First-principles studies of the structural,elastic, and lattice dynamical properties of ZrMo2 and HfMo2

A comprehensive investigation of the structural, elastic, and lattice dynamical properties for ZrMo₂ and HfMo₂ with C14, C15, C36, and CeCu₂ phases are conducted using density functional total energy calculations. The results have showed that C15 phase for both materials is energetically more stable than C14, C36 and CeCu₂ phases. We have also estimated the mechanical behaviours of these compounds, including mechanical stability, bulk modulus, Young's modulus, shear modulus, Poisson's ratio, ductility, and anisotropy. Additionally, the lattice dynamical properties are analyzed and discussed exhaustively for these phases. The calculated properties agree well with available experimental and theoretical data.     

The temperature-dependent elastic properties of B2-MgRE intermetallic compounds from first principles

The temperature-dependent elastic modulus of MgRE (RE=Y, Dy, Pr, Sc, Tb) intermetallics with B2-type structure are presented from first-principles quasistatic approach, in which the static volume-dependent elastic constants are obtained by the first-principles total-energy method within density functional theory and the thermal expansion is obtained from the quasiharmonic approach based on density-functional perturbation theory. The comparison between the predicted results and the available experimental data for a benchmark material NiAl provides good agreements. At T=0 K, our calculated values of lattice parameter and elastic moduli for MgRE intermetallics show excellent agreement with previous theoretical results and experimental data. With temperature increasing, we find that the elastic constants satisfy the stability conditions for B2 structures and follow a normal behavior with temperature, i.e., decrease and approach linearity at higher temperature and zero slope around zero temperature. In addition, the sound velocities as a function of temperature for the NiAl and MgRE intermetallics are calculated and the relations to phonon spectrums have also been discussed.     

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     

The structural, elastic and thermodynamical properties of zinc-blend structure InN from first principles

A theoretical study of the structural, elastic and thermodynamic properties of the cubic zinc-blende (ZB) structure InN are presented in this paper by performing first principles calculations within local density approximation. The values of lattice constant, bulk modulus and its pressure derivatives and elastic constants are in excellent agreement with the available experimental data and other theoretical results. It is found that the ZB structure InN should be unstable above 20 GPa mechanically. The pressure and temperature dependencies of the bulk modulus, the heat capacity and the thermal expansion coefficient and the entropy S, as well as the Grüneisen parameter are obtained by the quasi-harmonic Debye model in the ranges of 0-1500 K and 0-25 GPa.     

Anisotropic elastic properties and ideal uniaxial compressive strength of TiB_2 from first principles calculations

The structural, anisotropic elastic properties and the ideal compressive and tensile strengths of titanium diboride(TiB_2) were investigated using first-principles calculations based on density functional theory. The stress–strain relationships of TiB_2 under 1010, 1210, and 0001 compressive loads were calculated. Our results showed that the ideal uniaxial compressive strengths are |σ _(1210)| = 142.96 GPa, |σ_(0001)| = 188.75 GPa, and |σ_(1010)| = 245.33 GPa, at strains -0.16,-0.32, and-0.24, respectively. The variational trend is just the opposite to that of the ideal tensile strength withσ 1010 = 44.13 GPa, σ_(0001)= 47.03 GPa, and σ_(1210)= 56.09 GPa, at strains 0.14, 0.28, and 0.22, respectively. Furthermore, it was found that TiB_2 is much stronger under compression than in tension. The ratios of the ideal compressive to tensile strengths are 5.56, 2.55, and 4.01 for crystallographic directions 1010, 1210, and 0001, respectively. The present results are in excellent agreement with the most recent experimental data and should be helpful to the understanding of the compressive property of TiB_2.     

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.     

BaCoxZn2-xFe16O27六角铁氧体电子结构与介电特性的第一性原理研究

提出了W型六角铁氧体BaCo_xZn_2-xFe₁₆O₂₇的晶体结构模型,并通过基于密度泛函理论框架下Hubbard参数U修正的广义梯度近似密度泛函理论方法研究了该材料的基态电子结构、磁性和静电介电性特性.Co和Zn共掺杂引起BaFe₁₈O₂₇的导电性从半金属转换到半导体.BaCo_xZn_2-xFe₁₆O₂₇的能隙随x增加而增加,品格常数和原胞磁矩随之而变小.介电常数计算表明,BaCo_xZn_2-xFe₁₆O₂₇的静电介电常数随x增加而增加,在6.2—7.2范围而且显示各向异性.Bom电荷计算分析表明Co和Zn本身极化对材料的介电常数和其各向异性影响不大.     

Superconducting properties of MgB2 from first principles

Solid MgB(2) has rather interesting and technologically important properties, such as a very high superconducting transition temperature. Focusing on this compound, we report the first nontrivial application of a novel density-functional-type theory for superconductors, recently proposed by the authors. Without invoking any adjustable parameters, we obtain the transition temperature, the gaps, and the specific heat of MgB(2) in very good agreement with experiment. Moreover, our calculations show how the Coulomb interaction acts differently on sigma and pi states, thereby stabilizing the observed superconducting phase.     

First principles determination of structural,electronic and lattice dynamical properties of a model dipeptide molecular crystal

We present the results of ab initio calculations of the structural, electronic and lattice dynamical properties of the solid-state crystal of the glycyl-l-alanine dipeptide. Intramolecular bond lengths are found to be in good agreement with experimental values; lattice constants are in reasonable agreement, although it is found that discrepancies do exist. A hierarchy of hydrogen bond strengths is found, with those between (oppositely-charged) amine and carboxy functional groups being strongest. The crystal is found to be an indirect-bandgap material, with indirect bandgaps ≈4.95 eV, compared to a direct bandgap of 5.00 eV. Analysis of the electronic structure reveals that the electronic states in the near vicinity of the energy gap arise from carboxylate and amide oxygen atoms. The arrangement of both molecules and hydrogen bonds in the unit cell is found to manifest itself in increased bandwidth along specific reciprocal space directions, reflecting coupling brought about by hydrogen bonds. Determination of the zone-centre lattice dynamical behaviour permits the IR absorption spectrum to be explained. Intermolecular hydrogen bonds are found to couple intramolecular motions in adjacent moelcules, revealing the importance of an accurate treatment of intermolecular interactions, even for high-frequency vibronic modes.     

Mechanical and lattice dynamical properties of the Re2C compound

In this work, density functional theory calculations on the structural, mechanical, and lattice dynamical properties of Re₂C within ReB₂‐type structure are reported. The generalized gradient approximation has been used for modeling exchange–correlation effects. We have predicted the lattice constants, bulk modulus, bond distances, elastic constants, shear modulus, Young's modulus, Poisson's ratio, hardness, Debye temperature, and sound velocities of this compound. Furthermore, the band structure, phonon dispersion curves and corresponding density of states are computed. The obtained results are in good agreement with the available experimental and other theoretical data. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electronic and thermodynamic properties of B2-FeSi from first principles

The electronic and thermodynamic properties of B2-FeSi have been investigated using the first-principles method based on the plane-wave basis set. The calculated equilibrium lattice constant is in good agreement with available experimental and theoretical data. Our results have shown that B2-FeSi was a narrow gap semiconductor of above 0.055 eV and exhibited metallic characteristics. The density of states (DOS) can also describe orbital mixing. Using the quasi-harmonic Debye model, the thermodynamic properties of B2-FeSi have been analyzed. Variations of the Debye temperature Θ_D, thermal expansion α, heat capacity C_v, entropy S and the Grüneisen parameter γ on temperature T and pressure P were obtained successfully in the ranges of 0-2400 K and 0-140 GPa.     

Structural, electronic, magnetic and elastic properties of tetragonal layered diselenide KCo2Se2 from first principles calculations

The structural, elastic, magnetic and electronic properties of the layered tetragonal phase KCo₂Se₂ have been examined in details by means of the first-principles calculations and analyzed in comparison with the isostructural KFe₂Se₂ as the parent phase for the newest group of ternary superconducting iron-chalcogenide materials. Our data show that KCo₂Se₂ should be characterized as a quasi-two-dimensional ferromagnetic metal with highly anisotropic inter-atomic bonding owing to mixed ionic, covalent, and metallic contributions inside [Co₂Se₂] blocks, and with ionic bonding between the adjacent [Co₂Se₂] blocks and K sheets. This material should behave in a brittle manner, adopt enhanced elastic anisotropy rather in compressibility than in shear, and should show very low hardness.     

Electronic and lattice vibrational properties of cubic BaHfO3 from first principles calculations

Through first principles calculations, we investigated the electronic structure and lattice vibrational properties of BaHfO₃. The optimized lattice constant of BaHfO₃ is in agreement with experimental and theoretical results. Our results show that cubic BaHfO₃ is an insulator with an indirect band gap of 3.5 eV. Besides, the calculation using the screened exchange local density approximation (sX-LDA) has been performed with the predicted minimum gap of 5.3 eV. The phonon dispersion curves of BaHfO₃ were also calculated. All positive phonon frequencies in the Brillouin zone were found, indicating the stability of BaHfO₃ structure.     

Elastic,mechanical, electronic properties and hardness of Nb2AsC from first principles

The study aims at the elastic, mechanical, electronic properties and hardness of Nb₂AsC using first principles based on the density functional theory method within the generalised gradient approximation. The calculated lattice parameters of Nb₂AsC are in good agreement with the experimental data. The five independent elastic constants are firstly calculated as a function of pressure, and our results indicate that it is mechanically stable in the applied pressure. The elastic anisotropy is examined through the computation of the direction dependence of Young's modulus. The pressure dependences of the bulk modulus, shear modulus, average velocity of acoustic waves and Debye temperature of Nb₂AsC are systematically investigated. The band structure and density of states are discussed, and the results show that the strong hybridisations C p–Nb d and As p–Nb d would be beneficial to the structure stability of Nb₂AsC. Based on the Mulliken population analysis, the hardness of Nb₂AsC is predicted.     

Electronic structures and thermoelectric properties of La or Ce-doped Bi2Te3 alloys from first principles calculations

Thermoelectric properties of La or Ce-doped Bi₂Te₃ alloys were systematically investigated by ab initio calculations of electronic structures and Boltzmann transport equations. The Seebeck coefficient of p-type LaBi₇Te₁₂ and La₂Bi₆Te₁₂ was larger than that of Bi₂Te₃, because La doping increased the effective mass of carriers. On the other hand, the electrical conductivity of LaBi₇Te₁₂ and La₂Bi₆Te₁₂ decreased, which caused a reduction of power factor of these La-doped Bi₂Te₃ alloys in comparison with Bi₂Te₃. The influence of Ce doping on the band structure and thermoelectric properties of Bi₂Te₃ was similar to that of La doping. The theoretical calculation provided an insight into the transport properties of La or Ce-doped Bi₂Te₃-based thermoelectric materials.     

Structural,elastic, electronic properties and Fermi surface for superconducting Mo2GaC in comparison with V2GaC and Nb2GaC from first principles

First-principles calculations were performed to investigate structural, elastic and electronic properties of the first discovered superconducting nanolaminate Mo₂GaC in comparison with isostructural Ga-containing phases V₂GaC and Nb₂GaC. As a result, the optimized lattice parameters, independent elastic constants, bulk moduli, compressibility, shear moduli, Young’s moduli and Poisson’s ratio were evaluated. Besides, electronic bands, densities of states (DOS), total and site-projected l-decomposed DOS at the Fermi level, as well as the shapes of the Fermi surfaces for these Ga-containing nanolaminates were obtained and analyzed in comparison with the available theoretical and experimental data.     

Electronic properties of the Si/SiO2 interface from first principles

Unoccupied oxygen p-projected densities of states, calculated from first principles in a model Si/SiO(2) interface, are found to reproduce trends in recent atomic resolution electron energy-loss spectra [D. A. Muller et al., Nature (London) 399, 758 (1999)]. The shape of the unoccupied states and the magnitude of the local energy gap are explicitly related to the number of O second neighbors of a given oxygen atom. The calculated local energy gaps of the oxide become considerably smaller within 0.5 nm of the interface, suggesting that the electronic properties do not change abruptly at the interface.     

First-principles study of structural,elastic, lattice dynamical and thermodynamical properties of GdX (X = Bi,Sb)

The results are presented of first-principles calculations of the structural, elastic and lattice dynamical properties of GdX (X = Bi, Sb). In particular, the lattice parameters, bulk modulus, phonon dispersion curves, elastic constants and their related quantities, such as Young's modulus, Shear modulus, Zener anisotropy factor, Poisson's ratio, Kleinman parameter, and longitudinal, transverse and average sound velocities, were calculated and compared with available experimental and other theoretical data. The temperature and pressure variations of the volume, bulk modulus, thermal expansion coefficient, heat capacities, Grüneisen parameter and Debye temperatures were predicted in wide pressure (0?50 GPa) and temperature ranges (0–500 K). The plane-wave pseudopotential approach to the density-functional theory within the GGA approximation implemented in VASP (Vienna ab initio simulation package) was used in all computations.     

Calculation of the lattice dynamical properties of Ge

The lattice dynamical properties of Ge are studied employing a frozen phonon approach within an ab initio density-functional pseudopotential formalism. Using the atomic number, atomic mass, and crystal structure as the only input information, we calculate phonon frequencies and mode Grüneisen parameters at Γ and X and the third-order anharmonic parameter for the zone center optical mode. The results are in excellent agreement with available experimental data. An analysis of various energy contributions to the TA (X) mode shows that the proper inclusion of the exchange-correlation energy is crucial for accurate microscopic studies of lattice dynamics.     

A first-principle study of the structural, elastic, lattice dynamical and thermodynamic properties of PrX (X=P, As)

The structural, phase transition, elastic, lattice dynamic and thermodynamic properties of rare-earth compounds PrP and PrAs with NaCl (B1), CsCl (B2), ZB (B3), WC (B_h) and CuAu (L1₀) structures are investigated using the first principles calculations within the generalized gradient approximation (GGA). For the total-energy calculation, we have used the projected augmented plane-wave (PAW) implementation of the Vienna Ab-initio Simulation Package (VASP). Specifically, some basic physical parameters, e.g. lattice constants, bulk modulus, elastic constants, shear modulus, Young's modulus and Poison's ratio, are predicted. The obtained equilibrium structure parameters are in excellent agreement with the experimental and theoretical data. The temperature and pressure variations of the volume, bulk modulus, thermal expansion coefficient, heat capacity and Debye temperature are calculated in wide pressure and temperature ranges. The phonon dispersion curves and corresponding one-phonon density of states (DOS) for both compounds are also computed in the NaCl (B1) structure.