First-principles calculations for elastic properties of OsB2 under pressure

The structure, elastic properties and elastic anisotropy of orthorhombic OsB₂ are investigated by density functional theory method with the ultrasoft pseudopotential scheme in the frame of the generalized gradient approximation (GGA) as well as local density approximation (LDA). The obtained structural parameters, elastic constants, elastic anisotropy and Debye temperature for OsB₂ under pressure are consistent with the available experimental data and other theoretical results. It is found that the elastic constants, bulk modulus and Debye temperature of OsB₂ tend to increase with increasing pressure. It is predicted that OsB₂ is not a superhard material from our calculations.     

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.     

Elasticity and thermodynamic properties of RuB2 under pressure

First-principles calculations of the crystal structure and the elastic properties of RuB₂ have been carried out with the plane-wave pseudopotential density functional theory method. The calculated values are in very good agreement with experimental data as well as with some of the existing model calculations. The elastic constants c_ij, the aggregate elastic moduli (B, G, E), Poisson's ratio, and the elastic anisotropy with pressure have been investigated. Through the quasi-harmonic Debye model considering the phonon effects, the isothermal bulk modulus, the thermal expansions, Grüneisen parameters, and Debye temperatures depending on the temperature and pressure are obtained in the whole pressure range from 0 to 60 GPa and temperature range from 0 to 1100 K as well as compared to available data.     

Structural and elastic properties of LaTiO3 under pressure

We investigate the structural and elastic properties of LaTiO₃ by the plane-wave pseudopotential density functional theory method. The lattice constants, bulk modulus and its pressure derivative are obtained. These properties in the equilibrium phase are well consistent with the available experimental data. The pressure dependence of the elastic constants, ductility, mechanical stabilities, sound velocity and Debye temperatures are investigated for the first time. From the ratio G/B, we conclude that LaTiO₃ is ductile at 0 GPa and becomes more ductile at high pressure. In addition, the anisotropy factors for every symmetry plane and axis as well as linear bulk modulus at diverse pressures have been obtained.     

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.     

DFT - based study of electronic structures and mechanical properties of LiTaO3: ferroelectric and paraelectric phases

Abstract

By applying ab initio calculation within density functional theory (DFT), we study the structure parameters, electronic band structure, elastic coefficients, polycrystalline elastic properties, anisotropy factors and Debye temperature of ferroelectric and paraelectric phases of LiTaO₃ within the generalised gradient approximation at ambient pressure. The atomic structure in both phases is fully relaxed and the lattice constant, angle and atomic positions are well consistent with experimental values. The computed single-crystal elastic coefficients indicate that mechanical stability of LiTaO₃ in both phases is confirmed using the generalised Born criteria. The shear, bulk and Young’s modulus, Poisson’s ratio, and Vickers hardness were computed according to theoretical elastic constants by Voight–Reuss–Hill method. Several anisotropy factors and indexes are computed to illustrate mechanical anisotropy. Both phases are shown to be weakly anisotropic. The Debye temperature is estimated using the longitude and transverse elastic wave velocity of the ideal polycrystalline LiTaO₃ aggregates. We have found that LiTaO₃ in both phases has an indirect energy band gap. The differences in the electronic structure and density of states for both phases are quite small. Our results indicate that the mechanical and bonding properties of both phases are very similar. The obtained results were compared with the available experimental and theoretical values.     

First-principles calculations on elasticity of under pressure

First-principles calculations of the crystal structure and the elastic properties of OsN₂ have been carried out with the plane-wave pseudopotential density functional theory method. The calculated values are in very good agreement with experimental data as well as with some of the existing model calculations. The dependence of the elastic constants c_ij, the aggregate elastic moduli (B,G,E), Poisson’s ratio, and the elastic anisotropy on pressure has been investigated. Moreover, the variation of the Debye temperature and the compressional and shear elastic wave velocities with pressure P up to 60 GPa at 0 K have been investigated for the first time.     

Structural,elastic and mechanical properties of orthorhombic SrHfO3 under pressure from first-principles calculations

Structural, elastic and mechanical properties of orthorhombic SrHfO₃ under pressure have been investigated using the plane-wave ultrasoft pseudopotential technique based on the first-principles density functional theory. The calculated equilibrium lattice parameters and elastic constants of orthorhombic SrHfO₃ at zero pressure are in good agreement with the available experimental and calculational values. The lattice parameters, total enthalpy, elastic constants and mechanical stability of orthorhombic SrHfO₃ as a function of pressure were studied. With the increasing pressure, the lattice parameters and volume of orthorhombic SrHfO₃ decrease whereas the total enthalpy increases. Orthorhombic SrHfO₃ is mechanically stable with low pressure (<52.9 GPa) whereas that is mechanically instable with high pressure (>52.9 GPa). The bulk modulus, shear modulus, Young's modulus and mechanical anisotropy of orthorhombic SrHfO₃ as a function of pressure were analyzed. It is found that orthorhombic SrHfO₃ under pressure has larger bulk modulus, better ductility and less mechanical anisotropy than orthorhombic SrHfO₃ at 0 GPa.     

Elastic and electronic properties of YNi2B2C under pressure from first principles

The elastic and electronic structure properties of YNi₂B₂C under pressure are investigated by performing the generalized gradient approximation (GGA) and local density approximation (LDA) correction scheme in the frame of density functional theory (DFT). The pressure dependences of the normalized lattice parameters a/a₀ and c/c₀, the ratio c/a, and the normalized primitive volume V/V₀ of YNi₂B₂C are also obtained. The lattice constants and bulk modulus obtained are in agreement with the available experimental and other theoretical data. We have also studied the pressure dependences of elastic properties. It is found that, as pressure increases, the elastic constants C₁₁, C₃₃, C₆₆, C₁₂, and C₁₃ increase, the variation of elastic constant C₄₄ is not obvious. Moreover, our compressional and shear wave velocities V_L=6.99 km/s and V_S=3.67 km/s as well as the Debye temperature Θ=549.7 K at 0 GPa compare favorably with the available experimental data. The pressure dependences of band structures, energy gap and density of states are also investigated.     

Third-order elastic constants of ZnS and ZnSe

The complete set of non-vanishing third-order elastic constants of the semiconductors ZnS and ZnSe is obtained theoretically. The strain energy density is estimated using finite strain elasticity theory by considering the interactions up to two nearest neighbours of each atom in the unit cell of these compounds. This energy density is compared with the strain dependent lattice energy density from the continuum model approximation. The second-order parameter of the potential function φ is obtained from the measured principal axis C_ij. The third-order potential parameter is estimated by assuming a Lennard-Jones type of interatomic potential. The interlattice displacements as well as the second-order elastic constants are evaluated along with the six third-order elastic constants of ZnS and ZnSe. Using these second- and third-order elastic constants of ZnS, the pressure derivatives of second-order elastic constants are evaluated. The second- and third-order elastic constants of ZnSe are compared with the available experimental values. The third-order elastic constants show anisotropy in different directions.     

Elastic and thermodynamic properties of CaB6 under pressure from first principles

The elastic and thermodynamic properties of CsCl-type structure CaB₆ under high pressure are investigated by first-principles calculations based on plane-wave pseudopotential density functional theory method within the generalized gradient approximation (GGA). The calculated lattice parameters of CaB₆ under zero pressure and zero temperature are in good agreement with the existing experimental data and other theoretical data. The pressure dependences of the elastic constants, bulk modulus B (GPa), and its pressure derivative B′, shear modulus G, Young's modulus E, elastic Debye temperature Θ_B, Zener's anisotropy parameter A, Poisson ratios σ, and Kleinmann parameter ζ are also presented. An analysis for the calculated elastic constants has been made to reveal the mechanical stability of CaB₆ up to 100 GPa. The thermodynamic properties of the CsCl-type structure CaB₆ are predicted using the quasi-harmonic Debye model. The pressure-volume-temperature (P-V-T) relationship, the variations of the heat capacity C_V, Debye temperature Θ_D, and the thermal expansion α with pressure P and temperature T, as well as the Grüneisen parameters γ are obtained systematically in the ranges of 0-100 GPa and 0-2000 K.     

First-principles study of elastic and electronic properties of layered ternary nitride SrZrN2 under pressure

The structural, elastic, and electronic properties of SrZrN₂ under pressure up to 100?GPa have been carried out with first-principles calculations based on density functional theory. The calculated lattice parameters at 0?GPa and 0?K by using the GGA-PW91-ultrasoft method are in good agreement with the available experimental data and other previous theoretical calculations. The pressure dependence of the elastic constants and the elastic-dependent properties of SrZrN₂, such as bulk modulus B, shear modulus G, Young's modulus E, Debye temperature Θ, shear and longitudinal wave velocity V_S and V_L, are also successfully obtained. It is found that all elastic constants increase monotonically with pressure. When the pressure increases up to 140?GPa, the obtained elastic constants do not satisfy the mechanical stability criteria and a phase transition might has occurred. Moreover, the anisotropy of the directional-dependent Young's modulus and the linear compressibility under different pressures are analysed for the first time. Finally, the pressure dependence of the total and partial densities of states and the bonding property of SrZrN₂ are also investigated.     

Structural, electronic, elastic and optical properties of fluoro-perovskite KZnF3

We determine the structural, electronic, elastic and optical properties of fluoro-perovskite KZnF₃ using the full potential linear augmented plane wave approach (FP-LAPW) based on the density functional theory (DFT). The exchange-correlation potential is treated by the local density approximation (LDA) and the generalized gradient approximation (GGA). The calculated structural parameters are in good agreement with the available data. We have obtained an indirect band gap. The effect of the pressure on the band gaps is investigated. We evaluate the elastic constants (C_ij), elastic moduli and the Debye temperature. The imaginary and the real parts of the dielectric function ε(ω) and some optical constants are also calculated.     

First-principles study of the structural, elastic, electronic, optical, and vibrational properties of intermetallic Pd2Ga

The structural, elastic, electronic, optical, and vibrational properties of the orthorhombic Pd₂Ga compound are investigated using the norm-conserving pseudopotentials within the local density approximation in the frame of density functional theory. The calculated lattice parameters have been compared with the experimental values and found to be in good agreement with these results. The second-order elastic constants and the other relevant quantities, such as the Young's modulus, shear modulus, Poisson's ratio, anisotropy factor, sound velocity, and Debye temperature, have been calculated. It is shown that this compound is mechanically stable after analysing the calculated elastic constants. Furthermore, the real and imaginary parts of the dielectric function and the optical constants, such as the optical dielectric constant and the effective number of electrons per unit cell, are calculated and presented. The phonon dispersion curves are derived using the direct method. The present results demonstrate that this compound is dynamically stable.     

First-principles calculations for the elastic properties of Ni-base model superalloys: Ni/Ni3Al multilayers

A model system consisting of Ni[001](100)/Ni3Al[001](100) multi-layers are studied using the density functional theory in order to explore the elastic properties of single crystal Ni-based superalloys. Simulation results are consistent with the experimental observation that rafted Ni-base superalloys virtually possess a cubic symmetry. The convergence of the elastic properties with respect to the thickness of the multilayers are tested by a series of multilayers from 2γ′+2γ to 10γ′+10γ atomic layers. The elastic properties are found to vary little with the increase of the multilayer’s thickness. A Ni/Ni3Al multilayer with 10γ′+10γ atomic layers (3.54 nm) can be used to simulate the mechanical properties of Ni-base model superalloys. Our calculated elastic constants, bulk modulus, orientation-dependent shear modulus and Young’s modulus, as well as the Zener anisotropy factor are all compatible with the measured results of Ni-base model superalloys R1 and the advanced commercial superalloys TMS-26, CMSX-4 at a low temperature. The mechanical properties as a function of the γ′ phase volume fraction are calculated by varying the proportion of the γ and γ′ phase in the multilayers. Besides, the mechanical properties of two-phase Ni/Ni3Al multilayer can be well predicted by the Voigt-Reuss-Hill rule of mixtures.     

Theoretical predictions of the structural and thermodynamic properties of MgZn2 Laves phase under high pressure

The structural and thermodynamic properties of MgZn₂ Laves phase under hydrostatic pressure have been investigated by using a first-principles method based on the density functional theory within the generalized gradient approximation. The calculated equilibrium structural parameters are consistent with the previous experimental and theoretical data. Especially, we study the pressure dependence of the elastic constants, polycrystalline elastic moduli, Poisson’s ratio, elastic anisotropy, and theoretical Vickers hardness of MgZn₂. It is found that the pressure plays a significant role in the elastic properties of MgZn₂ due to the variations of inter-atomic distance. In addition, the density of states and Mulliken analysis are performed to reveal the bonding characteristics of MgZn₂. It is observed that the total density of states exhibits a certain offset with the increase of external pressure. Finally, the dependences of thermodynamic properties on pressure and temperature of MgZn₂ Laves phase have been also successfully predicted and analyzed within the quasi-harmonic Debye model for the first time.     

First-principles investigation on vibrational,anisotropic elastic and thermodynamic properties for L12 structure of Al3Er and Al3Yb under high pressure

To better clarify the physical properties for Al₃RE precipitates, first-principles calculations are performed to investigate the vibrational, anisotropic elastic and thermodynamic properties of Al₃Er and Al₃Yb. The calculated results agree well with available experimental and theoretical ones. The vibrational properties indicate that Al₃Er and Al₃Yb will keep their dynamical stabilities with L1₂ structure up to 100 GPa. The elastic constants are satisfied with mechanical stability criteria up to the external pressure of 100 GPa. The mechanical anisotropy is predicted by anisotropic constants A^G, A^U, A^Z and 3D curved surface of Young’s modulus. The calculated results show that both Al₃Er and Al₃Yb are isotropic at zero pressure and obviously anisotropic under high pressure. Further, we systematically investigate the thermodynamic properties and provide the relationships between thermal parameters and pressure. Finally, the pressure-dependent behaviours of density of states, Mulliken charge and bond length are discussed.     

Pressure effects on structural,elastic and electronic properties of BaZnO2: first-principles study

The structural, elastic and electronic properties of BaZnO₂ under pressure are investigated by the plane wave pseudopotential density functional theory (DFT). The calculated lattice parameters and unit cell volume of BaZnO₂ at the ground state are in good agreement with the available experimental data and other theoretical data. The pressure dependences of elastic constants C_ij, bulk modulus B, shear modulus G, B/G, Poisson’ s ratio σ, Debye temperature Θ and aggregate acoustic velocities V_P and V_S are systematically investigated. It is shown that BaZnO₂ maintains ductile properties under the applied pressures. Analysis for the calculated elastic constants has been made to reveal the mechanical stability and mechanical anisotropy of BaZnO₂. At the ground state, the calculated compressional and shear wave velocities are 8.26 km/s and 1.81 km/s, respectively, and the Debye temperature Θ is 240.8 K. The pressure dependences of the density of states and the bonding property of BaZnO₂ are also investigated.     

First-principles calculations for elastic and electronic properties of ZnSnO3 under pressure

The LiNbO₃ (LN)-type structure and the ilmenite (IL)-type structure of ZnSnO₃ are investigated with the ultrasoft pseudopotential scheme in the frame of the local density approximation (LDA). The calculated lattice parameters of ZnSnO₃ under zero pressure and zero temperature are in very good agreement with the existing experimental data. The pressure dependences of the elastic constants, Debye temperatures, Poisson's ratio, sound velocity, mechanical stability and mechanical anisotropy of the LN-type structure of ZnSnO₃ have also been investigated. We find that the LN-type structure of ZnSnO₃ is a mechanically stable phase under pressures up to 21 GPa; however, the mechanical anisotropy weakens with the increasing pressures. In addition, the calculated band structure indicates that LN-ZnSnO₃ has a direct band gap of 1.669 eV, and the total and partial densities of states, under diverse pressures of the LN-type structure ZnSnO₃ have also been obtained.     

Ground state and phonon spectrum of NiSi2

We have studied structural, electronic, elastic and dynamical properties of NiSi₂ by employing the plane wave pseudopotential method based on density functional theory within the local density approximation. The calculated lattice constant, bulk modulus and first-order pressure derivative of the bulk modulus are reported and compared with earlier available experimental and theoretical calculations. Numerical first-principles calculations of the elastic constants were used to calculate C₁₁, C₁₂ and C₄₄ for NiSi₂. The calculated electronic band structure has been compared with angle-resolved photoemission spectroscopy experimental data along the [100] and [111] symmetry directions. A linear response approach to density functional theory is used to derive the phonon dispersion curves and phonon partial density of states. Atomic displacement patterns for NiSi₂ at the Γ, X and L symmetry points are also presented.