First principles study of structural stability,electronic structure and mechanical properties of ReN and TcN

The crystal structure, structural stability, electronic and mechanical properties of ReN and TcN are investigated using first principles calculations. We have considered five different crystal structures: NaCl, zinc blende (ZB), NiAs, tungsten carbide (WC) and wurtzite (WZ). Among these ZB phase is found to be the lowest energy phase for ReN and TcN at normal pressure. Pressure induced structural phase transitions from ZB to WZ phase at 214 GPa in ReN and ZB to NiAs phase at 171 GPa in TcN are predicted. The electronic structure reveals that both ReN and TcN are metallic in nature. The computed elastic constants indicate that both the nitrides are mechanically stable. As ReN in NiAs phase has high bulk and shear moduli and low Poisson's ratio, it is found to be a potential ultra incompressible super hard material.     

First-principles study on the crystal, electronic structure and mechanical properties of hexagonal Al3RE (RE = La, Ce, Pr, Nd, Sm, Gd) intermetallic compounds

The structural properties, elastic properties and electronic structures of hexagonal Al₃RE intermetallic compounds are calculated by using first-principles calculations based on density functional theory. Since there exists strong on-site Coulomb repulsion between the highly localized 4f electrons of RE atoms, we present a combination of the GGA and the LSDA+U approaches in order to obtain the appropriate results. The GGA calculated lattice constants for the hexagonal Al₃RE intermetallic compounds are in good agreement with available experimental values. The results of cohesive energy indicate that these compounds can be stable under absolute zero Kelvin and the stability of Al₃Gd is the strongest in all of the hexagonal Al₃RE compounds. The densities of states for GGA and LSDA+U approaches are also obtained for the Al₃RE intermetallic compounds. The mechanical properties are calculated from the GGA method in this paper. According to the computed single crystal elastic constants, Al₃La, Al₃Sm and Al₃Gd are mechanically unstable, while Al₃Ce, Al₃Pr and Al₃Nd are stable. The polycrystalline elastic modulus and Poisson’s ratio have been deduced by using Voigt-Reuss-Hill (VRH) approximations, and the calculated ratio of bulk modulus to shear modulus indicates that Al₃La compound is ductile material, but Al₃Ce, Al₃Pr, Al₃Nd, Al₃Sm and Al₃Gd are brittle materials.     

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 of structural, elastic, electronic and optical properties of SrMO3 (M=Ti and Sn)

We present structural, elastic, electronic and optical properties of the perovskites SrMO₃ (M=Ti, and Sn) for different pressure. The computational method is based on the pseudo-potential plane wave method (PP-PW). The exchange-correlation energy is described in the generalized gradient approximation (GGA). The calculated equilibrium lattice parameters are in reasonable agreement with the available experimental data. This work shows that the perovskites SrTiO₃, and SrSnO₃ are mechanically stable and present an indirect band gaps at the Fermi level. Applied pressure does not change the shape of the total valence electronic charge density and most of the electronic charge density is shifted toward O atom. Furthermore, in order to understand the optical properties of SrMO₃, the dielectric function, absorption coefficient, optical reflectivity, refractive index, extinction coefficient and electron energy-loss are calculated for radiation up to 80 eV. The enhancement of pressure decreases the dielectric function and refractive indices of SrTiO₃ and SrSnO₃.     

The structural, elastic, and electronic properties of the pyrite-type phase for SnO2

We have studied some structural, thermodynamic, elastic, and electronic properties of pyrite-type SnO₂ polymorph by performing ab initio calculations within the LDA approximation. The basic physical properties, in particular lattice constant, bulk modulus, second-order elastic constants (C_ij), and the electronic structure, are calculated, and compared with the available experimental data. In order to gain some further information on the mechanical properties, we have also calculated the Young's modulus, Poison's ratio (ν), anisotropy factor (A), sound velocities, and Debye temperature for the same compound.     

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.     

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.     

Theoretical study of the structural, elastic, electronic and thermal properties of the MAX phase Nb2SiC

Structural, elastic, electronic and thermal properties of the MAX phase Nb₂SiC are studied by means of a pseudo-potential plane-wave method based on the density functional theory. The optimized zero pressure geometrical parameters are in good agreement with the available theoretical data. The effect of high pressure, up to 40 GPa, on the lattice constants shows that the contractions along the c-axis were higher than those along the a-axis. The elastic constants C_ij and elastic wave velocities are calculated for monocrystal Nb₂SiC. Numerical estimations of the bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, average sound velocity and Debye temperature for ideal polycrystalline Nb₂SiC aggregates are performed in the framework of the Voigt-Reuss-Hill approximation. The band structure shows that Nb₂SiC is an electrical conductor. The analysis of the atomic site projected densities and the charge density distribution shows that the bonding is of covalent-ionic nature with the presence of metallic character. The density of states at Fermi level is dictated by the niobium d states; Si element has a little effect. Thermal effects on some macroscopic properties of Nb₂SiC are predicted using the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. The variations of the primitive cell volume, volume expansion coefficient, bulk modulus, heat capacity and Debye temperature with pressure and temperature in the ranges of 0-40 GPa and 0-2000 K are obtained successfully.     

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.     

First-principles studies of the electronic and elastic properties of Ti2GeC

We have performed first-principles studies on electronic structure and elastic properties of Ti₂GeC. The calculated band structure shows that this compound is electrical conductor. From the pressure dependence of elastic constants, we find that Ti₂GeC is most stable in the pressure range from 0 to 100 GPa. The strong Ti 3d, Ge 4p and C 2p hybridization may stabilize the structure of Ti₂GeC. By analyzing the ratio between the bulk and shear moduli, we conclude that Ti₂GeC is brittle in nature, and the brittleness of Ti₂GeC originated from the large value of Ti atom occupying the internal parameter z.     

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.     

Pressure-induced structural phase transition in transition metal carbides TMC (TM = Ru,Rh, Pd,Os, Ir,Pt): a DFT study

First-principles calculations based on density functional theory was performed to analyse the structural stability of transition metal carbides TMC (TM = Ru, Rh, Pd, Os, Ir, Pt). It is observed that zinc-blende phase is the most stable one for these carbides. Pressure-induced structural phase transition from zinc blende to NiAs phase is predicted at the pressures of 248.5 GPa, 127 GPa and 142 GPa for OsC, IrC and PtC, respectively. The electronic structure reveals that RuC exhibits a semiconducting behaviour with an energy gap of 0.7056 eV. The high bulk modulus values of these carbides indicate that these metal carbides are super hard materials. The high B/G value predicts that the carbides are ductile in their most stable phase.     

First-principles study on the structural and electronic properties of AlNCx nanosheet

The structural and electronic properties of a hydrogen terminated hexagonally AlN nanoribbon with 6 zigzag Al-N chains across the ribbon width (6-ZAlNNR) and the hexagonally bonded hetero-sheets AlNC_x (x=2,4,6) consisting of AlN and graphite strips with zigzag shaped borders have been investigated systemically by using the first-principles. The results show that in 6-ZAlNNR, the states of the lowest unoccupied conduction band (LUCB) and the highest occupied valence band (HOVB) at zone boundary Z are edge states whose charges are localized at edge Al and N atoms, respectively. Introducing the graphite strip C_x and increasing its width lead to the LUCB and HOVB getting closer with each other especially in flat dispersion region around the zone boundary J_y, thus decreasing in the energy gap of the hetero-sheets AlNC₂, AlNC₄ and AlNC₆ successively. Similar to the edge states existing in zigzag edged AlNNR, the flat dispersion border states also exist in the zigzag borders of hexagonally networked hetero-sheets AlNC_x. Unlike the edge states whose charges are localized at one of the edge atoms, the border states are localized at two atoms of the borders with either bonding or antibonding character.     

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     

高压下MgS的弹性性质、电子结构和光学性质的第一性原理研究

采用基于密度泛函理论(density functional theory)基础上的第一性原理赝势平面波方法, 计算研究了MgS晶体B2构型在不同压强下的几何结构、弹性性质、电子结构和光学性质. 计算结果表明, 在高压作用下, 该结构的导带能级有向高能级移动的趋势, 而价带能级有向低能级移动的趋势. 同时, 对照态密度分布图及高压下能级的移动情况, 分析了MgS B2构型在高压作用下的光学性质, 发现高压作用下, 吸收光谱发生了明显的蓝移.     

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.     

Comparative first-principles calculations of electronic, optical and elastic anisotropy properties of CsXBr3 (X=Ca,Ge,Sn ) crystals

Detailed ab initio calculations of the structural, electronic, optical and elastic properties of CsCaBr₃, CsGeBr₃ and CsSnBr₃ crystals are presented in this paper. Based on the obtained results, CsCaBr₃ is characterized as a dielectric with an indirect band gap, whereas CsGeBr₃ and CsSnBr₃ are semiconductors with very narrow direct band gaps. The first theoretical estimations of the refractive indexes for all compounds are reported. Variations of the electron density difference distribution induced by changes of the second cation were analyzed and related to the type of chemical bonding between atoms. In addition, the complete set of elastic parameters (which includes the elastic constants, elastic compliance constants, bulk and Young’s moduli, elastic anisotropy) was obtained. Directional anisotropy of elastic properties was visualized; the directions in the crystal lattices, along which the maximal and minimal values of the Young’s moduli are realized, were identified.     

High pressure study of structural and electronic properties of magnesium telluride

The full-potential linear muffin-tin orbital method (FP-LMTO) within the local density approximation (LDA) is used to calculate the electronic band structures and the total energies of MgTe in its stable (NiAs-B8) and high pressure phases. The latter provide us with the ground state properties such us lattice parameter, bulk modulus and its pressure derivatives. The transition pressure at which this compound undergoes the structural phase transition from the NiAs to CsCl phase is calculated. The energy band gaps and their volume and pressure dependence in the stable NiAs-B8 phase are investigated. The ground state properties, the transition pressure are found to agree with the experimental and other theoretical results. The elastic constants at equilibrium in both NiAs and CsCl structure are also determined.     

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.     

高压下RhB的相变、弹性性质、电子结构及硬度的第一性原理计算

采用密度泛函理论中的赝势平面波方法系统地研究了高压下RhB的结构相变、弹性性质、电子结构和硬度.分析表明,RhB在25.3 GPa时从anti-NiAs结构相变到FeB结构,这两种结构的弹性常数、体弹模量、剪切模量、杨氏模量和弹性各向异性因子的外压力效应明显.电子态密度的计算结果显示,这两种结构是金属性的,且费米能级附近的峰随着压强的增大向两侧移动,赝能隙变宽,轨道杂化增强,共价性增强,非局域化更加明显.此外,硬度计算结果显示,anti-NiAs-RhB的金属性比较弱,有着较高的硬度,属于硬质材料.