第一性原理研究Mg2 Si同质异相体的结构、电子结构和弹性性质

在第一性原理框架下,采用赝势平面波方法研究了三种Mg₂Si同质异相体的晶胞结构、电子结构和弹性性质随压强的变化关系.研究发现,反萤石结构Mg₂Si、反氯铅矿结构Mg₂Si和Ni₂In型Mg₂Si分别在压强为0-7 GPa,7.5-20.2 GPa和21.9-40 GPa范围内能够保持结构稳定.计算获得了不同压强下Mg₂Si的弹性常数、体模量、剪切模量、杨氏模量、泊松比和各向异 关键词： 态密度 电子结构 弹性常数 第一性原理     

Au-Sn金属间化合物的第一性原理研究

采用基于密度泛函理论的第一性原理平面波赝势法,计算研究了Au-Sn二元系金属间化合物的生成焓、结合能、电子结构、弹性性质和结构稳定性. 计算结果表明：Au₅Sn合金的生成焓最小,说明Au₅Sn较容易生成,但Au₅Sn在热力学和力学上是不稳定的;AuSn₂和AuSn₄的键合作用较强,弹性模量、剪切模量均大于AuSn和Au₅Sn;从电子结构的角度,AuSn₂和AuSn₄ 的成键主要来自于Au原子d 轨道与Sn原子p轨道的杂化;而AuSn以Sn–Sn键的相互作用为主,Au₅Sn相中Au 的占比较大,导致Au–Au共价键发挥作用,抑制了Sn导带p电子的成键. 关键词： 电子结构 弹性性质 第一性原理 Au-Sn金属间化合物     

Mg8Sn4-xMx结构稳定性与弹性常数的第一性原理计算

利用基于密度泛函理论的CASTEP软件构建Mg₈Sn_4-xM_x(M=Al、Cu;x=0、1或2)晶体结构模型，采用第一性原理计算其晶格常数、结构稳定性和弹性常数，并分析不同量的M原子固溶于Mg₂Sn后体系的电子特性、弹性性能和本征硬度.计算结果表明，M原子能自发固溶于Mg₂Sn相，且所得Mg₈Sn_4-xM_x(x=1或2)晶体结构均可稳定存在；当2个M原子固溶于Mg₂Sn时，使其晶体结构由立方晶系转变为四方晶系.态密度分析表明，M原子固溶后体系原子存在明显轨道杂化现象，表现出较强的共价键，增加M原子固溶量不会改变各原子对态密度的贡献规律，但会提高该原子对电子对态密度的贡献度.弹性性能和本征硬度分析表明：Mg₂Sn中固溶M原子后，体系力学性能仍稳定，增加M原子固溶量，体系硬度逐渐降低，韧塑性不断提高，即M原子固溶量增加能提升体系的...     

Mg2Sn电子结构及热力学性质的第一性原理计算

采用基于第一性原理的赝势平面波方法系统地计算了Mg2Sn基态的电子结构、弹性常数和热力学性质.计算结果表明Mg2Sn的禁带宽度为0.1198eV.运用线性响应方法确定了声子色散关系和态密度,得出Mg2Sn的热力学性质如等容比热和德拜温度.计算Mg2Sn的热导率并与实验数据相比较.     

氧化钒晶体的半导体至金属相变的理论研究

本文利用第一性原理方法研究了金红石相和单斜相VO₂晶体的电子结构和热力学性质.在计算中采用局域密度近似结合Hubbard U模型(LDA+U)描述电子的局域强关联效应,同时也利用微扰密度泛函方法计算了两种相结构的声子谱.计算结果表明V原子3d电子轨道中x²-y²轨道能级分裂决定了VO₂晶体在不同相结构下的金属和绝缘体特性.零温状态方程计算揭示了在68 GPa时可以发生从单斜结构 关键词： 2')" href="#">VO₂ 相变 第一性原理     

高压下固态Kr弹性性质、电子结构和光学性质的第一性原理计算

采用密度泛函理论中平面波基矢,模守恒赝势结合局域密度近似以及广义梯度近似对固态Kr在高压下的结构以及弹性性质进行了研究, 通过计算发现弹性常数,Debye温度以及声速都随压力的增大而增大,所计算的弹性常数与实验和其他的理论符合的很好. 利用Debye模型得到了固态Kr的热力学性质, 熵随压力的增大而减小,随温度升高而升高;而定容热容C_v,定压热容C_p则随温度升高而升高,而且C_v在达到一定温度时趋于定值,所得的热力学性质和实验值是相符的．最后还预测了固态Kr在高压下的电子结构和光学性质, 计算结果表明随压力的增加固态Kr的前沿能带变窄,光吸收系数增大,吸收峰增宽,电子更容易发生跃迁,固态Kr有可能转化为半导体． 关键词： Kr 第一性原理 弹性常数 光学性质     

MAX相Nb2SnC和Nb2SnN的力学和热力学性质的第一性原理计算

利用基于密度泛函理论的第一性原理,在广义梯度近似下研究了MAX相Nb₂SnC和Nb₂SnN的力学、晶格动力学、电子以及热力学性质.通过弹性常数和声子的计算,研究了Nb₂SnC和Nb₂SnN两种结构的力学稳定性和动力学稳定性;通过对Nb₂SnC和Nb₂SnN的力学性质计算,证明了它们均具有较高的体积模量和剪切性,并且说明了Nb₂SnC和Nb₂SnN是具有弹性各向异性的韧性材料.此外,通过计算电子能带结构和态密度,研究了Nb₂SnC和Nb₂SnN的电子性质和成键性质,结果表明,两个化合物均具有金属导电性和较强的共价键,而且Nb₂SnN比Nb₂SnC具有更强的金属导电性.最后利用声子色散曲线预测了热容、自由能、焓和熵等热力学性质,结果标明,计算出的熵、焓和自由能值变化符合热力学第三定律.     

L12型铝合金的结构、弹性和电子性质的第一性原理研究

运用第一性原理方法研究了L1₂型铝合金相Al₃Sc和Al₃Zr的晶体结构、电子结构和弹性.结合能和形成能的计算表明,两种合金具有较强的合金化能力,且Al₃Zr较Al₃Sc具有更强的结构稳定性.电子结构分析表明,费米能级以下较多的价电子数决定了Al₃Zr具有较强的结构稳定性.计算并分析比较了两种合金相的单晶弹性常数（C₁₁,C₁₂和C₄₄）以及多晶弹性模量（体弹性模量B、剪切模量G、杨氏模量Y、泊松比ν和各向异性因子A）.通过对比实验和其他理论计算结果,进一步分析和解释了两种合金相的力学性质. 关键词： 铝合金 第一性原理 结构和电子性质 弹性     

LiGaX2(X=S,Se, Te)的电子结构,光学和弹性性质的第一性原理计算

采用基于第一性原理的密度泛函理论赝势平面波方法,对LiGaX₂(X=S, Se, Te)的能带结构、态密度、光学以及弹性性质进行了理论计算. 能带结构计算表明LiGaS₂ 的禁带宽度为4.146 eV, LiGaSe₂ 的禁带宽度为3.301 eV, LiGaTe₂ 的禁带宽度为2.306 eV; 其价带主要由Ga-4p 层电子和X- np 层电子的能态密度决定; 同时也对LiGaX< 关键词： 电子结构 光学性质 弹性性质 LGX     

Si基外延Ru2Si3电子结构及光学性质研究

采用基于第一性原理的赝势平面波方法,对异质外延关系为Ru₂Si₃ （100）//Si（001）,取向关系为Ru₂Si₃［010］//Si［110］正交相的Ru₂Si₃平衡体系下的能带结构、态密度和光学性质等进行了理论计算.计算结果表明：当晶格常数a取值为1093 nm时,正交相Ru₂Si₃处于稳定状态并且是具有带隙值 关键词： 外延 第一性原理 电子结构 光学性质     

Predictions of mechanical and thermodynamic properties of Mg17Al12 and Mg2Sn from first-principles calculations

Using first-principles calculations, we predict mechanical and thermodynamic properties of both Mg₁₇Al₁₂ and Mg₂Sn precipitates in Mg–Al–Sn alloys. The elastic properties including the polycrystalline bulk modulus, shear modulus, Young’s modulus, Lame’s coefficients and Poisson’s ratio of both Mg₁₇Al₁₂ and Mg₂Sn phases are determined with the Voigt–Reuss–Hill approximation. Our results of equilibrium lattice constants agree closely with previous experimental and other theoretical results. The ductility and brittleness of the two phases are characterized with the estimation from Cauchy pressure and the value of B/G. Mechanical anisotropy is characterized by the anisotropic factors and direction-dependent Young’s modulus. The higher Debye temperature of Mg₁₇Al₁₂ phase means that it has a higher thermal conductivity and strength of chemical bonding relative to Mg₂Sn. The anisotropic sound velocities also indicate the elastic anisotropies of both phase structures. Additionally, density of states and Mulliken population analysis are performed to reveal the bonding nature of both phases. The calculations associated with phonon properties indicate the dynamical stability of both phase structures. The temperature dependences of thermodynamic properties of the two phases are predicted via the quasi-harmonic approximation.     

Structural, vibrational and thermodynamic properties of Mg2 FeH6 complex hydride

Mg₂FeH₆, which has one of the highest hydrogen storage capacities among Mg based 3d-transitional metal hydrides, is considered as an attractive material for hydrogen storage. Within density-functional perturbation theory (DFPT), we have investigated the structural, vibrational and thermodynamic properties of Mg₂FeH₆. The band structure calculation shows that this compound is a semiconductor with a direct X-X energy gap of 1.96 eV. The calculated phonon frequencies for the Raman-active and the infrared-active modes are assigned. The phonon dispersion curves together with the corresponding phonon density of states and longitudinal-transverse optical (LO-TO) splitting are also calculated. Findings are also presented for the temperature-dependent behaviors of some thermodynamic properties such as free energy, internal energy, entropy and heat capacity within the quasi-harmonic approximation based on the calculated phonon density of states.     

Elastic and vibrational properties of Mg2Si1−xSnx alloy from first principles calculations

The structural, elastic and phonon properties of Mg₂Si_1?xSn_x alloy are investigated by performing density functional theory and density functional perturbation theory calculations. The calculated lattice parameter increases with the increase of Sn content obeying Vegard’s Law that is in good agreement with available experimental data. Shear modulus, Young’s modulus and sound velocities are determined from the obtained elastic constants. Phonon dispersion curves show a pronounced softening with increasing of Sn content. The softening mechanism has been discussed based upon the element mass and bond strength. Besides, phonon contribution to the Helmholtz free energy, the entropy and the constant-volume heat capacity are calculated within the harmonic approximation based on the calculated phonon density of states. Results show Mg₂Si_1?xSn_x is thermodynamically more stable with higher Sn content.     

Ab‐initio thermodynamic calculations of Mg2BIV (BIV = Si,Ge, Sn) solid solutions

The thermodynamic properties of ternary Mg₂B^IV (B^IV = Si, Ge, Sn) solid solutions were first calculated by the ab‐initio density functional method. The results showed that there exist composition regions with d²G /dx² < 0 in Mg₂Si_1–x Sn_x and Mg₂Ge_1–x Sn_x systems, implying the possibility of spinodal decomposition of the pseudobinary solid solutions. It is suggested that the spinodal decomposition would be a potential way to obtain Mg₂B^IV based bulk in‐situ nanocomposites with reduced grain sizes and enhanced phonon scattering, and hence an improved thermoelectric figure of merit. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Elastic constants and thermodynamic properties of Mg2SixSn1-x from first-principles calculations

<正>This paper stuides the elastic constants and some thermodynamic properties of Mg₂Si_xSn_n-1（x=0,0.25,0.5, 0.75,1） compounds by first-principles total energy calculations using the pseudo-potential plane-waves approach based on density functional theory,within the generalized gradient approximation for the exchange and correlation potential. The elastic constants of Mg₂Si_xSn_n-1 were calculated.It shows that,at 273 K,the elastic constants of Mg₂Si and Mg₂Sn are well consistent with previous experimental data.The isotropy decreases with increasing Sn content.The dependences of the elastic constants,the bulk modulus,the shear modulus and the Debye temperatures of Mg₂Si and Mg₂Si_0.5Sn_0.5 on pressure were discussed.Through the quasi-harmonic Debye model,in which phononic effects were considered,the specific heat capacities of Mg₂Si_xSn_1-x at constant volume and constant pressure were calculated.The calculated specific heat capacities are well consistent with the previous experimental data.     

High-pressure phase transitions of Mg2Ge and Mg2Sn: First-principles calculations

Phase transitions of the anti-fluorite compounds Mg₂Ge and Mg₂Sn under high pressure were investigated using the first-principles plane-wave method within the pseudopotential and generalized gradient approximations. The calculated results show that Mg₂Ge and Mg₂Sn undergo two first-order phase transitions at high pressure and the sequence of the pressure-induced phase transitions is from the anti-fluorite to the anti-cotunnite, and then to the Ni₂In-type structure. The high pressure behaviors of Mg₂Ge and Mg₂Sn are similar to Mg₂Si and the isostructural alkali-metal oxide Li₂O. Moreover, the electronic and optical properties of both the anti-fluorite and the high-pressure phases are presented.     

LiNH2 的晶格动力学、介电性质和热力学性质第一性原理研究

采用基于密度泛函理论的平面波赝势方法,在局域密度近似下采用线性响应的密度泛函微扰理论计算了LiNH₂的晶格动力学、介电性质和热力学性质,得到了布里渊区高对称方向上的声子色散曲线和相应的声子态密度,分析了 LiNH₂的红外和拉曼活性声子频率,同时给出它的介电张量和玻恩有效电荷张量. 研究表明,LiNH₂存在小的各向异性,计算所得结果与实验值和其他理论值符合较好.最后,利用得到的声子态密度进一步预测了LiNH₂的热力学性质 关键词： 密度泛函理论 晶格动力学 热力学性质 第一性原理计算     

Thermodynamics of phonon-stabilized Fermi distributions with application to uranium

Since phonons are built on the free energy of electrons, their frequencies can be altered by thermal electronic excitations, implying that thermal electronic excitations can alter the phonon entropy. The effect of this extra phonon entropy on electronic distribution functions and thermodynamic properties is calculated in the limit of classical vibrations. The phonon entropy stabilizes electrons above the Fermi level by more than the usual k _B T. The thermodynamic coupling of electron and phonon degrees of freedom allows far more heat capacity than in equivalent independent systems. The method developed is used to explain uranium data from the literature.     

First principles lattice dynamical study of the cubic antiperovskite compounds AsNBa3 and SbNBa3

An ab initio pseudopotential plane wave method using linear response approach has been employed to study the lattice dynamics of two cubic antiperovskites AsNBa₃ and SbNBa₃. The bulk properties, elastic constants, phonon dispersion curves, phonon density of states and temperature dependent thermodynamic quantities of both antiperovskites are obtained. The calculated lattice constants, elastic and bulk properties are compared with the available theoretical data. This is the first systematic and quantitative prediction of phonon and thermodynamical properties of these antiperovskite compounds.     

Raman scattering in Mg2Sn and Mg2Si

The Raman spectra of single crystal Mg₂Sn and Mg₂Si have been studied at room temperature. In Mg₂Sn we observe a single relatively strong line produced by the degenerate Raman active F_2g modes, seen previously by neutron inelastic scattering. Two weaker peaks are seen, one occuring at the F_2g overtone; the assignment for the other is uncertain. In Mg₂Si we find a strong line having a shift comparable to the value calculated by previous workers.