First-principles investigations on structural,elastic, thermodynamic and electronic properties of Ni3X (X = Al,Ga and Ge) under pressure

The structural, elastic, thermodynamic and electronic properties of L1₂-ordered intermetallic compounds Ni₃X (X = Al, Ga and Ge) under pressure range from 0 to 50 GPa with a step of 10 GPa have been investigated using first-principles method based on density functional theory (DFT). The calculated structural parameters of Ni₃X at zero pressure and zero temperature are consistent with the experimental data. The results of bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio v, anisotropy index A^U and Debye temperature Θ_D increase with the increase of external pressure. In addition, the Debye temperature of these compounds gradually reduce as the order of Ni₃Al > Ni₃Ga > Ni₃Ge. The ratio of shear modulus to bulk modulus G/B shows that the three binary compounds are ductile materials, and the ductility of Ni₃Al and Ni₃Ga can be improved with pressure going up, while Ni₃Ge is opposite. Finally, the pressure-dependent behavior of density of states, Mulliken charge and bond length are analyzed to explore the physical origin of the pressure effect on the structural, elastic and thermodynamic properties of Ni₃X.     

第一性原理研究Al-Y合金在压力作用下的晶胞结构、力学性质、热力学性质和电子结构

The influence of structural, elastic properties, thermodynamics and electronic properties Al-Y alloy were investigated by using first-principles. The equilibrium lattice constant, elastic constants, and elastic modulus as calculated here agree with results of previous studies. Calculated results of bulk modulus B, shear modulus G, Young’s modulus E, Poisson’s ratio v and Debye temperature all increase as pressure increase, but the opposite is true for heat capacity cp. In addition, the Debye temperature for the phases reduces gradually as follows: Al2Y > Al3Y> AlY. Additionally, the G/B ratio indicates that AlY and Al3Y are ductile materials, while Al2Y is a brittle material, and that the ductility of AlY and Al3Y can be improved with increased pressure, while the brittleness of Al2Y does not improve with increased pressure. Finally, the paper presents and discusses calculations of density of states and charge populations as they are affected by pressure.     

First-principle calculations of structural,elastic and thermodynamic properties of Fe–B compounds

The structural, elastic and thermodynamic properties of FeB, Fe₂B, orthorhombic and tetrahedral Fe₃B, FeB₂ and FeB₄ iron borides are investigated by first-principle calculations. The elastic constants and polycrystalline elastic moduli of Fe–B compounds are usually large especially for FeB₂ and FeB₄, whose maximum elastic constant exceeds 700 GPa. All of the six compounds are mechanically stable. The Vickers hardness of FeB₂ is estimated to be 31.4 GPa. Fe₂B and FeB₂ are almost isotropic, while the other four compounds have certain degree of anisotropy. Thermodynamic properties of Fe–B compounds can be accurately predicted through quasi-harmonic approximation by taking the vibrational and electronic contributions into account. Orthorhombic Fe₃B is more stable than tetrahedral one and the phase transition pressure is estimated to be 8.3 GPa.     

First-principle calculations of the electronic,elastic and thermoelectric properties of Ag doped CuGaTe2

To improve the performance of a thermoelectric material CuGaTe₂, element Ag is doped to replace element Ga and we investigate the electronic structure, phase stability, elastic and thermoelectric properties of CuGa_1−xAg_xTe₂ (x = 0, 0.25 and 0.5) via first-principles method. The phase stability of CuGa_1−xAg_xTe₂ is discussed by analyzing the formation energy, cohesive energy and elastic constants. The calculated sound velocities decrease with the increase of Ag content, which is favorable for reducing the lattice thermal conductivity. The analysis of band structures shows that the replacement of Ga by Ag makes CuGaTe₂ undergo a direct-indirect semiconductor transition. The Ag doping induces steep density of states in valence band edge, which is beneficial for increasing the carrier concentration and improving thermoelectric performance of CuGaTe₂.     

Temperature-dependent mechanical properties of alpha-/beta-Nb5Si3 phases from first-principles calculations

The temperature-dependent structural properties and anisotropic thermal expansion coefficients of α-/β-Nb₅Si₃ phases have been determined by minimizing the non-equilibrium Gibbs free energy as functions of crystallographic deformations. The results indicate that the crystal anisotropy of α-Nb₅Si₃ phase is much more temperature dependence than that of β-Nb₅Si₃ phase. The total/partial density of states of α-/β-Nb₅Si₃ phases are discussed in detail to analyze their electronic hybridizations. It is demonstrated that the bonding of the two phases is mainly contributed from the hybridization between Nb-4d and Si-3p electronic states. The temperature-dependent mechanical properties of α-/β-Nb₅Si₃ phases are further investigated via the quasi-harmonic approximation method in coupling with continuum elasticity theory. The calculated single-crystalline and polycrystalline elasticity shows that both phases are mechanically stable and exhibit the intrinsic brittleness. The results also suggest that α-Nb₅Si₃ phase possesses a superior ability of compression resistance but an inferior ability of high-temperature resistance of mechanical properties than those of β-Nb₅Si₃ phase. The bonding features of α-/β-Nb₅Si₃ phases are discussed by means of charge density difference analysis in order to explain the difference of the temperature-dependent mechanical properties between the two phases.     

Ab-initio calculation of enthalpies of formation of intermetallic compounds and enthalpies of mixing of solid solutions

A large number of ab-initio calculations of energies of formation of intermetallic compounds have been performed in the last 15 years. The currently used methods are listed. The paper presents a review of the aluminium based compounds which have been studied. Comparisons of calculated and experimental enthalpies of formation are provided for aluminim-3d and-4d transition metal alloys at equiatomic composition. The modelling of the enthalpies of mixing of solid solutions based on a given lattice is described.     

Enthalpies of formation of magnesium compounds from first-principles calculations

An energetics database of binary magnesium compounds has been developed from first-principles calculations. The systems investigated include Mg–X (X = As, Ba, Ca, Cd, Cu, Dy, Ga, Ge, La, Lu, Ni, Pb, Sb, Si, Sn and Y). The calculated lattice parameters and enthalpies of formation of binary compounds in these systems are compared with both experimental data and thermodynamic databases.     

Activity measurement of gallium in the B2 phase of the CoGa–Sb system by the EMF method with zirconia-based solid electrolyte

Gallium activity in the B2 phase regions of both binary Co–Ga and ternary Co–Ga–Sb systems was measured by EMF method with stabilized zirconia solid electrolyte The temperature range was 1073–1273 K and Sb concentrations were 1, 2 and 3 mol fractions. Ga activity at 1173 and 1273 K increases sharply in Ga rich region and the addition of Sb to the CoGa phase increases Ga activity. Activity change corresponds to the lattice parameter change with Sb addition to the CoGa phase.     

First-principle study of magnetic,elastic and thermal properties of full Heusler Co2MnSi

In this work, first principles calculation of structural, electronic magnetic and elastic properties of the half-metallic ferromagnetic Heusler compound Co₂MnSi are presented. We have applied the full-potential linearized augmented plane waves plus local orbitals (FP-L/APW+lo) method based on the density functional theory (DFT). For the exchange and correlation potential generalized-gradient approximation (GGA) is used. The computed equilibrium lattice parameters agree well with the available theoretical and experimental data. Elastic constants and their pressure dependence are also calculated. The calculated total magnetization of 5 μ_B is in excellent agreement with recent experiments. We also presented the thermal effects using the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. Temperature and pressure effects on the structural parameters, heat capacities, entropy, thermal expansion coefficient, and Debye temperatures are determined from the non-equilibrium Gibbs functions.     

Site preference and alloying effect on elastic properties of ternary B2 RuAl-based alloys

The structural and elastic properties of ternary B2 RuAl-based alloys are studied using first-principles calculations. Single-crystal elastic constants, atomic volumes, transfer energies, and electronic densities for RuAl-TM are computed, considering all possible transition-metal solute species TM. Calculated elastic constants are used to compute values of some commonly considered elasticity parameters, such as bulk modulus, shear modulus, Yong's modulus, Pugh ratio, and Cauchy pressure. The present results suggest that the bulk modulus of RuAl-TM increase approximately linearly with increasing electron density. Calculated elastic properties are in favorable accord with available experimental and theoretical data.     

Synergistic effect of co-alloying elements on site preferences and elastic properties of Ni3Al: A first-principles study

The site preferences of co-alloying elements (Mo–Ta, Mo–Re, Mo–Cr) in Ni₃Al are studied using first-principles calculations, and the effects of these alloying elements on the elastic properties of Ni₃Al are evaluated by elastic property calculations. The results show that the Mo–Ta, Mo–Re and Mo–Cr atom pairs all prefer Al–Al sites and the spatial neighbor relation of substitution sites almost has no influence on the site preference results. Furthermore, the Young's modulus of Ni₃Al increases much higher by substituting Al–Al sites with co-alloying atoms, among which Mo–Re has the best strengthening effect. The enhanced chemical bondings between alloying atoms and their neighbor host atoms are considered to be the main strengthening mechanism of the alloying elements in Ni₃Al.     

Electronic structure,phase stability and elastic properties of ruthenium based four intermetallic compounds: Ab-initio study

The structural, electronic and elastic properties of four RuX (X = Sc, Ti, V and Zr) intermetallic compounds have been investigated by using density functional theory within full potential linearized augmented plane wave method and using generalized gradient approximations in the scheme of Perdew, Burke and Ernzrhof (PBE), Wu and Cohen (WC) and Perdew et al. (PBEsol) for the exchange correlation potential. The relative phase stability in terms of volume-energy and enthalpy-pressure for these compounds is presented for the first time in three different (B₁, B₂ and B₃) structures. The total energy is computed as a function of volume and fitted to Birch equation of states to find the ground state properties such as lattice constant (a₀), bulk modulus (B) and its pressure derivative (B′). It is found that the lattice parameters in B₂-phase agree well with the existing experimental and previous theoretical results. The second order elastic constants (SOECs) are also predicted for the above compounds. All the four compounds show ductile behavior. The ductility of these compounds has been analyzed using Pugh's rule. From the plots of electronic density of states (DOS), it can be concluded that these intermetallic compounds are metallic in nature.     

A first principles study of structural and mechanical properties of Ti2AlZr intermetallic

The present work describes the structural and mechanical behaviour of three phases namely B2, D0₁₉ and O phases of Ti₂AlZr intermetallic using first principles density functional theory (DFT) within generalized gradient approximation (GGA). The equilibrium lattice constant values of B2, D0₁₉ and O phases are in good agreement with the experimental and theoretical data, respectively. Formation energy of O phase is minimum followed by D0₁₉ and B2. Bonding characteristics of these phases have been explained based on electronic density of states and charge density. All the three phases satisfy the Born stability criteria in terms of elastic constants and are associated with ductile behaviour based on G/B ratios. The B2 phase exhibits very high anisotropy in comparison to those of the D0₁₉ and O.     

Thermodynamic properties of the ternary system InSb–NiSb–Sb in the temperature range 640–860 K

The ternary InSb–NiSb–Sb system has been studied by X-ray diffraction and by potentiometry. The electromotive forces (EMF) have been measured in the temperature range 640<T/K<860 by using the following galvanic cell:

with x (0.075<x<0.498) and y (0<y<0.359). The investigated samples are located on the following lines of the Gibbs triangle: InSb–Ni_0.33Sb_0.66, InSb–Ni_0.48Sb_0.52, InSb–NiSb, Sb–(InSb)_0.75(NiSb)_0.25, Sb–(InSb)_{0. 5}(NiSb)_0.5, Sb–(InSb)_0.25(NiSb)_0.75. From these measurements, the values of the partial molar thermodynamic functions (Δμ°_m,In, ΔH°_m,In, ΔS°_m,In) (data at reference pressure p⁰=10⁵ Pa), for the liquid InSb alloy, for the three solid heterogeneous regions InSb–NiSb₂–Sb, InSb–NiSb_1±δ?–NiSb₂, InSb–NiSb_1±δ, for six ternary liquid–solid alloys, have been calculated.     

Effect of the tetragonal distortion on the electronic structure,phonons and superconductivity in the Mo3Sb7 superconductor

Effect of tetragonal distortion on the electronic structure, dynamical properties and superconductivity in Mo₃Sb₇ is analyzed using first principles electronic structure and phonon calculations. Rigid muffin tin approximation (RMTA) and McMillan formulas are used to calculate the electron–phonon coupling constant λ and superconducting critical temperature. Our results show, that tetragonal distortion has small, but beneficial effect on superconductivity, slightly increasing λ, and the conclusion that the electron–phonon mechanism is responsible for the superconductivity in Mo₃Sb₇ is supported. The spin-polarized calculations for the ordered (ferromagnetic or antiferromagnetic), as well as disordered (disordered local moment) magnetic states yielded non-magnetic ground state. We point out that due to its experimentally observed magnetic properties the tetragonal Mo₃Sb₇ might be treated as noncentrosymmetric superconductor, which could have influence for the pairing symmetry. In this context the relativistic band structure is calculated and spin–orbit interaction effects are discussed.     

Density functional study of the mechanical and phonon properties of Al12X (X = Mo,Tc, Ru,W, Re,and Os) compounds

By means of first principles calculations, we have studied the structural, elastic, and phonon properties of the Al₁₂X (X = Mo, Tc, Ru, W, Re, and Os) compounds in cubic structure. The elastic constants of these compounds are calculated, then bulk modulus, shear modulus, Young's modulus, Possion's ratio, Debye temperature, hardness, and anisotropy value of polycrystalline aggregates are derived and relevant mechanical properties are compared with the available theoretical ones. Furthermore, the phonon dispersion curves, mode Grüneisen parameters, and thermo-dynamical properties such as free energy, entropy and heat capacity are computed and the obtained results are discussed in detail.     

Structural,elastic, electronic,and thermodynamic properties of intermetallic Zr2Cu: A first-principles study

Three competing structures (C11_b, C16 and E9₃) of intermetallic Zr₂Cu have been systematically investigated by first-principles calculations and quasi-harmonic Debye model. Both the calculated equation of states (EOS) and pressure–enthalpy results indicate a structural phase transition from C11_b to C16 phase at around 11–14 GPa. The calculated equilibrium crystal parameters and elastic constants are in consistence with available experimental or theoretical data. All three phases are mechanically stable according to the elastic stability criteria, and ductile according to Pugh's ratio, while the ambient-stable C11_b phase shows a higher elastic anisotropy. Furthermore, differences in the nature of bonding between three competing structures are uncovered by electron density topological analysis. C11_b Zr₂Cu possesses an intriguing pseudo BaFe₂As₂-type structure with the charge density maxima at Zr tetrahedral interstices serving as Fe-position pseudoatoms; C16 Zr₂Cu contains Zr-pair configurations bonded through bifurcated Zr–Zr bonding paths; while the E9₃ phase has only conventional straight bonding. Additionally, through quasi-harmonic Debye model, the pressure and temperature dependences of the bulk modulus, specific heat, Debye temperature, Grüneisen parameter and thermal expansion coefficient for three phases are obtained and discussed.     

First principle calculations of elastic and thermodynamic properties of Ir3Nb and Ir3V with L12 structure under high pressure

The structural and elastic properties of the L1₂ structure Ir₃Nb and Ir₃V under pressure have been investigated by means of the first principles calculations based on the density functional theory within the generalized gradient approximation. The lattice parameters of Ir₃Nb and Ir₃V obtained by minimization of the total energy are consistent with the available experimental and other theoretical results. In addition, the elastic constants (C₁₁, C₁₂, C₄₄) of Ir₃Nb and Ir₃V show that they are mechanical stable structures under pressure. The values of B/G exhibit an upward trend with increasing pressure, which means its ductility increased. When the pressure reaches 45 GPa, the Cauchy pressures and B/G values reveal that Ir₃Nb and Ir₃V change from brittle to ductile. Finally, through quasi-harmonic Debye model, the temperature and pressure dependences of thermodynamic properties are predicted in a wide pressure (0–50 GPa) and temperature (0–1200 K) ranges.