Fe2+取代对MgSiO3钙钛矿高温高压物性的影响

(Mg, Fe)SiO₃钙钛矿是下地幔中最主要的候选矿物成分之一, 关于其高温高压特性的研究对于深层地幔状态的理解和地震波变化规律的探索具有重要意义. 应用第一性原理计算了MgSiO₃和(Mg_0.75, Fe_0.25)SiO₃在0–140 GPa静水压范围内的晶体结构和弹性模量, 并由Voigt-Reuss-Hill方程计算了地震波速随压力的变化, 利用准简谐近似下的Debye模型模拟了高温效应, 分析了Fe²⁺取代Mg²⁺后镁铁钙钛矿弹性和热学性质的变化, 推断Fe²⁺取代行为软化了MgSiO₃等含镁的地球深部矿物的地震波速. 为解释地幔中某些区域的地震波速软化现象提供了一个有力的理论依据. 关键词： 钙钛矿 铁 高压 波速     

Ab initio calculations of the stability and lattice dynamics of the MgSiO3 post-perovskite

Using the first-principles computations we have calculated phonon-dispersion relations and the phonon density of states for perovskite and post-perovskite phases of MgSiO₃. From them using the quasiharmonic approximation we have estimated the free energies, bulk modulus and volume expansion coefficients of both structures at various pressures and temperatures. Our calculations indicate that the thermal expansivity of MgSiO₃ changes very little across the phase transition. We have determined the P-T coexisting line of perovskite and post-perovskite phases.     

硅酸镁钙钛矿弹性及热力学特性的第一性原理计算

基于密度泛函理论的第一性原理计算,结合准谐德拜模型研究了高压下硅酸镁钙钛矿的弹性及热力学特性. 计算得到的物态方程数据、热容、热膨胀系数等在宽广的温度和压力范围与实验结果及其他理论计算结果吻合. 根据有限应变理论计算了硅酸镁钙钛矿的弹性常数,并讨论了杨氏模量、泊松比、德拜温度、晶体各向异性随压力的变化.     

First-Principles Calculations of Elastic Properties of LaNi5 Compound

The equilibrium lattice constants, temperature dependence of bulk modulus, the pressure dependence of the normalized volume V/V0, elastic constants Cij and bulk modulus of LaNi5 crystal are obtained using the firstprincipies piane-wave pseudopotential method in the GGA-PBE generalized gradient approximation as well as the quasi-harmonic Debye model. We analyse the relationship between bulk modulus and temperature up to 2000 K and obtain the relationship between bulk modulus B and pressure at diFFerent temperatures. It is found that the bulk modulus B increases monotonously with increasing pressure. Moreover, the pressure dependences of Debye temperatures and the pressure derivatives of lattice constants are also successfully obtained. The calculated results are in agreement with the experimental data and the other theoretical results.     

MgSiO_3钙钛矿高温高压特性的第一性原理研究

本文采用第一性原理方法,计算了MgSiO_3钙钛矿在零温和0～150 GPa静水压范围内的晶体结构和弹性模量,并利用准简谐近似Debye模型,拟合三阶Birch-Murnaghan物态方程得到了其高温高压下的热力学性质.通过与现有的理论和实验的结果数据比较,确认在0～2000 K的温度区间内,第一性原理计算结合Debye模型能够较可靠地模拟在下地幔压力范围内MgSiO_3钙钛矿的热力学性质.     

MgSiO3钙钛矿高温高压特性的第一性原理研究

本文采用第一性原理方法,计算了MgSiO3钙钛矿在零温和0 ~ 150 GPa静水压范围内的晶体结构和弹性模量,并利用准简谐近似Debye模型,拟合三阶Birch-Murnaghan物态方程得到了其高温高压下的热力学性质。通过与现有的理论和实验的结果数据比较,确认在0 ~ 2000 K的温度区间内,第一性原理计算结合Debye模型能够较可靠地模拟在下地幔压力范围内MgSiO3钙钛矿的热力学性质。     

Theoretical investigations of the physical properties of tetragonal CaSiO3 perovskite

The first principles study of the physical properties of tetragonal CaSiO₃ perovskite is reported using the pseudopotential plane wave method within the local density approximation. The calculated equilibrium volume is in good agreement with the available experimental data. A complete elastic tensor of the tetragonal CaSiO₃ perovskite is determined in the wide pressure range. The geologically important quantities: sound velocity, Young’s modulus, Poisson’s ratio, and crystal anisotropy, are derived from the calculated data. A systematic decrease of crystal’s anisotropy with pressure is noticed except symmetry plane (001). Finally, by using a quasiharmonic Debye model, the Debye temperature, the heat capacity, the coefficient of thermal expansion, and the Grüneisen parameter are also obtained in the present work.     

Structural Transition of Gd2O3：Eu Induced by High Pressure

The structural transition of bulk and uano-size Gd2O3：Eu are studied by high pressure energy disperse x-ray diffraction （XRD） and high pressure photoluminescence. Our results show that in spite of different size of Gd2O3 particles, the cubic structure turns into a possible hexagonal one above 13.4 GPa. When the pressure is released, the sample reverses to the monoclinic structure. No cubic structure presents in the released samples. That is to say, the compression and relaxation of the sample leads to the cubic Gd2O3：Eu then turns into the monoclinic one.     

The structural, elastic and thermodynamic properties of thorium tetraboride

The structural, elastic and thermodynamic properties of thorium tetraboride (ThB₄) have been investigated by using first-principles plane-wave pseudopotential density functional theory with generalized gradient approximation. The behaviors of structural parameters under 0-70 GPa hydrostatic pressure are studied by means of Broyden, Fletcher, Goldfarb, and Shanno (BFGS) geometry optimization scheme. By using the stress-strain method, single crystal elastic constants are calculated to test the mechanical stability of the crystal structure and to determine mechanical properties such as bulk modulus at each pressure. However, in order to study the thermodynamic properties of ThB₄, the quasi-harmonic Debye model is used. Then, the dependencies of bulk modulus, heat capacities, thermal expansions, Grüneisen parameters and Debye temperatures on the temperature and pressure are obtained in the whole pressure range 0-70 GPa and temperature range 0-1500 K.     

First-Principles Calculations of Elastic Properties of LaNi4.75Sn0.25 Alloys under Pressure

The equilibrium lattice constants, bulk modulus, shear modulus, elastic constants and Debye temperature of LaNi4.75 Sn0.25 under pressure are calculated using the full-potential linearized augmented plane wave （FP-LAPW） method as well as the quasi-harmonic Debye model. The results at zero pressure are in excellent agreement with the experimental data. The Sn atom is found to occupy the equivalent 3g site （0.5a, 0.75b, 0.5c） in the quadruple cell. The Debye temperature of LaNi4.75Sn0.25 is lower than that of LaNi5. The dependences of bulk modulus on finite temperature and on finite pressure are also investigated. The results show that the bulk modulus B increases monotonously as pressure increases.     

FP-LMTO calculations of the structural,elastic, thermodynamic,and electronic properties of the ideal-cubic perovskite BiGaO3

Using the first-principles full-potential linear muffin-tin orbital method within the local density approximation, we have studied the structural, elastic, thermodynamic, and electronic properties of the ideal-cubic perovskite BiGaO₃. It is found that this compound has an indirect band gap. The valence band maximum (VBM) is located at Γ-point, whereas the conduction band minimum (CBM) is located at X-point. The pressure and volume dependences of the energy band gaps have been calculated. The elastic constants at equilibrium are also determined. We derived the bulk and shear moduli, Young’s modulus, and Poisson’s ratio. The thermodynamic properties are predicted through the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. The variation of the bulk modulus, heat capacities, and Debye temperature with pressure and temperature are successfully obtained.     

First-Principles Calculations of Elastic Properties of Cubic Ni2MnGa

Dependence of bulk modulus on both pressure and temperature, the elastic constants Cij and the pressure and temperature dependence of normalized volume V/Vo of cubic Ni2MnGa alloy are successfully obtained using the first-principles plane-wave pseudopotential （PW-PP） method as well as the quasi-harmonic Debye model. We analyse the relationship between bulk modulus and temperature up to 800 K and obtain the relationships between bulk modulus B and pressures at different temperatures. It is found that the bulk modulus B increases monotonically with increasing pressure. Moreover, the temperature dependences of the Debye temperature are also analysed. The calculated results are in agreement with the available experimental data and the previous theoreticM results.     

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 calculations of structural, elastic, electronic and optical properties of the antiperovskite AsNMg3

The density functional theory (DFT) calculations of structural, elastic, electronic and optical properties of the cubic antiperovskite AsNMg₃ has been reported using the pseudo-potential plane wave method (PP-PW) within the generalized gradient approximation (GGA). The equilibrium lattice, bulk modulus and its pressure derivative have been determined. The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk and shear moduli, Young's modulus and Poisson's ratio for ideal polycrystalline AsNMg₃ aggregate. We estimated the Debye temperature of AsNMg₃ from the average sound velocity. This is the first quantitative theoretical prediction of the elastic properties of AsNMg₃ compound, and it still awaits experimental confirmation. Band structure, density of states and pressure coefficients of energy gaps are also given. The fundamental band gap (Γ-Γ) initially increases up to 4 GPa and then decreases as a function of pressure. Furthermore, the dielectric function, optical reflectivity, refractive index, extinction coefficient, and electron energy loss are calculated for radiation up to 30 eV. The all results are compared with the available theoretical and experimental data.     

Theoretical Study of Compressibility and Thermoelasticity of LaNi5-xAlx （x= 0.3, 0.5 and 1.0）

The compressibility, the temperature dependence of bulk modulus, the pressure dependence of normalized volume V/V0, thermal expansion coefficient and Debye temperature of LaNi5-xAlx compounds are successfully obtained using the first-principles plane-wave pseudopotential （PW-PP） method, the EOSFIT6.0 software and the quasiharmonic Debye model. The rapid decrease of relative lattice constant a/a0 shows that the deformation is easier in directions normal to the c-axis than that along it. The relationships between bulk modulus B and pressure at different temperatures are also analysed. It is found that the bulk modulus B increases monotonically with increasing pressure. Moreover, the pressure dependences of thermal expansion and Debye temperature are also successfully obtained. The calculated results are in agreement with the experimental data.     

Preparation of Mg55Ni35Si10 Amorphous Powders by Mechanical Alloying and Consolidation by Vacuum Hot Pressing

Amorphous Mg55Ni35Si10 powders are fabricated by using a mechanical alloying technique. The amorphous powders are found to exhibit a relatively high crystallization temperature of 380℃. The as-milled amorphous Mg55Ni35Si10 powders are consolidated successfully into bulk body by vacuum hot pressing technique. Limited nanocrystallization is noticed. The Vickers microhardness range of the Mg55Ni35Si10 bulk sample is 7834 to 8048 MPa. Its bending strength and compressive strength are 529 MPa and 1466 MPa, respectively.     

High-pressure and high-temperature bulk modulus of cubic fluorite-type MgF2 from quasi-harmonic Debye model

A detailed theoretical study of the isothermal and adiabatic bulk moduli of MgF₂ with a fluorite structure under high pressure and temperature has been carried out by means of first-principles density functional theory calculations combined with the quasi-harmonic Debye model in which the phononic effects are considered. Particular attention is paid to the prediction of the isothermal bulk modulus and its first and second pressure derivatives for the first time. The calculated ground state properties agree well with other theoretical values. At extended pressure and temperature ranges, the variation of the bulk modulus which plays a central role in the formulation of approximate equations of state has also been predicted. The properties of MgF₂ with a fluorite structure are summarized in the pressure range of 0–135 GPa and the temperature up to melting temperature 1500 K.     

Structural and elastic properties of TiN under high pressure

We have investigated the structural and elastic properties of TiN at high pressures by the first-principles plane wave pseudopotential density functional theory method at applied pressures up to 45.4 GPa. The obtained normalized volume dependence of the resulting pressure is in excellent agreement with the experimental data investigated using synchrotron radial x-ray diffraction (RXRD) under nonhydrostatic compression up to 45.4 GPa in a diamond-anvil cell. Three independent elastic constants at zero pressure and high pressure are calculated. From the obtained elastic constants, the bulk modulus, Young's modulus, shear modulus, acoustic velocity and Debye temperature as a function of the applied pressure are also successfully obtained.     

顽火辉石(Mg0.92,Fe0.08)SiO3的冲击相变和高压状态方程及其地球物理意义

 用阻抗匹配法和电探针技术在48~140 GPa冲击压力范围内对化学组分为(Mg_0.92, Fe_0.08)SiO₃、初始密度为3.06 g/cm³的天然顽火辉石进行了冲击压缩实验。根据本工作13发实验数据，结合McQueen等人的数据可以看出，(Mg_0.92, Fe_0.08)SiO₃顽火辉石在冲击压缩过程中，大约经历三个明显区域：低压相区，压力范围为0~40 GPa；混合相区，压力范围为40～67 GPa；高压相区，压力范围为68～140 GPa。在低压相区，D-u关系已由McQueen给出；而在高压相区（68～140 GPa），可由本实验数据得到。由叠加原理计算得到的混合物(Mg_0.92, Fe_0.08)O(Mw)+SiO₂(St)的D-u关系及p-ρ关系曲线明显偏离了实验数据的拟合曲线，从而排除了在高达140 GPa冲击压力下，钙钛矿结构的(Mg_0.92, Fe_0.08)SiO₃发生向氧化物化学分解相变的可能性。对高压相区的实验数据进行拟合，可以得到(Mg_0.92, Fe_0.08)SiO₃钙钛矿的Grüneisen参数γ。通过三阶Birch-Murnaghan有限应变状态方程，由冲击波实验数据得到了零压等熵体积模量K_0S=259.6(9) GPa及其对压力的一阶偏导数K′_0S=4.20(5)，其ρ₀=4.19 g/cm³。(Mg_0.92, Fe_0.08)SiO₃钙钛矿冲击压缩下的密度数据与PREM密度剖面吻合很好，支持钙钛矿为主要成分的下地幔模型。     

Cu_3N弹性和热力学性质的第一性原理研究

利用基于密度泛函理论的第一性原理全势线性缀加平面波方法,研究了立方反ReO3结构Cu3N在零温(0K)零压下的平衡晶格常数、体弹模量及其对压强的一阶导数,计算结果与其他实验及理论结果基本相符.同时得出Cu3N的弹性常数,Poisson比等,并分析出Cu3N在零温零压下是稳定的.通过准谐Debye模型计算Cu3N的热力学性质,得到了Cu3N的晶格常数、等压比热容、等容比热容、热胀系数与温度和压强之间的关系,同时计算出不同温度不同压强下其体弹模量及Debye温度的值。