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.     

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.     

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.     

Lithium solubility limit in ZnGa2O4:Bi0.0013+,Li+ phosphor

The lithium solubility limit, photoluminescence (PL) and photoluminescence excitation (PLE) properties of lithium ion co-activated ZnGa₂O₄:Bi³⁺,Li⁺ phosphor have been investigated. A LiGaO₂ second phase began to appear from 3 mol% Li⁺ ion co-activated ZnGa₂O₄:Bi³⁺,Li⁺ phosphor. The enhanced brightness of blue (λ_ex = 254 nm) and white (λ_ex = 315 nm) colors of bismuth ions doped ZnGa₂O₄:Bi³⁺,Li⁺ phosphor was assigned to the formation of LiGaO₂. Bi³⁺ activated lithium zinc gallate phosphor showed a more enhanced PLE peak around 315 nm than that of lithium zinc gallate phosphor when λ_em = 520 nm. Thus, we observed that the PL intensity of ZnGa₂O₄:Bi³⁺,Li⁺ phosphor with λ_em = 520 nm was much greater than that of ZnGa₂O₄:Li⁺ phosphor. Also, ZnGa₂O₄:Bi³⁺,Li⁺ phosphor exhibited a shorter decay time than that of ZnGa₂O₄:Li⁺ phosphor by about a factor of about 2.     

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.     

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.     

Synthesis of mesoporous Zn–Al spinel oxide nanorods with membrane like morphology

A well-aligned ZnAl₂O₄ spinel oxide nanorods with hierarchical pore structure was synthesized by a simple route using polycarbonate membrane as the sacrificial template. Spinel formation and the template removal occurred in one-step by calcination at 550 °C. SEM analysis of the calcined sample showed a membrane morphology made up of aligned rods with micron sized pores in between them. TEM analysis revealed that individual rods are 130 ± 30 nm in diameter and are nanoporous in nature. These rods are made up of nanoparticles of the sizes ranging from 5 to 12 nm. HRTEM analysis further showed that these nanoparticles are crystalline with the lattice spacing in commensurate with the ZnAl₂O₄ spinel oxide. Adsorption–desorption study shows Type IV isotherm with the pore size around 3.4 nm and high specific surface area of about 140 m²/g.     

Structural,elastic, and lattice dynamical properties of Germanium diiodide (GeI2)

To investigate the structural, elastic, and lattice dynamical properties of the germanium diiodide, we have performed the first-principles calculations by using the local density approximation method based on density-functional theory. Some basic physical parameters such as lattice constant, bulk modulus and its first derivatives, elastic constants, shear modulus, Young’s modulus, and Poisson’s ratio are calculated. The phonon dispersion curves, electronic band-structures, and total and partial density of states have also been calculated for ground state C6 phase of GeI₂. Our results show that this structure has got 1.72 eV direct band gap. Our secondary results on the temperature-dependent behavior of thermodynamical properties such as entropy, heat capacity, internal energy, and free energy are also presented for the same compounds. The obtained results are in good agreement with the available experimental and other theoretical data.     

Structural stabilities,elastic and thermodynamic properties of Scandium Chalcogenides via first-principles calculations

The high-pressure structural (B1–B2) phase transition and the elastic properties of ScS and ScSe are studied using the full-potential augmented plane wave plus local orbitals method (FP-APW + LO) with the generalized-gradient approximation (GGA) exchange-correlation functional. The elastic constants and their pressure dependence are calculated following the total energy variation with strain technique. The stability and the ductility mechanisms for these compounds are discussed via the electronic density of states (DOS) and the elastic constants C_ij. The thermodynamic properties of (B1) structure are predicted through the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. The variation of bulk moduli, the thermal expansion coefficient, the heat capacities and the Debye temperature with pressure and temperature are successfully obtained. To our knowledge this is the first quantitative theoretical prediction of the elastic, high pressure and thermal properties for the investigated compounds and still awaits experimental confirmations.     

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.     

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.     

Modulations in structural and ferroelectric properties due to tensile strain in BiFeO3 films on MgAl2O4 substrates induced by thermal-expansion

The effect of tensile strain on structural and ferroelectric properties of BiFeO₃ epitaxial films was investigated. The films grown by pulsed laser deposition on MgAl₂O₄ (0 0 1) substrates revealed monoclinic structure deviated from the bulk rhombohedral structure due to a tensile strain along the in-plane direction. The strain is induced by the difference in thermal expansion coefficients between the film and the substrate. A Poisson ratio is calculated from the in-plain and out-of-plain lattice constants at different temperatures measured by reciprocal space maps of X-ray diffraction. The small Poisson ratio compared to the bulk suggests a weaker elastic response at high temperature. The ferroelectric polarization of the tensile-strained film along the (0 0 1) is also decreased from the bulk value.     

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.     

Theoretical investigation on the transition-metal borides with Ta3B4-type structure: A class of hard and refractory materials

Based on density functional theory, we have systematically studied the structural stability, mechanical properties and chemical bonding of the transition-metal borides M₃B₄ (M = Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) for the first time. All the present studied M₃B₄ have been demonstrated to be thermodynamically and mechanically stable. The bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, microhardness, Debye temperature and anisotropy have been derived for ideal polycrystalline M₃B₄ aggregates. In addition, the relationship between Debye temperature and microhardness has been discussed for these isostructral M₃B₄. Furthermore, the results of the Cauchy pressure, the ratio of bulk modulus to shear modulus, and Poisson’s ratio suggest that the valence electrons of transition metals play an important role in the ductility of M₃B₄. The calculated total density of states for M₃B₄ indicates that all these borides display a metallic conductivity. By analyzing the electron localization function, we show that the improvement of the ductility in these M₃B₄ might attribute to the decrease of their angular bonding character.     

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.     

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.     

Microwave dielectric properties of ZnAl2O4–TiO2 spinel-based composites

The microstructures, phase compositions and microwave dielectric properties of ZnAl₂O₄–TiO₂ spinel-based composites have been investigated. It is found that ZnAl₂O₄ cannot form a solid solution with TiO₂. As TiO₂ content increases, the ε_r and τ_f values increase gradually, while the Q · f values degrade by degrees. Under the same amount of TiO₂ content, the ε_r and Q · f values increase initially and then decrease slightly with increasing sintering temperature, while the τ_f values increase slowly. The optimal microwave dielectric properties are achieved in (1 − x)ZnAl₂O₄–xTiO₂ (x = 0.21) sintered at 1500 °C for 3 h with ε_r value of 11.4, Q · f value of 71,810 GHz (at about 6.5 GHz), and τ_f value of − 0.5 ppm/°C.     

Synthesis and characterization of ultrafine Ln2Ti2O7 (Ln = Sm,Gd, Dy,Er) pyrochlore oxides by stearic acid method

Stearic acid method (SAM) was developed to synthesize series of pyrochlore Ln₂Ti₂O₇ (Ln = Sm, Gd, Dy, Er) nanocrystals. The synthesis process was monitored by X-ray diffraction, Thermal–gravimetric–differential thermal analysis and Fourier Transform InfraRed methods. Comparing with traditional solid-state reaction (SSR), Ln₂Ti₂O₇ can be synthesized at relatively low temperature (700–800 °C) with shortened reaction time (2–4 h). The average particle size of Ln₂Ti₂O₇ was greatly reduced (ca. 40 nm) and the BET surface area was increased (ca. 12 m²/g) by using SAM. From the X-ray diffraction patterns, we found that Ln has an effect on the crystal structure of Ln₂Ti₂O₇, every lattice peak shifted to larger angle slightly with the increasing atomic number of Ln. Also, the lattice constant of Ln₂Ti₂O₇ was calculated by Jade.5 and found it decreased along with the decrease of ionic radius of Ln³⁺. The morphology of obtained Ln₂Ti₂O₇ was determined by transmission electron microscopy technique. Results showed that the obtained Ln₂Ti₂O₇ were all square-like and the interplanar distance of Ln₂Ti₂O₇ (Ln = Sm, Gd, Dy, Er) according to (111) plane was 0.65, 0.64, 0.63, and 0.62 nm respectively, which was measured from High Resolution Transmission Electron Microscopy images. Possible reason for this phenomenon was presented.     

Structural and thermodynamic properties of antiperovskite SbNMg3

The full potential augmented plane wave plus local orbital (FP-LAPW + lo) method using the generalized-gradient approximation within the framework of density functional theory is applied to the study of the lattice constant, bulk modulus, pressure derivative of the bulk modulus and elastic constants of antiperovskite semiconductor SbNMg₃. The quasi-harmonic Debye model, in which the phononic effects are considered, is applied to the study of the thermodynamic properties. The temperature effect on the structural parameters, bulk modulus, thermal expansion coefficient, specific heats and Debye temperatures in the whole pressure range from 0 to 30 GPa and temperature range from 0 to 1200 K.