First principles study of the structural,electronic, mechanical and superconducting properties of WX (X=C,N)

The structural, electronic, mechanical and superconducting properties of tungsten carbide (WC) and tungsten nitride (WN) are investigated using first principles calculations based on density functional theory (DFT). The computed ground state properties, such as equilibrium lattice constant and cell volume, are in good agreement with the available experimental data. A pressure induced structural phase transition is observed in both tungsten carbide and nitride, from a tungsten carbide phase (WC) to a zinc blende phase (ZB), and from a zinc blende phase (ZB) to a wurtzite phase (WZ). The electronic structure reveals that these materials are metallic at ambient conditions. The calculated elastic constants obey the Born-Huang criteria, suggesting that they are mechanically stable at normal and high pressure. Also, the superconducting transition temperature is estimated for the WC and WN in stable structures at atmospheric pressure.     

First principles study on the structural, electronic, and elastic properties of Na-As systems

We have performed the first principles calculation by using the plane-wave pseudopotential approach with the generalized gradient approximation for investigating the structural, electronic, and elastic properties Na-As systems (NaAs in NaP, LiAs and AuCu-type structures, NaAs₂ in MgCu₂-type structure, Na₃As in Na₃As, Cu₃P and Li₃Bi-type structures, and Na₅As₄ in A₅B₄-type structure). The lattice parameters, cohesive energy, formation energy, bulk modulus, and the first derivative of bulk modulus (to fit to Murnaghan’s equation of state) of the related structures are calculated. The second-order elastic constants and the other related quantities such as Young’s modulus, shear modulus, Poisson’s ratio, sound velocities, and Debye temperature are also estimated.     

Structural,electronic and mechanical properties of alkaline earth metal oxides MO (M=Be,Mg, Ca,Sr, Ba)

The structural, electronic and mechanical properties of alkaline earth metal oxides MO (M=Be, Mg, Ca, Sr, Ba) in the cubic (B1, B2 and B3) phases and in the wurtzite (B4) phase are investigated using density functional theory calculations as implemented in VASP code. The lattice constants, cohesive energy, bulk modulus, band structures and the density of states are computed. The calculated lattice parameters are in good agreement with the experimental and the other available theoretical results. Electronic structure reveals that all the five alkaline earth metal oxides exhibit semiconducting behavior at zero pressure. The estimated band gaps for the stable wurtzite phase of BeO is 7.2 eV and for the stable cubic NaCl phases of MgO, CaO, SrO and BaO are 4.436 eV, 4.166 eV, 4.013 eV, and 2.274 eV respectively. A pressure induced structural phase transition occurs from wurtzite (B4) to NaCl (B1) phase in BeO at 112.1 GPa and from NaCl (B1) to CsCl (B2) phase in MgO at 514.9 GPa, in CaO at 61.3 GPa, in SrO at 42 GPa and in BaO at 14.5 GPa. The elastic constants are computed at zero and elevated pressures for the B4 and B1 phases for BeO and for the B1 and B2 phases in the case of the other oxides in order to investigate their mechanical stability, anisotropy and hardness. The sound velocities and the Debye temperatures are calculated for all the oxides using the computed elastic constants.     

Structural stability,electronic structure and mechanical properties of 4d transition metal nitrides TMN (TM=Ru,Rh, Pd)

Ab initio calculations are performed to investigate the structural stability, electronic, structural and mechanical properties of 4d transition metal nitrides TMN (TM=Ru, Rh, Pd) for five different crystal structures, namely NaCl, CsCl, zinc blende, NiAs and wurtzite. Among the considered structures, zinc blende structure is found to be the most stable one among all three nitrides at normal pressure. A structural phase transition from ZB to NiAs phase is predicted at a pressure of 104 GPa, 50.5 GPa and 56 GPa for RuN, RhN and PdN respectively. The electronic structure reveals that these nitrides are metallic. The calculated elastic constants indicate that these nitrides are mechanically stable at ambient condition.     

First principles study of structural stability, electronic and related properties of (NH4)2SO4

The electronic structure and related physical properties of crystalline ammonium sulfate, (NH₄)₂SO₄, have been studied using the first principles code CRYSTAL06 at the B3LYP level of theory. The title compound has been found to possess one stable and three metastable configurations, all within the polar space group Pna2₁ (no. 33). Two of the metastable polymorphs are newly predicted and have not yet been observed experimentally. The different configurations show considerably varying magnitudes of the spontaneous polarization P_s. All coefficients of the elastic stiffness tensor, c_kl, and elasto-electrical tensor, e_ki have been calculated for the first time and have been found to agree satisfactorily with experimental data, as far as available.     

Structural phase stability,electronic structure and mechanical properties of alkali metal hydrides AMH4 (A=Li,Na; M=B,AL)

The structural stability of Alkali metal hydrides AMH₄ (A=Li, Na; M=B, Al) is analyzed among the various crystal structures, namely hexagonal (P6₃mc), tetragonal (P4₂/nmc), tetragonal (P-42₁c), tetragonal (I4₁/a), orthorhombic (Pnma) and monoclinic (P2₁/c). It is observed that, orthorhombic (Pnma) phase is the most stable structure for LiBH₄, monoclinic (P2₁/c) for LiAlH₄, tetragonal (P4₂/nmc) for NaBH₄ and tetragonal (I4₁/a) for NaAlH₄ at normal pressure. Pressure induced structural phase transitions are observed in LiBH₄, LiAlH₄, NaBH₄ and NaAlH₄ at the pressures of 4 GPa, 36.1 GPa, 26.5 GPa and 46 GPa respectively. The electronic structure reveals that these metal hydrides are wide band gap insulators. The calculated elastic constants indicate that these metal hydrides are mechanically stable at normal pressure.     

First principle study of structural,electronic, mechanical,dynamic and optical properties of half-Heusler compound LiScSi under pressure

In this study, several physical properties of LiScSi compound with MgAgAs phase were investigated via the plane-wave pseudo-potential technique in density functional theory (DFT). The calculated total energy-atomic volume was fitted to the Murnaghan equation of state in order to obtain bulk modulus, their first derivatives and the lattice constant. These results were compared to findings of recent literature. Afterwards, the partial density of states (PDOS) and charge density differences were used to evaluate the electronic band structure of LiScSi under pressure. By analysing elastic properties (shear modulus, Poisson ratio, Young’s modulus, etc.) of the material, it has been shown that MgAgAs phase of the compound is mechanically stable under pressure. Moreover, the dynamical stability of this compound is calculated by means of the phonon dispersion curves and one-phonon DOS. Finally, the optical properties and related parameters (refractive index, dielectric function, and loss function) of LiScSi were examined with subject to different pressures.     

Mn掺杂Fe2B晶体结构、力学性能及电子结构的第一性原理研究

基于密度泛函理论的第一性原理计算方法,本文系统地研究了Fe8-xMnxB4 (x = 0, 0.25, 0.5, 1, 2, 3, 4, 5, 6, 7 ,8)的晶体结构、机械性能和电子结构。计算得到Fe2B的晶格常数与实验值相符,所有相都具有良好的热力学稳定性和机械稳定性。随着Mn掺杂浓度逐渐增大,Fe8-xMnxB4的各向异性先减弱后增强,Fe7.75Mn0.25B4的各向异性最弱。当Mn掺杂浓度较低时,Fe8-xMnxB4的硬度略微降低,韧性增强。除了Fe7Mn1B4、Fe6Mn2B4、Fe5Mn3B4、Fe4Mn4B4之外,其余的Fe8-xMnxB4相的韧性均比Fe2B好。由电子结构可以发现,Fe8-xMnxB4的力学性能主要由Fe-B键或Mn-B键决定。Mn掺杂到Fe2B中会使得B-B共价键增强,Fe2B的本征脆性得到改善,同时Fe2B的磁性不断减弱。     

First-principles study on the electronic structure, magnetic properties and phase stability of alloyed cementite with Cr or Mn

Using the first-principles technique, the electronic structures, magnetic properties and phase stability of alloyed cementite with Cr or Mn were investigated. The calculations show that the chemical and mechanical stability of alloyed cementite can be strengthened by the use of Cr/Mn-doped method. The Magnetic Moments (Ms) of Mn₁Fe₂C, Mn₂Fe₁C, Cr₁Fe₂C and Cr₂Fe₁C are 5.274, 0.941, 1.864 and 0.736 μ_B/f.u, respectively. The Ms of Cr in Fe₂CrC (−1.374 μ_B) and Cr₂FeC (−0.032 μ_B) are different due to replacing different sites Fe atoms. The magnetic behaviors of Mn are different from Cr in alloyed cementite. The Ms of Mn in Fe₂MnC and Mn₂FeC are 2.300 μ_B and −0.147 μ_B, respectively.     

First-principles study on alloying stability, electronic structure, and mechanical properties of Al-based intermetallics

First-principles calculations were performed to study on alloying stability, electronic structure, and mechanical properties of Al-based intermetallic compounds (AlCu₃, AlCu₂Zr, and AlZr₃). The calculated results show that the lattice parameters obtained after full relaxation of crystalline cells are consistent with experimental data. The calculation of cohesive energies indicated that the structure stability of these Al-based intermetallics will become higher with increasing Zr element in crystal. The calculations of formation energies showed that AlCu₂Zr has the strongest alloying ability, followed by AlZr₃ and finally the AlCu₃. The further analysis find out that single-crystal elastic constants at zero-pressure satisfy the requirement of mechanical stability for cubic crystals. The calculations on the ratio of bulk modulus to shear modulus reveal that AlCu₂Zr can exhibit a good ductility, followed by AlCu₃, whereas AlZr₃ can have a poor ductility; however, for stiffness, these intermetallics show a converse order. The calculations on Poisson's ratio show that AlCu₃ is much more anisotropic than the other two intermetallics. In addition, calculations on densities of states indicate that the valence bonds of these intermetallics are attributed to the valence electrons of Cu 3d states for AlCu₃, Cu 3d, and Zr 4d states for AlCu₂Zr, and Al 3s, Zr 5s and 4d states for AlZr₃, respectively; in particular, the electronic structure of the AlZr₃ shows the strongest hybridization, leading to the worst ductility.     

Structural,mechanical, and electronic properties of P3m1-BCN

The mechanical and electronic properties of P3m1-BCN have been studied by using first principles calculations. The anisotropy studies of Young's modulus, shear modulus and Poisson's ratio show that P3m1-BCN exhibits a large anisotropy. Electronic structure study shows that P3m1-BCN is an indirect semiconductor with band gap of 4.10 eV. Unusually, the band gap of P3m1-BCN increase with increasing pressure.     

外压下VN相变和力学性质的第一性原理研究

运用基于赝势平面波基组的密度泛函程序VASP并结合Quantum ESPRESSO,Phonopy软件包对压力下VN的结构、力学性质、声子色散关系进行了第一性原理的研究.分别对NaCl型（B1）,CsCl型（B2）,WC型（Bh）三种构型的VN进行了计算,三种结构的体积能量曲线、焓压关系和声子谱表明在常压下六角WC结构与立方结构相比更稳定.随着压力增加VN由Bh结构到B1结构的相变点发生在30GPa左右,而B1结构到B2结构的相变点可能发生在150GPa左右.常压下三种结构的VN是力学稳定的,其弹性常数和弹性模量都有随压强的增大而增加的趋势,三者都是脆性材料.B1结构和B2结构坐标基矢方向上的杨氏模量数值与体对角线方向上的差距较大,体现出明显的各向异性.随压力的增加B1结构各向异性程度增大而B2结构各向异性程度减小     

ZrRh晶体结构、弹性性质和电子结构的第一性原理研究

运用基于密度泛函理论的第一性原理计算方法研究了ZrRh的晶体结构、弹性性质和电子结构性质.结果表明:计算的B2、B19'和ZrIr结构的平衡晶格常数与相应的实验参数符合很好.从形成焓和态密度的角度来看,ZrRh的相稳定顺序是ZrIrFe BB19'B2,ZrIr结构是最稳定的.ZrIr结构的形成焓最小,说明ZrIr结构最容易生成.利用应力-应变的方法计算了ZrRh的弹性常数,表明B2、FeB和ZrIr结构是力学稳定的.B/G和泊松比均表明ZrRh具有很好的延展性.对ZrRh的态密度研究发现,增强的Rh4d态与Zr4d态杂化作用是ZrIr结构稳定的主要原因.     

Structural, electronic properties and stability of tungsten mono- and semi-carbides: A first principles investigation

First principles calculations have been performed with the purpose to understand the comparative peculiarities of the structural, electronic properties and stability for all phases formed in the tungsten-carbon system: hexagonal and cubic mono-carbides WC and four polymorphs (α, β, γ and ε) of semi-carbide W₂C. All calculations were performed by means of the full-potential linearized augmented plane wave method (FLAPW). The generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE) formalism was used for the exchange and correlation energy functional. The geometries of all WC and W₂C phases were optimized and their structural parameters and theoretical density were established. Besides, we have evaluated the formation energies (E_form) of all the tungsten carbides. Based on our estimations we can arrange all investigated W-C phases depending on their stability in the following sequence: h-WC>ε-W₂C>β-W₂C>γ-W₂C>α-W₂C>c-WC. Here three carbides (h-WC, ε-W₂C and β-W₂C) are stable (E_form<0), γ-W₂C belongs to metastable systems (E_form∼0), whereas α-W₂C and c-WC appear to be unstable (E_form>0). Moreover, band structures, total and partial densities of states were obtained and analyzed systematically for all W-C phases in comparison with other available theoretical and experimental data.     

First-principles study of structural stability, electronic and elastic properties of ZrC compounds

We theoretically study the possible pressure-induced structural phase transition, electronic and elastic properties of ZrC by using first-principles calculations based on density functional theory (DFT), in the presence and absence of spin-orbit coupling (SOC). The calculations indicate that there exists a phase transition from the NaCl-type (B1) structure to CsCl-type (B2) structure at the transition pressure of 313.2 GPa (without SOC) and 303.5 GPa (with SOC). The detailed structural changes during the phase transition were analyzed. The band structure shows that B1-ZrC is metallic. A pseudogap appears around the Fermi level of the total density of states (DOS) of the B1 phase of ZrC, which may contribute to its structural stability.     

First-principles investigation of structural,elastic, electronic and magnetic properties of Be0.75Co0.25Y (YS,Se and Te) compounds

We have theoretically investigated the structural, elastic, electronic and magnetic properties of Be_0.75Co_0.25Y (YS, Se and Te) alloys, in their zinc-blend phase. This study is carried out by using the full-potential augmented plane wave plus local orbitals method within the density functional theory. Foe computing the exchange-correlation potential, the Wu and Cohen generalized gradient approximation is employed to calculate structural and elastic properties whereas the modified Becke and Johnson potential local density approximation is utilized to examine electronic and magnetic properties. By minimizing the total energy in paramagnetic (PM) and ferromagnetic (FM) phases, it is found the studied compounds are stable in FM structure. The mechanical behavior of the studied compounds is reported with the calculation of shear modulus, Young's modulus, and Poisson's ratio provides. Such mechanical aspects might be useful for the experimentalists to study the mechanical properties upon alloying BeY compounds with Co. We also compute electronic structures, density of states (total and partial), pd-exchange splitting and magnetic moments. Moreover, bond nature is studied by estimating the spin polarized charge densities of Be_0.75Co_0.25Y (YS, Se and Te).     

First principles study of electronic structure and optical properties of LiMTe2 (M=Al, Ga, In) crystals

The results of first principles calculations of the electronic band structure, density of states and frequency dependent dielectric functions of LiAlTe₂, LiGaTe₂ and LiInTe₂ chalcopyrite crystals are reported. The calculations have been carried out within the density functional theory using norm-conserving pseudopotentials and a plane-wave basis. The peculiarities of the imaginary part ε₂(ω) of the complex permittivity are discussed and interpreted on the basis of the obtained band spectra. Our calculations show that the Ga-containing compound is characterized by the largest optical anisotropy compared to Al- and In-containing compounds and, therefore, is the most promising candidate for nonlinear optical applications among considered crystals.     

First-principle study of phase stability,electronic structure and thermodynamic properties of cadmium sulfide under high pressure

By employing first principle and a quasi-harmonic Debye model, we study the phase stability, phase transition, electronic structure and thermodynamic properties of cadmium sulfide (CdS). The results indicate that CdS is a typical ionic crystal and that the zinc-blende phase in CdS is thermodynamically unstable. Moreover, the heat capacity of the wurtzite and rocksalt phases of CdS decreases with pressure and increases with temperature, obeying the rule of the Debye T³ law at low temperature and the Dulong–Petit limit at high temperature.     

Polymorphism in Cu-substituted GaV4S8: structural and electronic properties

The hexagonal and cubic phases of Ga_1−xCu_xV₄S₈ are obtained by different methods of preparation. The reaction of elements above 900 °C gives hexagonal phases for large range of x=0.02-0.5. These are metallic and show enhanced paramagnetism. The reduction of oxides by H₂S at a lower temperature of 700 °C gives non-stoichiomertic compositions of cubic-V₄ cluster compound GaV₄S₈. The solubility of Cu-atoms in cubic phase is less than 10% and above x=0.2 the samples contain a mixture of phases, Cu_xVS₂, Ga_xVS₂ and CuGaS₂. The cubic phases are insulating and show Mott's Variable Range Hopping conduction. The non-stoichiomerty and the Cu-substitution reduce the resistivity and thermopower. For x=0.15 and 0.20, the additional peaks are observed in X-ray patterns. These compositions showed a sharp metal to insulator transition on cooling below 180 K. The transitional behaviour is very similar to that previously reported intercalated VS₂ compound Al_xVS₂. The transport and magnetic properties of these phases are discussed in terms of the clustering interactions among V-atoms and the localisation of carriers on the metallic clusters frequently found in V-chalcogenides.     

First principles study of structural,electronic, mechanical and thermal properties of A15 intermetallic compounds Ti3X (X=Au,Pt, Ir)

The structural, electronic, elastic, mechanical and thermal properties of Ti₃Au, Ti₃Pt and Ti₃Ir intermetallic compounds crystallizing in A15 structure have been studied using density functional theory within generalized gradient approximation (GGA) for the exchange correlation potential. Elastic properties such as Young's modulus (E), rigidity modulus (G), bulk modulus (B), Poisson's ratio (σ) and elastic anisotropic factor (A) have been calculated. From the present study it is noted that Ti₃Ir is the hardest compound among the three materials studied due to its larger bulk modulus. Also, it is more ductile in nature.