NbSi2奇异高压相及其热力学性质的第一性原理研究

采用基于粒子群优化算法的结构预测程序CALYPSO, 并结合第一性原理的VASP程序, 在175 GPa发现NbSi₂的奇异立方高压相. 在此结构中, Nb原子形成金刚石结构, 而Si原子则形成正四面体镶嵌在金刚石结构中. 声子谱计算结果表明该结构是动力学稳定的. 电子结构分析表明, 六角相和立方相NbSi₂均为金属, 对金属性贡献较大的是Nb原子, 而且Nb和Si原子之间存在明显的p-d杂化现象, 电荷更多地聚集在Si四面体中. 利用“应力应变”方法, 计算了NbSi₂的弹性常数, 分析了其体积模量、剪切模量、杨氏模量和德拜温度等热动力学性质随压力的变化并进行了详细的讨论. 根据剪切模量和体积模量的比值分析了NbSi₂两种相结构的脆性和延展性, 发现压力会导致六角相NbSi₂的延展性增加, 但对立方相结构的延展性影响较小; 采用经验算法计算了NbSi₂两种相结构硬度变化情况, 结合这一比值进行了详细的分析. 弹性各向异性计算结果表明, 随着压力增加, 六角结构的各向异性增强, 而立方结构的各向异性减小.     

Anisotropy of mechanical and thermal properties of perovskite LaYbO3: first-principles calculations

LaYbO₃ ceramic material with a perovskite structure has the advantages of a high melting point, sintering resistance and high-temperature phase stability. It is a promising candidate for a structurally and functionally integrated material. However, the anisotropy of physical properties of LaYbO₃ has rarely been studied. Herein, the anisotropy of the mechanical and thermal properties of LaYbO₃ was studied by first-principles calculations. The elastic coefficients, Young’s modulus, Poisson’s ratio and minimum thermal conductivity of LaYbO₃ were found to exhibit anisotropic characteristics. In particular, the sound velocity of the longitudinal wave was nearly twice that of the transverse wave. Especially, the minimum thermal conductivity of LaYbO₃ at high temperature was found to be as low as 0.88?W/m?K, indicating that the compound has potential for use in thermal insulation applications.     

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.     

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. 关键词： 第一性原理 电子结构 弹性常数 热力学性质     

β-PtO2: Phononic,thermodynamic, and elastic properties derived from first-principles calculations

β-PtO₂ is a useful transition metal dioxide, but its fundamental thermodynamic and elastic properties remain unexplored. Using first-principles calculations, we systematically studied the structure, phonon, thermodynamic and elastic properties of β-PtO₂. The lattice dynamics and structural stability of β-PtO₂ under pressure were studied using the phonon spectra and vibrational density of states. The vibrational frequencies of the optical modes of β-PtO₂ increase with elevating pressure; this result is comparable with the available experimental data. Then, the heat capacities and their pressure responses were determined based on the phonon calculations. The pressure dependence of the Debye temperature was studied, and the results were compared in two distinct aspects. The elastic moduli of β-PtO₂ were estimated through the Voigt–Reuss–Hill approximation. The bulk modulus of β-PtO₂ increases linearly with pressure, but the shear modulus is nearly independent of pressure. Our study revealed that the elastic stiffness coefficients C₄₄, C₅₅ and C₆₆ play a primary role in the slow variation of the shear modulus.     

Nb掺杂对Mo2FeB2弹性、硬度和电子结构影响的第一性原理计算

Mo₂FeB₂具有耐高温、耐磨、高强度,是一种良好的硼基金属陶瓷材料,在模具领域有很广阔的应用前景.本文采用第一性原理计算的方法,研究了Nb元素掺杂Mo₂FeB₂合金的结构稳定性、弹性、硬度和电子结构.结合能和生成焓的计算结果表明,Nb在Mo₂FeB₂中更容易占据Fe位置,并且在Fe位掺Nb的Mo₂FeB₂比在Mo位处掺Nb具有更好的力学性能.此外,计算结果还表明,Nb掺杂可以提高Mo₂FeB₂的剪切模量、杨氏模量、体积模量和硬度,但塑性略有下降,合适的掺杂浓度应为2.5 at.%.电子结构计算结果表明,Nb掺杂Mo₂FeB₂力学性能的提高可归因于Nb-B共价键的形成.     

A first-principles study on structural stability and mechanical properties of polar intermetallic phases CaZn2 and SrZn2

Structural stability and electronic properties of polar intermetallic CaZn₂ and SrZn₂ in both CeCu₂-type and MgZn₂-type structures have been investigated using first-principles method. The calculated equilibrium lattice parameters agree closely with the available experimental and other theoretical results. In terms of formation enthalpy, it is discovered that the present compounds with CeCu₂-type structure are energetically more stable than that with MgZn₂-type. They are all mechanically stable according to the criteria of elastic stability. In particular, we have investigated the pressure effect on the compressive behaviour and structural stability of each compound. Subsequently, the bulk modulus, shear modulus, Young’s modulus, theoretical hardness, Poisson’s ratio and Debye temperature in the ground state can be estimated using Voigt–Reuss–Hill homogenization method. Mechanical anisotropy is characterized by the anisotropic factors and direction-dependent Young’s modulus. Finally, the electronic structures are determined to reveal the bonding characteristics of considered phases.     

PtN2的结构和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计算了PtN₂的坐标、平衡态的晶格常数、体弹模量、剪切模量和弹性常数,计算结果与已有的实验值和理论值符合较好.通过能量与体积曲线,可知ST_AA结构比黄铁矿结构具有更低的能量.根据计算结果和Pugh提出的经验判据,PtN₂是易脆的硬物质,随着压强增加PtN₂的脆性逐渐过渡到延性.两种结构的能带结构和态密度表明了黄铁矿结构的PtN₂是半导体而ST_AA 关键词： 2')" href="#">PtN₂ 第一性原理 力学性质     

Elastic and thermodynamic properties in CdO at high pressures from first-principles calculations

In this study, we report first-principles calculations of the elastic and thermodynamic properties for CdO in both the B1 (rocksalt) phase and B2 (cesium chloride) phase. The calculations are performed within the framework of density functional theory, using the pseudopotential plane-wave method. From the theoretical results, we find that the high pressure structural phase transition of CdO from B1 structure to B2 structure is 90.31 GPa. The calculated values are, generally speaking, in good agreement with experiments and with similar theoretical calculations. According to the quasi-harmonic Debye model, we investigate the sound velocity and Debye temperature of CdO under pressures in the range of 0<P<150 GPa.     

Lattice stabilities,mechanical and thermodynamic properties of Al_3Tm and Al_3Lu intermetallics under high pressure from first-principles calculations

The effects of high pressure on lattice stability, mechanical and thermodynamic properties of L1_2 structure Al_3Tm and Al_3Lu are studied by first-principles calculations within the VASP code. The phonon dispersion curves and density of phonon states are calculated by using the PHONONPY code. Our results agree well with the available experimental and theoretical values. The vibrational properties indicate that Al_3Tm and A_3Lu keep their dynamical stabilities in L1_2 structure up to 100 GPa. The elastic properties and Debye temperatures for Al_3Tm and Al_3 Lu increase with the increase of pressure. The mechanical anisotropic properties are discussed by using anisotropic indices AG, AU, AZ, and the threedimensional(3D) curved surface of Young's modulus. The calculated results show that Al_3Tm and Al_3Lu are both isotropic at 0 GPa and anisotropic under high pressure. In the present work, the sound velocities in different directions for Al_3Tm and Al_3Lu are also predicted under high pressure. We also calculate the thermodynamic properties and provide the relationships between thermal parameters and temperature/pressure. These results can provide theoretical support for further experimental work and industrial applications.     

Thermodynamics and elastic properties of Ta from first-principles calculations

<正>Within the framework of the quasiharmonic approximation,the thermodynamics and elastic properties of Ta, including phonon density of states(DOS),equation of state,linear thermal expansion coefficient,entropy,enthalpy, heat capacity,elastic constants,bulk modulus,shear modulus,Young’s modulus,microhardness,and sound velocity, are studied using the first-principles projector-augmented wave method.The vibrational contribution to Helmholtz free energy is evaluated from the first-principles phonon DOS and the Debye model.The thermal electronic contribution to Helmholtz free energy is estimated from the integration over the electronic DOS.By comparing the experimental results with the calculation results from the first-principles and the Debye model,it is found that the thermodynamic properties of Ta are depicted well by the first-principles.The elastic properties of Ta from the first-principles are consistent with the available experimental data.     

Mechanical stability and thermodynamic properties of OsC from first-principles calculations

The elastic and thermodynamic characteristics of OsC crystal have been predicted through a method of density functional theory within the generalized gradient approximation (GGA). Compared with WC-type OsC, NaCl-type OsC is not only energy unfavorable but also mechanics unstable. The five independent elastic constants (C_ij), bulk modulus (B₀), the dependence of bulk modulus on temperature and pressure as well as the thermal expansion coefficient (α_V) at various temperatures for WC-type OsC are discussed. According to our calculations, WC-type OsC should be an ultra-incompressible material with high bulk modulus about 381 GPa. In addition, the bulk modulus will increase with increasing pressure while decrease with increasing temperature. The researches on the thermal expansion coefficient indicate that there will be a knee point during the process of thermal expansion coefficient variation versus increasing temperature. Our results may provide useful information for theoretical and experimental investigation of OsC.     

Magnetic properties of Co-doped SnO: first-principles calculations

Based on density functional theory calculations,the electronic and magnetic properties of Co-doped SnO are investigated.It is found that the spin-polarized state,with a magnetic moment of about 1.0 μ B per Co-dopant,is more favorable in energy than the non-spin-polarized state.Moreover,the origin of the ferromagnetism in Co-doped SnO is found to be the double exchange mechanism.Our results indicate that Co-doped SnO is a possible candidate of the p-type spintronics material.     

Prediction of scandium tetraboride from first-principles calculations:Crystal structures,phase stability,mechanical properties,and hardness

Using the evolutionary methodology for crystal structure prediction, we have predicted the orthorhombic Cmcm and Pnma phases for ScB_4. The earlier proposed Cr B_4~-, Fe B_4~-, Mn B_4~-, and Re P_4~- type structures for ScB_4 are excluded.It is first discovered that the Cmcm phase transforms to the Pnma phase at about 18 GPa. Moreover, both phases are dynamically and mechanically stable. The large bulk modulus, shear modulus, and Young's modulus of the two phases make it an optimistic low compressible material. Moreover, the strong covalent bonding nature of ScB_4 is confirmed by the ELF analysis. The strong covalent bonding contributes greatly to its stability.     

Elastic and thermodynamic properties of c-BN from first-principles calculations

The elastic constants and thermodynamic properties of c-BN are calculated using the first-principles plane wave method with the relativistic analytic pseudopotential of the Hartwigen, Goedecker and Hutter (HGH) type in the frame of local density approximation and using the quasi-harmonic Debye model, separately. Moreover, the dependences of the normalized volume V/V₀ on pressure P, as well as the bulk modulus B, the thermal expansion α, and the heat capacity C_V on pressure P and temperature T are also successfully obtained.     

Structural and thermodynamic properties of Os from first-principles calculations

We investigate the structural, thermodynamic and electronic properties of Os by plane-wave pseudopotential density functional theory method. The obtained lattice constants, bulk modulus and cell volumes per formula unit are well consistent with the available experimental data. Especially, from our calculated bulk modulus, we conclude that Os is more compressible than diamond. Moreover, the temperature induced phase transition of Os from HCP structure to FCC structure has been obtained. It is found that the transition temperature of Os at zero pressure is 2702 K. However no transition pressure is found in our calculations. The effect of bulk modulus B as well as other thermodynamic properties of Os (including the thermal expansion α and the Grüneisen constant γ) on temperatures have also been studied. Our calculated thermal expansion α=1.510×10⁻⁵ K⁻¹ and the Grüneisen constant γ=2.227 for HCP structure at room temperature agree very well with the experimental data. The density of states for HCP structure at 0 K and FCC structure at transition temperature 2702 K are also investigated in our work.     

First-principles calculations of optical properties of Mg2Pb

Based on the density functional theory(DFT),the electronic structures and optical properties of Mg2Pb are calculated by using the local density approximation(LDA)and plane wave pseudo-potential method.The calculation results show that the indirect band gap width of Mg2Pb is 0.02796 eV.The optical properties of Mg2Pb have isotropic characteristics,the static dielectric function of Mg2Pb is1(0)=10.33 and the refractive index is n0=3.5075.The maximum absorption coefficient is 4.8060×105cm 1.The absorption in the photon energy range of 25–40 eV approaches to zero,shows the optical colorless and transparent behaviors.     

Effects of chromium on structure and mechanical properties of vanadium:A first-principles study

Stability and diffusion of chromium （Cr） in vanadium （V）, the interaction of Cr with vacancies, and the ideal me- chanical properties of V are investigated by first-principles calculations. A single Cr atom is energetically favored in the substitution site. Vacancy plays a key role in the trapping of Cr in V. A very strong binding exists between a single Cr atom and the vacancy with a binding energy of 5.03 eV. The first-principles computational tensile test （FPCTT） shows that the ideal tensile strength is 19.1 GPa at the strain of 18% along the [100] direction for the ideal V single crystal, while it decreases to 16.4 GPa at a strain of 12% when one impurity Cr atom is introduced in a 128-atom V supercell. For shear deformation along the most preferable { 110} （111） slip system in V, we found that one substitutional Cr atom can decrease the cleavage energy （7cl） and simultaneously increase the unstable stacking fault energy （]＇us） in comparison with the ideal V case. The reduced ratio of ]＇cl/]＇us in comparison with pure V suggests that the presence of Cr can decrease the ductility of V.     

Stability,electronic structures,and mechanical properties of Fe–Mn–Al system from first-principles calculations

The stability, electronic structures, and mechanical properties of the Fe–Mn–Al system were determined by firstprinciples calculations. The formation enthalpy and cohesive energy of these Fe–Mn–Al alloys are negative and show that the alloys are thermodynamically stable. Fe_3Al, with the lowest formation enthalpy, is the most stable compound in the Fe–Mn–Al system. The partial density of states, total density of states, and electron density distribution maps of the Fe–Mn–Al alloys were analyzed. The bonding characteristics of these Fe–Mn–Al alloys are mainly combinations of covalent bonding and metallic bonds. The stress-strain method and Voigt–Reuss–Hill approximation were used to calculate the elastic constants and moduli, respectively. Fe_(2.5)Mn_(0.5)Al has the highest bulk modulus, 234.5 GPa. Fe_(1.5)Mn_(1.5)Al has the highest shear modulus and Young's modulus, with values of 98.8 GPa and 259.2 GPa, respectively. These Fe–Mn–Al alloys display disparate anisotropies due to the calculated different shape of the three-dimensional curved surface of the Young's modulus and anisotropic index. Moreover, the anisotropic sound velocities and Debye temperatures of these Fe–Mn–Al alloys were explored.     

Gd掺杂ZnO纳米线磁耦合性质的第一性原理研究

本文利用基于密度泛函理论的第一性原理方法计算了钆(Gd)掺杂氧化锌(ZnO)纳米线的磁耦合特性. 讨论了两个Gd原子替换ZnO纳米线中不同位置Zn原子的各种可能情况. 计算发现, ZnO中掺杂的Gd原子处于相邻的位置时它们之间的相互作用是铁磁性的, 并且体系的铁磁性可以通过注入合适数目的电子来得到加强. 同时发现Gd掺杂ZnO纳米线后s-f耦合作用变得显著, 使得体系的铁磁性变得更加稳定, 这也是Gd掺杂ZnO纳米线呈现铁磁性的原因. 这些结果为实验上发现的Gd掺杂ZnO纳米线呈铁磁性提供了理论依据.