations on elastic and thermodynamic properties of zinc-blende structure BeS

In this paper the elastic and thermodynamic properties of the cubic zinc-blende structure BeS at different pressures and temperatures are investigated by using \textit{ab initio} plane-wave pseudopotential density functional theory method within the generalized gradient approximation (GGA). The calculated results are in excellent agreement with the available experimental data and other theoretical results. It is found that the zinc-blende structure BeS should be unstable above 60GPa. The thermodynamic properties of the zinc-blende structure BeS are predicted by using the quasi-harmonic Debye model. The pressure-volume-temperature ($P-V-T$) relationship, the variations of the thermal expansion coefficient $\alpha$ and the heat capacity $C_{V}$ with pressure $P$ and temperature $T$, as well as the Gr\"{u}neisen parameter-pressure-temperature ($\gamma -P-T$) relationship are obtained systematically in the ranges of 0--90GPa and 0--2000K.