First-principles investigations on structural,elastic and electronic properties of SnO2 under pressure

We investigate the structural, elastic, and electronic properties of rutile-type SnO₂ by plane-wave pseudopotential density functional theory method. The lattice constants, bulk modulus and its pressure derivative are all calculated. These properties at equilibrium phase are well consistent with the available experimental and theoretical data. Especially, we study the pressure dependence of elastic properties such as the elastic constants, elastic anisotropy, aggregate acoustic velocities and elastic Debye temperature Θ. It is concluded that this structure becomes more ductile with increasing pressure up to 28 GPa. Moreover, our compressional and shear wave velocities V_P = 7.02 km/s and V_S = 3.84 km/s, as well as elastic Debye temperature Θ = 563 K at 0 GPa compare favorably with the experimental values. The pressure dependences of band structures, energy gap and density of states are also investigated.     

Ab initio study of AlCu2M (M = Sc,Ti and Cr) ternary compounds under pressures

The structural, electronic and elastic properties of the AlCu₂M (M = Sc, Ti and Cr) compounds in the pressure range of 0–100 GPa was investigated based on density functional theory. The calculated lattice parameters of the AlCu₂M compounds at zero pressure and zero temperature are in very good agreement with the existing experimental data. The bulk modulus, shear modulus and Young’s modulus increases with the increase of pressure, which indicates that higher materials hardness may be obtained when increasing pressures. The bulk modulus and Young’s modulus of AlCu₂Cr is the greatest under pressure. The shear modulus of AlCu₂Ti is the highest above 30 GPa, while that of the AlCu₂Sc is the strongest below 30 GPa. The calculated B/G values at zero and higher pressure indicated that they are ductile materials. The electronic densities of states and bonding charge densities have been discussed in details, revealing these compounds exhibit half-metallic behavior. In addition, the pressure dependences of Debye temperatures of AlCu₂M compounds have also been calculated. The results indicate that Debye temperatures increase with increasing pressure.     

Structural,electronic, elastic and thermodynamic properties of AlSi2RE (RE = La,Ce, Pr and Nd) from first-principle calculations

It is of academic interest to study the ternary intermetallic compounds of the Al–Si–RE system for the development of both structural and functional materials. In this work, the structural, electronic, elastic and thermodynamic properties of the AlSi₂RE (RE = La, Ce, Pr and Nd) compounds was investigated using first-principle calculations based on density functional theory. The calculated structural parameters of AlSi₂RE compounds are consistent with the experimental data. Due to the fact that there is strong Coulomb correlation among the partially filled 4f electron for RE atoms, we present a combination of the GGA and the LSDA + U approaches to investigate the electronic structures of Al₃RE compounds in order to obtain the appropriate results. The elastic constants were determined from a linear fit of the calculated stress–strain function according to Hooke’s law. The bulk modulus B, shear modulus G, Young’s modulus E, and Poisson’s ratio ν of polycrystalline AlSi₂RE compounds were determined using the Voigt–Reuss–Hill (VRH) averaging scheme. The Debye temperature of AlSi₂RE compounds can be obtained from elastic constants. The temperature dependence of the internal energy, free energy, entropy and heat capacity for AlSi₂RE compounds were also calculated by using the quasi-harmonic approximation.     

First-principles calculations of the structural and elastic properties of OsSi2 at high pressure

We investigated the pressure dependence of the structural and elastic properties of OsSi₂ in the range 0–60 GPa using first-principles calculations based on density functional theory. Calculations were performed within the local density approximation as well as the generalized gradient approximation to the exchange correlation potential. The calculated lattice constants and atomic fractional coordinates are in good agreement with previous experimental results. The pressure dependence of nine independent elastic constants, c₁₁, c₂₂, c₃₃, c₄₄, c₅₅, c₆₆, c₁₂, c₁₃, and c₂₃, of orthorhombic OsSi₂ has been evaluated. The isotropic bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, elastic anisotropy, and Debye temperature of polycrystalline OsSi₂ under pressure are also presented.     

FP-LAPW study of the structural,elastic and thermodynamic properties of spinel oxides ZnX2O4 (X = Al,Ga, In)

We have performed density functional self-consistent calculations based on the full-potential augmented plane wave plus local orbital method with the local density approximation to investigate the structural, elastic and thermal properties of three spinel oxides: ZnAl₂O₄, ZnGa₂O₄ and ZnIn₂O₄. The computed ground state structural parameters, i.e. lattice constant, free internal parameter, bulk modulus and its pressure derivative, are in good agreement with the available theoretical an experimental works. Single and polycrystalline elastic parameters and their pressure dependence are calculated and compared with the previous theoretical results. Thermal and pressure effects on some macroscopic properties of ZnAl₂O₄, ZnGa₂O₄ and ZnIn₂O₄ are predicted using the quasi-harmonic Debye model in which the lattice vibrations are taken into account. We have computed the variations of the lattice constant, bulk modulus, volume expansion coefficient, heat capacities and Debye temperature with pressure and temperature in the ranges of 0–30 GPa and 0–1600 K.     

Structural,elastic, electronic and optical properties of new layered semiconductor BaGa2P2

We report the results of a detailed first-principles based density functional theory study of the structural, elastic, electronic and optical properties of a recently synthesized layered semiconductor BaGa₂P₂. The optimized structural parameters are in excellent agreement with the experimental structural findings, which validates the used theoretical method. The single crystal and polycrystalline elastic constants are numerically estimated using the strain–stress method and Voigt–Reuss–Hill approximations. Predicted values of the elastic constants suggest that the considered material is mechanically stable, brittle and very soft material. The three-dimensional surface and its planar projections of Young’s modulus are visualized to illustrate the elastic anisotropy. It is found that Young’s modulus of BaGa₂P₂ show strong dependence on the crystallographic directions. Band structure calculation reveals that BaGa₂P₂ is a direct energy band gap semiconductor. The effective masses of electrons and holes at the minimum of the conduction band and maximum of the valence band are numerically estimated. The density of state, charge density distribution and charge transfers are calculated and analyzed to determine the chemical bonding nature. Dielectric function, refractive index, extinction coefficient, absorption coefficient, reflectivity and electron-loss energy function spectra are computed for a wide photon energy range up to 20 eV. Calculated optical spectra exhibit a noticeable anisotropy.     

Mechanical and electronic properties of Ag3Sn intermetallic compound in lead free solders using ab initio atomistic calculation

First principle calculations based on density functional theory (DFT) are used to calculate the structural, elastic and electronic properties of tin–silver intermetallic compound (Ag₃Sn), found mainly in lead free solder joints. In present work, for the exchange-correlation energy, generalized gradient approximation (GGA) functional is used. The calculated lattice constants are found to be within 2% error of the experimental values. All single crystal elastic constants are computed from which values of shear modulus, bulk modulus, Young's modulus and Poisson's ratio for polycrystalline Ag₃Sn are calculated using Voigt and Hill approximations. To explain the scatter in the experimentally determined values of elastic constants, directional dependence of bulk modulus and Young's modulus are estimated. The values of Young's modulus calculated along different planes are found to be in same range as experimentally determined values. Various anisotropic indices like universal anisotropic index, Zener anisotropic index, shear anisotropic index and others are calculated to study elastic anisotropy. Further anisotropy in Poisson's ratio is studied by calculating their values along six lower-index planes. The value of Debye temperature calculated using elastic data of present work is found be to higher than the values obtained using resistivity measurement, which can be attributed to temperature dependence. Electronic properties are studied via the band structure and total and partial density of states. The density of state (DOS) of Ag₃Sn has a characteristic main peak which is mainly dominated by Ag-d states. At the Fermi level, the total DOS value is found to be 1.97 states/eV with major contribution coming from Sn p states and minor contribution from Sn s and Ag s, p and d states.     

First-principles calculations of mechanical and thermodynamic properties of YAlO3

First-principles calculations are performed to investigate the crystal structure, electronic properties, the elastic properties, hardness and thermodynamic properties of YAlO₃. The calculated ground-state quantities such as lattice parameter, bulk modulus and its pressure derivative, the band structure and densities of states were in favorable agreement with previous works and the existing experimental data. The elastic constants C_ij, the aggregate elastic moduli (B, G, E), the Poisson’s ratio, and the elastic anisotropy have been investigated. YAlO₃ exhibits a slight elastic anisotropy according to the universal elastic anisotropy index A^U = 0.24. The estimated hardness for YAlO₃ is consistent with the experimental value, and Al–O bond in AlO₆ octahedra plays an important role in the high hardness. The Y–O bonds in YO₁₂ polyhedra exhibit different characteristic. Using the quasi-harmonic Debye model considering the phonon effects, the temperature and pressure dependencies of bulk modulus, heat capacity and thermal expansion coefficient are investigated systematically in the ranges of 0–20 GPa and 0–1300 K.     

Investigated optical and elastic properties of Porous silicon: Theoretical study

Compatibility between experimental and theoretical works is achieved. Empirical Pseudopotential Method (EPM) is used to calculate the energy gap of Si which is found to be indirect. Features such as refractive index, optical dielectric constant, bulk modulus, elastic constants and short-range force constants have been investigated. In addition to the shear modulus, Young’s modulus, Poisson’s ratio and Lame’s constants for both bulk Si (p = 0%) and Porous silicon (PS) are derived. The calculated results are found to be in good agreement with other experimental and theoretical ones. Also, the Debye temperature of PS is estimated from the average sound velocity. To our knowledge, the optical properties using specific models and elasticity of PS are reported for the first time.     

Structural,elastic, and lattice dynamical properties of YB2 compound

In this work, density functional theory calculations on the structural, mechanical, lattice dynamical, and thermodynamical properties of YB₂ in AlB₂-type and monoclinic (C2/m) structures are reported. The local density approximation has been used for modeling exchange–correlation effects. We have predicted the lattice constants, bulk modulus, elastic constants, shear modulus, Young’s modulus, Poison’s ratio, Debye temperature, and sound velocities. Furthermore, the phonon dispersion curves, corresponding phonon density of states, some thermodynamical quantities such as internal energy, entropy, heat capacity, and their temperature-dependent behaviors are presented. Our structural and some other results are in agreement with the available experimental and theoretical data.     

Dependence of pressure on elastic,electronic and optical properties of CeO2 and ThO2: A first principles study

The phase transformation of CeO₂ and ThO₂ from fluorite to cotunnite-type structure under pressure is predicted within the density functional theory implemented with the GGA-PW91 method, the pressure induced structural phase transition occurs at 28.9 GPa for CeO₂ and 29.8 GPa for ThO₂. These values are in excellent agreement with the experimentally measured data. The elastic, electronic and optical properties at normal as well as for high-pressure phase have been calculated, particular attention is devoted to the cotunnite phase. Further, the dependence of the elastic constants, the bulk modulus B, the energy band gaps and the dielectric function on the applied pressure are presented.     

First-principles calculations of structural,elastic and electronic properties of TaB2 with AlB2 structure under high pressure

We present a systematic theoretical study for the structural, elastic and electronic properties of TaB₂ with AlB₂ structure under pressures ranging from 0 GPa to 120 GPa within the framework of density-functional theory in this paper. The results at zero pressure are in good agreement with available theoretical and experimental values. Our attention has been focused on high pressure behavior of TaB₂. The pressure dependence of structure, elastic constants, Debye temperature, and density of states (DOS) are successfully calculated and discussed.     

First principles study of structural,electronic and elastic properties of lutatium mono-pnictides

The structural, electronic, elastic and thermal properties of two lutatium mono-pnictides (LuAs and LuSb) have been studied using the density functional theory within the generalized gradient approximation. The calculations indicate that there is a structural phase transition from their ambient NaCl – (B₁) to CsCl – (B₂) structure at 56.7 and 25.2 GPa along with the volume collapse percentage of 3% and 5%, respectively. Structural parameters like lattice constant (a₀), bulk modulus (B) and pressure derivative of the bulk modulus (B′) are presented. The calculated band structures indicate that B₁ and B₂ phase of these compounds are metallic. We have calculated the second order elastic constants for these compounds. We also compare the ground state (a₀ and B) and high pressure phase transition (P_t) properties for three members of lanthanide series.     

Structural,electronic, elastic properties and stabilities of hexagonal ZrNiAl alloy and its hydride ZrNiAlH0.67 under pressure

The structural, electronic, elastic properties and stabilities of hexagonal prototype alloy ZrNiAl and its saturated hydride ZrNiAlH_0.67 are investigated using the pseudopotential plane wave method within the generalized gradient approximation (GGA). The calculated structural parameters are in good agreement with the available experimental data. Partial covalent characters on ZrNiAl and ZrNiAlH_0.67 are verified by the calculations of PDOS (partial density of states) and overlap population. Band structures show both ZrNiAl and ZrNiAlH_0.67 belong to metals. The elastic constants and their pressure dependences are calculated using the static finite strain technique. From the analysis of the mechanical stabilities, hexagonal ZrNiAl is unstable at higher pressure than 29.34 GPa; that its hydride ZrNiAlH_0.67 is stable up to 50 GPa is similar with the experimental result of isostructural LaNiInD_1.63−x. Hydrogenation not only leads to strong lattice anisotropy but also leads to strong mechanical anisotropy.     

FP-LAPW investigation of structural,electronic, linear and nonlinear optical properties of ZnIn2Te4 defect-chalcopyrite

A theoretical study of structural, electronic, linear and nonlinear optical properties of ZnIn₂Te₄ defect-chalcopyrite is presented using the full-potential linearized augmented plane-wave (FP-LAPW) method. The exchange and correlation potential is treated by the generalized-gradient approximation (GGA). Moreover, the Engel and Vosko GGA formalism (EV-GGA) is also used to improve the band gap results. The lattice parameters (a, c) and the atomic positions (x, y and z) are optimized and found in good agreements with the available experimental data. Our calculations performed for band structure and density of state show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Γ resulting in a direct energy gap of about 0.89 eV for GGA and 1.20 eV for EV-GGA. The linear optical properties namely, the real and imaginary parts of the dielectric function and the reflectivity spectrum are calculated. This compound possesses a considerable negative birefringence. Based on the density functional theory the nonlinear optical properties are calculated and their spectra are analyzed.     

First principle calculation of physical properties of barium based chalcogenides BaM4S7 (M = Ga,Al); a DFT,DFT-D and hybrid functional HSE06 study

The electronic structure, elastic and optical properties have been calculated for the novel nonlinear optical (NLO) crystals BaQ₄S₇ (Q = Ga, Al) using plane wave pseudo-potential density functional theory (DFT) method as implemented in CASTEP and ABINIT codes. In this study we used both hybrid HSE06 and DFT-D functionals with GGA approximation. These NLO compounds, which belong to the mm2 point group, are particularly interesting because of their transparency in the mid-infrared region and wide energy band gap. We present results for electronic structure, elastic tensor coefficients, refractive indices and second order nonlinear optical susceptibilities. The calculated energy band gap and frequency dependent refractive indices as well as the NLO coefficients of BaGa₄S₇ are in good agreement with the experimental values. With no reported theoretical or experimental energy band gap and optical properties of BaAl₄S₇, we present for the first time its electronic structure and above mentioned optical coefficients. This compound has higher direct band gap with 3.74 eV, better optical birefringence and second-order NLO coefficients than most NLO compounds. The second-order NLO coefficients for BaAl₄S₇ have been calculated as d₃₁ = 3.15 pm/V, d₃₁ = 2.20 pm/V, d₃₃ = −6.31 pm/V.     

First principles calculations of structural,electronic, optical and thermodynamic properties of PbS,SrS and their ternary alloys Pb1−xSrxS

Using first-principle method, we investigate the structural, electronic, optical and thermodynamic properties of the strontium semiconductors Pb_1?xSr_xS with 25%, 50% and 75% of Sr. The calculations are performed by using the full-potential linearized augmented plane wave (FP-LAPW) method. As exchange–correlation potential we used the generalized gradient approximation (GGA) of Perdew et al. The variation of the calculated equilibrium lattice constant versus concentration shows that a small deviation from Vegard’s law is clearly visible with downward bowing parameter equal to 0.009 Å. The bulk modulus as a function of x for Pb_1?xSr_xS alloy shows a significant deviation from the linear concentration dependence (LCD) with downward bowing equal to 6.62 GPa. The different roles of structural and chemical effects on the gap bowing and its variation with composition are identified and discussed. In addition, density of states and optical properties were calculated and compared to the available experimental data and previous theoretical works. On the other hand, the thermodynamic stability of these alloys was investigated by calculating the excess enthalpy of mixing ΔH_m as well as the phase diagram.     

Effects of pressure on the elasticity and stability of zircon (ZrSiO4): First-principle investigations

We investigate the effects of pressure on the elasticity of zircon using the Local Density Approximation (LDA) within the density functional theory. Our LDA calculations predict the elastic constants (C_ij) with positive pressure derivatives, except C₆₆ whose pressure derivatives are nearly zero, showing small positive to negative departures with increasing pressure. We calculated Young’s moduli, E₁ and E₃ along the crystallographic axes a and c, respectively. At low pressures (<4 GPa) E₁ < E₃, but the relation reverses at pressures >4 GPa, implying a directional change of axial stiffness of zircon crystals. Our LDA calculated Hill’s average bulk (B) and shear (G) moduli show contrasting variations with pressure. B is more sensitive to pressure with positive derivatives varying from 2.25 to 5.63, as compared to much lower pressure derivatives (0.4) of G, which drops to negative values (?0.404) at pressures >7 GPa, and then take up positive values (1.3). The zircon to reidite phase transition, involving “bond switching mechanisms” [1] increases the magnitudes of both B and G, but shows relatively weak effects on their pressure derivatives. This study shows the effects of pressure on the degrees of shear and directional bulk modulus anisotropy of zircon. We also calculated its Debye temperature (708 K) from the elastic wave velocities, which fairly agrees to the available experimental data. Our study predicts that increasing pressure widens the electronic band gap of zircon, implying its greater insulation property at high pressures.     

Effects of Si addition on microstructure and mechanical properties of Mg–8Gd–4Y–Nd–Zr alloy

Effects of Si addition (1.0 wt.%) on microstructure and mechanical properties of Mg–8Gd–4Y–Nd–Zr alloy have been investigated using scanning electron microscopy (SEM) equipped with energy dispersive spectrum (EDS), X-ray diffraction (XRD), hardness measurements and tensile testing. The results indicated that the addition of Si led to the formation of Mg₂Si and (RE + Si)-rich particles, which enhanced the Young’s modulus of the alloy by 7 GPa while decreased the yield strength and ultimate strength by 10 MPa and 31 MPa, respectively. The tensile properties of the Mg–8Gd–4Y–Nd–Zr–Si alloy are as follows: Young’s modulus E = 51 GPa, yield strength σ_0.2 = 347 MPa, ultimate strength σ_b = 392 MPa and elongation δ = 2.7%. The increase in Young’s modulus was attributed to the formation of particles with high Young’s modulus, while the decrease in strength was ascribed to the decrease in volume fraction of metastable β′ precipitates caused by the consumption of rare earth atoms due to the formation of the rare earth containing particles.     

Ab initio calculation of vibrational and thermodynamic properties of SrX (S,Se, Te) in the B1 (NaCl) and B2 (CsCl) structures

Using density–functional perturbation theory, we have investigated the lattice dynamics of SrX (X = S, Se and Te) and their pressure dependence in both B1 (NaCl structure) and B2 (CsCl structure) phases. Static calculations predict the B1–B2 transition to occur at 17.7, 13.6 and 12.3 GPa for SrS, SrSe and SrTe respectively. The inclusion of zero-point vibrations lowers these values. The splitting varies little with pressure. The results compare well with available experiments. We have also used the calculated dispersions in conjunction with quasi-harmonic approximation to predict temperature and pressure dependence of various quantities such as the thermal expansion coefficient, the bulk modulus and the heat capacity. Our results for the thermal expansion coefficient show that it decreases with pressure and does not show any negative behaviour. The predicted transition pressure decreases with temperature.