Strain effects on electronic structure and superconductivity in the iron telluride

The influence of tensile strain in the ab-plane on crystal and electronic structure of FeTe has been studied ab initio. In superconducting FeSe the Fermi surface nesting with a vector q ∼ (0.5,0.5) × (2π/a) is believed to be crucial for rising superconductivity mediated by spin-fluctuations. The results presented here indicate that tensile-strained FeTe also exhibits such conditions. Furthermore, the Fermi surface changes, related to the increase of the lattice parameter a of this telluride, are opposite to analogous effects reported for FeSe. Since a recently transition from the double-stripe to the single-stripe magnetic order in FeTe under tensile strain in the ab-plane is associated with an occurrence of superconductivity in corresponding thin films, these findings allow for drawing a consistent picture of superconductivity in FeSe_1−xTe_x systems, in general.     

Electronic structure of non-centrosymmetric superconductor LaPdSi3 and its reference compound LaPdGe3

Electronic structures of a superconductor without inversion symmetry, LaPdSi₃, and its non-superconducting counterpart, LaPdGe₃, have been calculated employing the full-potential local-orbital method within the density functional theory. The investigations were focused on analyses of densities of states at the Fermi level in comparison with previous experimental heat capacity data and an influence of the antisymmetric spin–orbit coupling on the band structures and Fermi surfaces (FSs) being very similar for both considered here compounds. Their FSs sheets originate from four bands and have a holelike character, but exhibiting pronounced nesting features only for superconducting LaPdSi₃. It may explain a relatively strong electron–phonon coupling in the latter system and its lack in non-superconducting LaPdGe₃.     

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.     

First-principles study of electronic structure and magnetic properties of orthorhombic CrGa2Sb2 and MnGa2Sb2

By the first-principles calculations, we present the results of electronic structure and magnetic properties on bulk CrGa₂Sb₂ and MnGa₂Sb₂ in an orthorhombic structure with the linear chains of transition-metal Cr and Mn atoms, using four different exchange correlation potentials: the local density approximation (LDA), the generalized gradient approximation (GGA), GGA + U, and the Tran-Blaha modified Becke-Johnson functional (mBJ). The electronic structure calculations from four exchange correlation potentials show that CrGa₂Sb₂ is a pseudogap (negative gap) material with very small density of states (DOS) at the Fermi level, while MnGa₂Sb₂ has notably higher DOS at the Fermi level compared to CrGa₂Sb₂, exhibiting stronger metallic conductivity, although the mBJ potential obtains lower DOS at the Fermi level than LDA and GGA for both CrGa₂Sb₂ and MnGa₂Sb₂. The GGA + U method with a small value (1 eV) of the on-site Coulomb interaction parameter U obtains lower DOS at the Fermi level compared to the large value of U. In agreement with the measurement data, the total energy calculations reveal that both CrGa₂Sb₂ and MnGa₂Sb₂ have a stable ferromagnetic ground state with lower energies relative to antiferromagnetic state. Based on the Heisenberg model, the magnetic exchange constants between the nearest-neighbor Cr–Cr and Mn–Mn along transition-metal linear chains are calculated to be 48.6 meV and 27.5 meV for CrGa₂Sb₂ and MnGa₂Sb₂, respectively. By the mean-field approximation method, we calculated the Curie temperature of two compounds to be above room-temperature.     

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.     

Theoretical prediction of the electronic structure,bonding behavior and elastic moduli of scandium intermetallics

We report the structural, electronic, bonding, elastic and mechanical properties of nine scandium intermetallic compounds, ScTM (TM = Co, Rh, Ir, Ni, Pd, Pt, Zn, Cd and Hg), using ab initio density functional theory with the generalized gradient approximation for exchange and correlation potentials. The calculated structural parameters, such as the lattice constant (a₀), bulk modulus (B) and its pressure derivative (B₀) and elastic constants, are calculated using the CsCl-(B₂ phase) structure. The electronic and bonding properties of the ScX compounds are quantitatively analyzed using band structures, DOS, Fermi surfaces and contour plots. The mechanical properties and ductile behaviors of these compounds are also predicted based on the calculated elastic constants.     

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.     

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.     

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.     

Cluster expansion approach for modeling strain effects on alloy phase stability

We demonstrate applicability of the proposed extended cluster expansion (CE) technique, enabling complete representation of strain effects on alloy configurational energy on single and multiple lattices. Complete and orthonormal basis function to describe configurational energy is constructed in terms of spin variable ω on virtual lattice and σ on base lattice, where the former specifies the strain of a given cell from the base cell. We estimate formation energy of superlattice composed of alternate stacking of ordered structures for Cu–Au binary alloys where the strain effects should play significant role. The proposed CE is shown to precisely estimate the strain effects on total energy for alloys, which cannot be essentially handled by the current CE.     

Electronic structure and ferromagnetism of Fe11NiSi4, Fe11CoSi4, Fe11CrSi4 and Fe3Si from first principles

Based on the density functional theory (DFT), the plane-wave pseudopotential method was used to calculate structural stabilities, electronic structures, and ferromagnetism of Fe₃Si, Fe₁₁NiSi₄, Fe₁₁CoSi₄ and Fe₁₁CrSi₄ intermetallic compound. This study showed that the Fe₁₁NiSi₄ and Fe₁₁CrSi₄ phase are more stable than Fe₃Si phase, especially Fe₁₁NiSi₄, but decreased with Fe₁₁CoSi₄ phase. Calculating the density of states and the Mulliken electronic populations showed that Fe₁₁NiSi₄ had the highest structural stability because of its Fermi level, which was close to the bottom of the pseudo-gap. Fe₁₁NiSi₄ also had the largest Mulliken population, which increased the metallic bonding of the alloying system. The total magnetic moments of Fe₁₁NiSi₄, Fe₁₁CoSi₄ and Fe₁₁CrSi₄ were 20.04μ_B, 19.98μ_B, and 18.81μ_B, respectively. These magnetic moments mainly originated from the 3d spin polarization of Fe and those of additional atoms.     

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.     

Structural stability of some CsCl structure HfTM (TM = Co, Rh, Ru, Fe) compounds

In order to investigate Hf-TM (TM = Fe, Co, Rh, Ru) phase diagrams in the region of 50:50% atomic ratio, we performed ab initio Full-Potential Linearized Augmented Plane Waves calculations of the most stable Hf and TM elemental phases and HfTM compounds of the CsCl and CuAu structure types. The obtained electronic structures, cohesive energies and enthalpies of formation are discussed and compared to some of the existing models and available experimental data. The non-existing compound HfFe is found to be at least metastable, and the reason for its absence from the phase diagram is discussed.     

Electronic and transport properties of Mg2Si under isotropic strains

Density functional theory and Boltzmann theory calculations of the electronic and thermoelectric properties of Mg₂Si subjected to isotropic strains have been performed. The electrical conductivity, the Seebeck coefficient and the power factor have been evaluated at two temperatures (300 K and 900 K) and two charge carrier concentrations (10¹⁸ cm⁻³ and 1.2 × 10²⁰ cm⁻³ electrons and holes). Up to 3% of both compressive and tensile strains have been applied to the material. From our results, we can highlight that a significant improvement of both the Seebeck coefficient (S) and the power factor (PF) is obtained at low temperature and moderate n-doping. The increase in S and PF amounts to 40% and 100%, respectively, compared to the unstrained Mg₂Si.     

Ab initio based study of finite-temperature structural,elastic and thermodynamic properties of FeTi

We employ density functional theory (DFT) to calculate pressure dependences of selected thermodynamic, structural and elastic properties as well as electronic structure characteristics of equiatomic B2 FeTi. We predict ground-state single-crystalline Young's modulus and its two-dimensional counterpart, the area modulus, together with homogenized polycrystalline elastic parameters. Regarding the electronic structure of FeTi, we analyze the band structure and electronic density of states. Employing (i) an analytical dynamical matrix parametrized in terms of elastic constants and lattice parameters in combination with (ii) the quasiharmonic approximation we then obtained free energies, the thermal expansion coefficient, heat capacities at constant pressure and volume, as well as isothermal bulk moduli at finite temperatures. Experimental measurements of thermal expansion coefficient complement our theoretical investigation and confirm our theoretical predictions. It is worth mentioning that, as often detected in other intermetallics, some materials properties of FeTi strongly differ from the average of the corresponding values found in elemental Fe and Ti. These findings can have important implications for future materials design of new intermetallic materials.     

Stability,elastic and electronic properties of the Rh–Zr compounds from first-principles calculations

A systematic investigation on structural, elastic and electronic properties of Rh–Zr intermetallic compounds is conducted using first-principles electronic structure total energy calculations. The equilibrium lattice parameters, enthalpies of formation (E_for), cohesive energies (E_coh) and elastic constants are presented. Of the eleven considered candidate structures, Rh₄Zr₃ is most stable with the lowest E_for. The two orthogonal-type, relative to the CsCl-type, are the competing ground-state structures of RhZr. The result is in agreement with the experimental reports in the literature. The analysis of E_for and mechanical stability excludes the presence of Rh₂Zr and RhZr₄ at low temperature mentioned by .Curtarolo et al. [Calphad 29, 163 (2005)]. It is found that the bulk modulus B increases monotonously with Rh concentration, whereas all other quantities (shear modulus G, Young's modulus E, Poisson's ratio σ and ductility measured by B/G) show nonmonotonic variation. RhZr₂ exhibits the smallest shear/Young's modulus, the largest Poisson's ratio and ductility. Our results also indicate that all the Rh–Zr compounds considered are ductile. Furthermore, the detailed electronic structure analysis is implemented to understand the essence of stability.     

First principles calculations of the stability of the T2 and D88 phases in the V–Si–B system

The crystal and electronic structures of D8_l-V₅SiB₂ and D8₈-V₅Si₃B ternary compounds have been investigated by means of first principle calculations. The calculated structural parameters are in very good agreement with the experimental data. The calculated values of the enthalpies of formation at T = 0 K of the D8_l-V₅SiB₂ and D8₈-V₅Si₃B ternary compounds are −67.1 and −62.1 kJ/mol of atoms respectively. The total and partial electronic densities of states show a strong hybridization between the B p states and V d states. The defect enthalpies of formation as well as the mixing enthalpies have been computed. These data are essential for the modeling of the D8_l and D8₈ phases in the V–Si–B ternary system. A partial V–VSi₂–VB isothermal section at 298 K is proposed.