First-principles investigation on the elastic stability and thermodynamic properties of Ti₂SC

Using Vanderbilt-type plane-wave ultrasoft pseudopotentials within the generalized gradient approximation(GGA) in the frame of density functional theory(DFT),we have investigated the crystal structures,elastic,and thermodynamic properties for Ti2SC under high temperature and high pressure.The calculated pressure dependence of the lattice volume is in excellent agreement with the experimental results.The calculated structural parameter of the Ti atom experienced a subtle increase with applied pressures and the increase suspended under higher pressures.The elastic constants calculations demonstrated that the crystal lattice is still stable up to 200 GPa.Investigations on the elastic properties show that the c axis is stiffer than the a axis,which is consistent with the larger longitudinal elastic constants(C 33,C 11) relative to transverse ones(C 44,C 12,C 13).Study on Poisson’s ratio confirmed that the higher ionic or weaker covalent contribution in intra-atomic bonding for Ti2SC should be assumed and the nature of ionic increased with pressure.The ratio(B/G) of bulk(B) and shear(G) moduli as well as B/C 44 demonstrated the brittleness of Ti2SC at ambient conditions and the brittleness decreased with pressure.Moreover,the isothermal and adiabatic bulk moduli displayed opposite temperature dependence under different pressures.Again,we observed that the Debye temperature and Gru¨neisen parameter show weak temperature dependence relative to the thermal expansion coefficient,entropy,and heat capacity,from which the pressure effects are clearly seen.     

First-principles calculation of the lattice compressibility, elastic anisotropy and thermodynamic stability of V2GeC

We investigate the elastic and the thermodynamic properties of nanolaminate V₂GeC by using the ab initio pseudopotential total energy method. The axial compressibility shows that the c axis is always stiffer than the a axis. The elastic constant calculations demonstrate that the structural stability is within 0-800 GPa. The calculations of Young's and shear moduli reveal the softening behaviour at about 300 GPa. The Possion ratio makes a higher ionic or a weaker covalent contribution to intra-atomic bonding and the degree of ionicity increases with pressure. The relationship between brittleness and ductility shows that V₂GeC is brittle in ambient conditions and the brittleness decreases and ductility increases with pressure. Moveover, we find that V₂GeC is largely isotropic in compression and in shear, and the degree of isotropy decreases with pressure. The Gr黱eisen parameter, the Debye temperature and the thermal expansion coefficient are also successfully obtained for the first time.     

Cr2MC(M=Al, Ga)的电子结构、弹性和热力学性质的第一性原理研究

本文使用第一性原理的GGA/RPBE方法研究了Cr₂MC(M=Al, Ga)的电子结构、弹性和热力学性质. 研究发现两个化合物的体积压缩性几乎相同, 并且证实了在0—50 GPa范围内c轴始终较a轴更难以压缩并且结构始终是稳定的. 通过对内坐标的研究发现了Cr₂AlC中Cr离子的内坐标始终大于Cr₂GaC中Cr离子的内坐标. 使用准谐德拜模型得到的体弹模量在0 GPa下随着温度的升高而减小, 而在300 GPa下则随着温度的升高而增大. 对德拜温度的研究发现Cr₂GaC的值小于Cr₂AlC的值, 而对热膨胀系数、Grüneisen参数、熵和热容的计算发现Cr₂GaC的值大于Cr₂AlC的值. 对电子结构的分析发现Cr₂GaC的s和p电子在费米能级处的值大于Cr₂AlC的s和p电子的值, 而其他离子的电子分布几乎一致.     

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。     

First-principles calculation of elastic and thermodynamic properties of Ni2MnGa Heusler alloy

The equilibrium lattice parameter, heat capacity, thermal expansion coefficient and bulk modulus of Ni 2 MnGa Heusler 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 B and temperature T up to 800 K and obtain the relationship between bulk modulus B and pressure at different temperatures. It is found that the bulk modulus B increases monotonically with increasing pressure and decreases with increasing temperature. The pressure dependence of heat capacity C v and thermal expansion α at various temperatures are also analysed. Finally, the Debye temperature of Ni 2 MnGa is determined from the non-equilibrium Gibbs function. Our calculated results are in excellent agreement with the experimental data.     

First-principles study on electronic structure and elastic properties of Ti2SC

We have performed first-principles study on electronic structure and elastic properties of Ti₂SC. The absence of band gap at the Fermi level and the finite value of the density of states at the Fermi energy reveal the metallic behavior of this compound. The five independent elastic constants were derived and the bulk modulus, Young's modulus, shear modulus, and Poisson's ratio were determined. The high bulk modulus and hardness was found to be originated from the strong Ti 3d-S 2p hybridization. Such strong MA bonding is unusual in the MAX phases studied so far. Ti₂SC is elastically stable and exhibits highly elastic isotropy.     

First-principles calculation of the lattice compressibility, elastic anisotropy and thermodynamic stability of V2GeC

We investigate the elastic and the thermodynamic properties of nanolaminate V2GeC by using the ab initio pseudopotential total energy method.The axial compressibility shows that the c axis is always stiffer than the a axis.The elastic constant calculations demonstrate that the structural stability is within 0-800 GPa.The calculations of Young’s and shear moduli reveal the softening behaviour at about 300 GPa.The Possion ratio makes a higher ionic or a weaker covalent contribution to intra-atomic bonding and the degree of ionicity increases with pressure.The relationship between brittleness and ductility shows that V2GeC is brittle in ambient conditions and the brittleness decreases and ductility increases with pressure.Moveover,we find that V2GeC is largely isotropic in compression and in shear,and the degree of isotropy decreases with pressure.The Grüneisen parameter,the Debye temperature and the thermal expansion coefficient are also successfully obtained for the first time.     

Structural, elastic and electronic properties of a new ternary-layered Ti2SiN

The structural, elastic and electronic properties of Ti₂SiN were studied by first-principle calculations. The calculated bond lengths of Ti-Si and Ti-C are 2.65 and 2.09 Å, respectively. The results show Ti₂SiN is mechanically stable, and its bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio μ and anisotropy factor A are determined to be 182 GPa, 118 GPa, 291 GPa, 0.233 and 1.57, respectively. The calculated electronic structure indicates that Ti₂SiN is anisotropic and conductive.     

Effect of hydrogen-doping on bonding properties of Ti3SiC2

Using the first principles method based on the density functional theory, we investigated the effect of hydrogen-doping on bonding properties of Ti₃SiC₂. The formation energies of hydrogen interstitials in three possible positions were calculated. The results show that hydrogen favors residing near the (0 0 1) Si plane. In these positions, hydrogen is hybridized most with 1s states of lattice atoms (Si and C), instead of Ti. The presence of hydrogen does not substantially influence the bonding nature of Ti₃SiC₂; chemical bonding is characterized by the hybridizations of Ti d-Si p and Ti d-C p states, and yields high strength. This is contrary to hydrogen-doping in transition metals, where the electron of hydrogen fills in the d bands of the metals and, as a consequence, decreases the cohesive strength of the lattice.     

N掺杂锐钛矿TiO2光学性能的第一性原理研究

用平面波赝势方法(PWP)计算了N掺杂锐钛矿型TiO₂前后的光学特性,即介电函数虚部ε₂(ω)，光学吸收系数I(ω)和反射率R(ω). 并从能带结构上解释了为什么掺N后锐钛矿型TiO₂的光学谱在2.93，3.56和3.97eV处相对掺杂前会出现3个峰值的原因. 从光谱图上分析得出，掺杂后TiO₂要发生红移现象，实验现象证实了这一结果. 关键词： N掺杂 2')" href="#">锐钛矿型TiO₂ 光学性能 第一性原理     

First-principles study of structural,elastic, electronic and thermodynamic properties of topological insulator Bi2Se3 under pressure

The structural, elastic, electronic and thermodynamic properties of the rhombohedral topological insulator Bi₂Se₃ are investigated by the generalized gradient approximation (GGA) with the Wu–Cohen (WC) exchange-correlation functional. The calculated lattice constants agree well with the available experimental and other theoretical data. Our GGA calculations indicate that Bi₂Se₃ is a 3D topological insulator with a band gap of 0.287 eV, which are well consistent with the experimental value of 0.3 eV. The pressure dependence of the elastic constants C_ij, bulk modulus B, shear modulus G, Young’s modulus E, and Poisson’s ratio σ of Bi₂Se₃ are also obtained successfully. The bulk modulus obtained from elastic constants is 53.5 GPa, which agrees well with the experimental value of 53 GPa. We also investigate the shear sound velocity V_S, longitudinal sound velocity V_L, and Debye temperature Θ_E from our elastic constants, as well as the thermodynamic properties from quasi-harmonic Debye model. We obtain that the heat capacity C_v and the thermal expansion coefficient α at 0 GPa and 300 K are 120.78 J mol^?1 K^?1 and 4.70 × 10^?5 K^?1, respectively.     

高压下Ti_2AlX(X=C,N)的结构、力学性能及热力学性质

采用基于密度泛函理论的第一性原理方法,计算研究了压力对Ti_2AlC与Ti_2AlN结构、力学性能的影响.研究发现压力的增大会使体系的体积比降低,Ti_2AlC压缩性较Ti_2AlN好.力学性能研究发现,压力的增大使材料抵抗变形能力增强,体系的延展性有了很大的提升,当压力超过40 GPa后,Ti_2AlC与Ti_2AlN从脆性材料转变为延性材料,体模量与剪切模量的比值达到1.75,延展性有了很大的提升.通过准谐德拜模型,分析了压力与温度对Ti_2AlC与Ti_2AlN体模量、热容及热膨胀系数的影响.结果表明,随着温度的升高,Ti_2AlN与Ti_2AlC的体模量下降.定容热容与定压热容的变化趋势相同,但在高温下,定容热容遵循Dulong-Petit极限,温度对热容的影响效果较压力明显.温度与压力对Ti_2AlN与Ti_2AlC线膨胀系数的影响主要发生在低温区域.     

高压下TiAl3结构及热动力学性质的第一性原理研究

采用平面波赝势密度泛函理论研究了钛铝系金属间化合物TiAl₃的结构性质, 计算值与实验值及其他理论值相符合. 通过准谐德拜模型研究了TiAl₃的热动力学性质, 计算得到了相对体积(V/V₀)与压强和温度的关系, 以及不同温度和压强下的热膨胀系数和热容. 与TiAl的计算结果进行对比, 发现随着温度的升高, TiAl的热膨胀系数增大的速度高于TiAl₃, 且随着压强的增大温度效应减弱; TiAl₃的热容值近似为TiAl的热容值的2倍. 关键词： 结构性质 热动力学性质 第一性原理 高压     

First-principles study of elastic and thermo-physical properties of kesterite-type Cu2ZnSnS4

The lattice constants and elastic constants of the kesterite-type Cu₂ZnSnS₄ have been calculated using density-functional theory (DFT). The calculated lattice constants are in good agreement with the experimental data. The calculated elastic constants indicate that the bonding strength along the [1 0 0] and [0 1 0] directions is as strong as the one along the [0 0 1] direction. The high B/G ratio shows that the kesterite-type Cu₂ZnSnS₄ compound has ductile behavior. Finally, using the Debye model, the volume, bulk modulus and heat capacity as a function of temperature for the kesterite-type CZTS have been estimated at different pressures. The Debye temperature and Gruneisen parameter are 157 K and 2.28 at 300 K temperature, respectively. The present results can give some information for the design of the kesterite-type CZTS compounds, and these can also be used to stimulate future experimental and theoretical work.     

First-principles investigations on elastic, phonon and thermodynamic properties of SrB6 under pressure

The elastic, phonon and thermodynamic properties of the divalent alkaline-earth hexaboride SrB₆ are investigated by using plane-wave pseudopotential density functional theory method. The calculated structure parameters and bulk modulus are well consistent with the available experiment and theoretical data. The pressure dependences of elastic constants C_ij, bulk modulus B₀, shear modulus G, Young's modulus E and Poisson's ratio σ are also presented. With these elastic parameters, we investigate the mechanical stability and compressibility of SrB₆. For the thermodynamic properties, both phonon and quasi-harmonic Debye model methods are adopted. Through the comparison with experimental and other theoretical results, we found the method of quasi-harmonic Debye model is a little better. Moreover, the phonon dispersion relations are also obtained. It is found that there are two LO/TO splitting around 5 THz and 26 THz, respectively.     

BC5力学性质的第一性原理计算

采用平面波赝势密度泛函理论方法对0—60 GPa静水压下BC₅ 六角晶系P3m1和四方晶系I4m2结构的平衡态晶格常数、弹性常数、各向异性以及泊松比与Cauchy扰动进行了研究. 研究结果表明, BC₅的两种结构在高压下是稳定的, 且不可压缩性随着压强的增加而增大. 另外, 对其电子结构也进行了计算, 计算结果表明, BC₅存在一个较宽的带隙, 两种原子间有较强的共价杂化, 材料的性质主要由B的2p¹和C的2p²态电子共同决定. 压强对材料带隙和费米能级附近的态密度几乎没有影响, 只引起微小的漂移, 可推断其很好的高压稳定性.     

Ab initio study of phase stability,thermodynamic and elastic properties of C3N2 derived from cubic C20

In this work, we report a quite different conclusion from Tian et al. [Phys. Rev. B 78 (2008) 235431]. It is proved that β-C₃N₂ is the only phase under high pressure, and α-C₃N₂ does not exist. β-C₃N₂ is a covalent crystal composed of strong CC and CN covalent bonds. Band gap of β-C₃N₂ increases with pressure. The width of antibonding state, shown in partial density of states (PDOS), keeps about 5 eV with rising pressures, which brings stable CN or CC covalent bonds. At sufficiently low temperatures, heat capacity (C_v) is proportional to T³; and at intermediate temperatures, C_v is governed by the details of vibrations of the atoms; finally, C_v reaches to β-C₃N₂'s Dulong–Pettit limit (about 120 J/mol K). Though thermal expansion coefficient (α) increases with temperature, α is less than 1×10⁻⁵ K⁻¹. Elastic constants rise with pressure, but shear moduli is quite steady which increases just a little with pressures.     

高压下ErNi2B2C弹性性质、电子结构和热力学性质的第一性原理研究

采用密度泛函理论中的赝势平面波方法研究了高压下超导材料 ErNi₂B₂C 的弹性性质、电子结构和热力学性质.分析表明, 弹性常数、体弹模量、剪切模量、杨氏模量和弹性各向异性因子的外压力效应明显. 电子态密度(DOS)的计算结果显示, 在费米能级(E_F)处的 DOS 峰随外界压强的增大显著降低, 由于 ErNi₂B₂C 相对较高的超导温度(T_c)起因于E_F处的 DOS 峰, 因此推测压强增大可能会降低 ErNi₂B₂C 的 T_c.类似的现象在超导材料 MgB₂和 SrAlSi 中已被发现.此外, 基于准谐德拜模型, 对 ErNi₂B₂C 在高温高压下的热力学性质的研究表明, 在一定范围内, 温度和压强将对其热膨胀系数和热容产生明显的影响. 关键词： 高压 弹性性质 电子结构 热力学性质     

First-principles study of the elastic and thermodynamic properties of thorium hydrides at high pressure

The high pressure behaviors of Th_4H_(15) and ThH_2 are investigated by using the first-principles calculations based on the density functional theory(DFT). From the energy–volume relations, the bct phase of ThH_2 is more stable than the fcc phase at ambient conditions. At high pressure, the bct ThH_2 and bcc Th_4H(15) phases are more brittle than they are at ambient pressure from the calculated elastic constants and the Poisson ratio. The thermodynamic stability of the bct phase ThH_2 is determined from the calculated phonon dispersion. In the pressure domain of interest, the phonon dispersions of bcc Th_4H(15) and bct ThH_2 are positive, indicating the dynamical stability of these two phases, while the fcc ThH_2 is unstable. The thermodynamic properties including the lattice vibration energy, entropy, and specific heat are predicted for these stable phases. The vibrational free energy decreases with the increase of the temperature, and the entropy and the heat capacity are proportional to the temperature and inversely proportional to the pressure. As the pressure increases, the resistance to the external pressure is strengthened for Th_4H_(15) and ThH_2.     

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