Electronic and elastic properties of the superconducting nanolaminate Ti<Subscript>2</Subscript>InC

The electronic properties and elastic parameters of the superconducting nanolaminate Ti₂InC are analyzed using the ab initio full-potential linearized augmented-plane-wave (FLAPW) method with the generalized gradient approximation (GGA) of the local spin density. The equilibrium parameters of the crystal lattice, the band structure, the total and partial densities of states, and the Fermi surface are determined within a unified approach. The independent elastic constants, the bulk modulus, and the shear modulus are calculated, and the elastic parameters are numerically estimated for the first time for polycrystalline Ti₂InC.     

External strain effect on the electronic and mechanical properties of the superconductor Nb2InC

We use first-principles method to investigate the effects of external strain ε on the structural, mechanical and electronic properties for the superconductor Nb₂InC. The results show that the tensile strain induces an isostructural phase transition in Nb₂InC. The elastic constants C_ij, bulk modulus B, shear modulus G, Young's moduli E, and Poisson ratio v_ij of Nb₂InC were also investigated in the range from ε=−10% to ε=10%. It indicates that Nb₂InC is mechanically stable under external strain, and its brittle–ductile transition occurs at ε=3.5%. Moreover, Nb₂InC gets a negative Poisson ratio at ε=4%. The calculated electronic structures indicate that the Nb–C bonding is stronger than Nb–In bonding in Nb₂InC. The energy band structures and densities of states of strained Nb₂InC were also calculated and discussed in detail. From these calculations, it is clear that the related properties of Nb₂InC can be easily tuned by strain. Therefore, our findings are very useful to tailor the physical properties of Nb₂InC by using strain engineering.     

Ab initio calculation of the electronic structure,Fermi surface,and elastic properties of the new 7.5-K superconductor Nb<Subscript>2</Subscript>InC

Ab initio calculations were performed to investigate electronic and elastic properties of the newly discovered 7.5 K superconductor: layered Nb₂InC. As a result, electronic bands, total and site-projected l—decomposed density of states at the Fermi level, shape of the Fermi surface for Nb₂InC were obtained for the first time. Besides, independent elastic constants, bulk modulus, compressibility, shear modulus, Young’s modulus, Poisson’s ratio together with the elastic anisotropy parameters and indicator of brittle/ductile behavior of Nb₂InC were evaluated and analyzed in comparison with the available data.     

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.     

Pressure effect on the structural and elastic property of Hf2InC

Using plane-wave pseudopotential (PW-PP) method based on the density functional theory (DFT) within the Local Density Approximation (LDA), we have performed a study of the structural and elastic properties of selected Hf₂InC compound belonging to the so-called MAX phases. The calculated equilibrium lattice parameters are in accordance with the experimental results. The result of high pressures on the lattice parameter shows that the contractions along the c-axis were more than along the a-axis. In order to gain further information on the mechanical properties, we have also calculated the anisotropy factor, Poison's ratio, Young's modulus, sound velocities and Debye temperature for Hf₂InC.     

Insight into structural,mechanical, electronic and thermodynamic properties of intermetallic phases in Zr–Sn system from first-principles calculations

The structural, phase stabilities, mechanical, electronic and thermodynamic properties of intermetallic phases in Zr–Sn system are investigated by using first-principles method. The equilibrium lattice constants, enthalpy of formation (ΔH^form) and elastic constants are obtained and compared with available experimental and theoretical data. The configuration of Zr₄Sn is measured with reasonable precision. The ΔH^form of five hypothetical structures are obtained in order to find possible metastable phase for Zr–Sn system. The mechanical properties, including bulk modulus, shear modulus, Young's modulus and Poisson's ratio, are calculated by Voigt–Reuss–Hill approximation and the Zr₅Sn₄ and Zr₅Sn₃ show excellent mechanical properties. The electronic density of states for Zr₅Sn₄, Zr₅Sn₃ and cP8-Zr₃Sn are calculated to further investigate the stability of intermetallic compounds. Through the quasi-harmonic Debye model, the Debye temperature, heat capacity and thermal expansion coefficient under temperature of 0–300 K and pressure of 0–50 GPa for Zr₅Sn₃ and Zr₅Sn₄ are deeply investigated.     

Superconductivity in Nb2InC

In this work the Nb₂InC phase is investigated by X-ray diffraction, heat capacity, magnetic and resistivity measurements. Polycrystalline samples with Nb₂InC nominal compositions were prepared by solid state reaction. X-ray powder patterns suggest that all peaks can be indexed with the hexagonal phase of Cr₂AlC prototype. The electrical resistance as a function of temperature for Nb₂InC shows superconducting behavior below 7.5 K. The M(H) data show typical type-II superconductivity with H_C1 ～ 90 Oe at 1.8 K. The specific heat data are consistent with bulk superconductivity. The Sommerfeld constant is estimated as γ ～ 12.6 mJ mol^?1 K^?1.     

First-principles study on electronic structure and elastic properties of Ti2SC

We have performed first-principles study on electronic structure and elastic properties of Ti₂SC. The absence of band gap at the Fermi level and the finite value of the density of states at the Fermi energy reveal the metallic behavior of this compound. The five independent elastic constants were derived and the bulk modulus, Young's modulus, shear modulus, and Poisson's ratio were determined. The high bulk modulus and hardness was found to be originated from the strong Ti 3d-S 2p hybridization. Such strong MA bonding is unusual in the MAX phases studied so far. Ti₂SC is elastically stable and exhibits highly elastic isotropy.     

Full potential calculation of structural, elastic properties and high-pressure phase of binary noble metal carbide: ruthenium carbide

We present in this paper the results of an ab initio theoretical study within the local density approximation (LDA) to determine in rock-salt (B1), cesium chloride (B2), zinc-blende (B3), and tungsten carbide (WC) type structures, the structural, elastic constants, hardness properties and high-pressure phase of the noble metal carbide of ruthenium carbide (RuC).The ground state properties such as the equilibrium lattice constant, elastic constant, the bulk modulus, its pressure derivative, and the hardness in the four phases are determined and compared with available theoretical data. Only for the three phases B1, B3, and WC, is the RuC mechanically stable, while in the B2 phase it is unstable, but in B3 RuC is the most energetically favourable phase with the bulk modulus 263 GPa, and at sufficiently high pressure (P_t=19.2 GPa) the tungsten carbide (WC) structure would be favoured, where ReC-WC is meta-stable.The highest bulk modulus values in the B3, B2, and WC structures and the hardnesses of H(B3)=36.94 GPa, H(B1)=25.21 GPa, and H(WC)=25.30 GPa indicate that the RuC compound is a superhard material in B3, and is not superhard in B1 and WC structures compared with the H(diamond)=96 GPa.     

Structures, phase transition, elastic properties of SnO2 from first-principles analysis

We investigate the structural, phase transition and elastic properties of SnO₂ in the rutile-type, pyrite-type, ZrO₂-type and cotunnite-type phases by the plane-wave pseudopotential density functional theory method. The lattice constants, bulk modulus and its pressure derivative are well consistent with the available experimental and other theoretical data. Also, we find that the rutile→pyrite, pyrite→ZrO₂ and ZrO₂→cotunnite phase transition occur at 12.9, 59.1 and 111.1 GPa, which are in better agreement with the experimental results than those of Gracia et al. (2007). Moreover, we obtain the pressure dependences of elastic constants for the four structures.     

AuB2 in comparison to superconducting MgB2-an ab initio study

The noble metal diboride AuB₂, a potential candidate for superconductor, is studied by an ab initio method in comparison to the superconducting MgB₂. The results, described in terms of equilibrium lattice constants, bulk modulus, pressure derivative of bulk modulus and their in- and out-of-plane linear values, volume coefficient of T_c, density of states, band structure, show some similarity as well as dissimilarity between the behaviour of the two compounds. The implications for the behaviour are discussed.     

Phase transition,elastic and electronic properties of topological insulator Sb_2Te_3 under pressure: First principle study

The phase transition, elastic and electronic properties of three phases(phase Ⅰ,Ⅱ, and Ⅲ) of Sb_2Te_3 are investigated by using the generalized gradient approximation(GGA) with the PBESOL exchange–correlation functional in the framework of density-functional theory. Some basic physical parameters, such as lattice constants, bulk modulus, shear modulus,Young's modulus, Poisson's ratio, acoustic velocity, and Debye temperature Θ are calculated. The obtained lattice parameters under various pressures are consistent with experimental data. Phase transition pressures are 9.4 GPa(Ⅰ→Ⅱ) and 14.1 GPa(Ⅱ→Ⅲ), which are in agreement with the experimental results. According to calculated elastic constants, we also discuss the ductile or brittle characters and elastic anisotropies of three phases. Phases Ⅰ and Ⅲ are brittle, while phaseⅡ is ductile. Of the three phases, phaseⅡ has the most serious degree of elastic anisotropy and phase Ⅲ has the slightest one.Finally, we investigate the partial densities of states(PDOSs) of three phases and find that the three phases possess some covalent features.     

高温高压下Zr2AlC的结构及热学性质的第一性原理

利用密度泛函理论研究了高温高压下Zr₂AlC的结构和热力学性质,计算得到Zr₂AlC的晶格参数与实验值符合较好．研究了Zr₂AlC的弹性常数、体模量、剪切模量和杨氏模量等力学性质随压力变化的趋势．同时研究了维氏硬度随压力的变化趋势．通过计算得到的杨氏模量预测了Zr₂AlC的弹性各向异性．最后,基于准简谐德拜模型,成功预测了Zr₂AlC的德拜温度、热容、热膨胀系数和Grüneisen参数随着压强和温度的变化关系.     

First-principles study of elastic and electronic properties of layered ternary nitride SrZrN2 under pressure

The structural, elastic, and electronic properties of SrZrN₂ under pressure up to 100?GPa have been carried out with first-principles calculations based on density functional theory. The calculated lattice parameters at 0?GPa and 0?K by using the GGA-PW91-ultrasoft method are in good agreement with the available experimental data and other previous theoretical calculations. The pressure dependence of the elastic constants and the elastic-dependent properties of SrZrN₂, such as bulk modulus B, shear modulus G, Young's modulus E, Debye temperature Θ, shear and longitudinal wave velocity V_S and V_L, are also successfully obtained. It is found that all elastic constants increase monotonically with pressure. When the pressure increases up to 140?GPa, the obtained elastic constants do not satisfy the mechanical stability criteria and a phase transition might has occurred. Moreover, the anisotropy of the directional-dependent Young's modulus and the linear compressibility under different pressures are analysed for the first time. Finally, the pressure dependence of the total and partial densities of states and the bonding property of SrZrN₂ are also investigated.     

Structural, electronic, elastic and thermal properties of Mg2Si

First-principles calculations of the lattice parameter, electron density maps, density of states and elastic constants of Mg₂Si are reported. The lattice parameter is found to differ by less than 0.8% from the experimental data. Calculations of density of states and electron density maps are also performed to describe the orbital mixing and the nature of chemical bonding. Our results indicate that the bonding interactions in the Mg₂Si crystal are more covalent than ionic. The quasi-harmonic Debye model, by means of total energy versus volume calculations obtained with the plane-wave pseudopotential method, is applied to study the elastic, thermal and vibrational effects. The variations of bulk modulus, Grüneisen parameter, Debye temperature, heat capacity C_v, C_p and entropy with pressure P up to 7 GPa in the temperature interval 0-1300 K have been systemically investigated. Significant differences in properties are observed at high pressure and high temperature. When T<1300 K, the calculated entropy and heat capacity agree reasonably with available experimental data. Therefore, the present results indicate that the combination of first-principles and quasi-harmonic Debye model is an efficient approach to simulate the behavior of Mg₂Si.     

FP-LMTO investigations of mechanical stability and high pressure of platinum nitride compounds

We report local density functional calculations using the full potential linear muffin-tin orbital (FP-LMTO) method for binary platinum nitride (PtN), in five different crystal structures, the rock salt (B1), zinc-blende (B3), wurtzite (B4), nickel arsenide (B8), and PbS (B10) phases. The ground state properties such as the equilibrium lattice constant, elastic constants, the bulk modulus and its pressure derivative of PtN in these phases are determined and compared with the other available experimental and theoretical works.Our calculations confirm in the B3 structure that PtN is found to be mechanically stable with a large bulk modulus B=232.45 GPa and at a sufficiently high pressure the B8₁ structure would be favoured.The theoretical transition pressure from zinc blende (B3) to NiAs (B8₁), zinc-blende (B3) to rock-salt (B1) and zinc-blende (B3) to PbO (B10) is determined to be 9.10 GPa, 9.85 GPa and 69.35 GPa, respectively. Our calculation shows also in five different structures for PtN a high bulk modulus is a good indicator of a hard material.     

Prediction study of structural and elastic properties under pressure effect of M2SnC (M=Ti, Zr, Nb, Hf)

Using first-principles calculations, we have studied the structural and elastic properties of M₂SnC, with M=Ti, Zr, Nb and Hf. Geometrical optimization of the unit cell is in good agreement with the available experimental data. The effect of high pressures, up to 20 GPa, on the lattice constants shows that the contractions along the a-axis were higher than those along the c-axis. We have observed a quadratic dependence of the lattice parameters versus the applied pressure. The elastic constants and their pressure dependence are calculated using the static finite strain technique. A linear dependence of the elastic stiffnesses on the pressure is found. We derived the bulk and shear moduli, Young's moduli and Poisson's ratio for ideal polycrystalline M₂SnC aggregates. We estimated the Debye temperature of M₂SnC from the average sound velocity. This is the first quantitative theoretical prediction of the elastic properties of Ti₂SnC, Zr₂SnC, Nb₂SnC, and Hf₂SnC compounds.     

Mechanical and lattice dynamical properties of the Re2C compound

In this work, density functional theory calculations on the structural, mechanical, and lattice dynamical properties of Re₂C within ReB₂‐type structure are reported. The generalized gradient approximation has been used for modeling exchange–correlation effects. We have predicted the lattice constants, bulk modulus, bond distances, elastic constants, shear modulus, Young's modulus, Poisson's ratio, hardness, Debye temperature, and sound velocities of this compound. Furthermore, the band structure, phonon dispersion curves and corresponding density of states are computed. The obtained results are in good agreement with the available experimental and other theoretical data. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural, electronic and elastic properties of Ti2TlC, Zr2TlC and Hf2TlC

Using First-principle calculations, we have studied the structural, electronic and elastic properties of M₂TlC, with M = Ti, Zr and Hf. Geometrical optimization of the unit cell is in good agreement with the available experimental data. The effect of high pressures, up to 20 GPa, on the lattice constants shows that the contractions are higher along the c-axis than along the a axis. We have observed a quadratic dependence of the lattice parameters versus the applied pressure. The band structures show that all three materials are electrical conductors. The analysis of the site and momentum projected densities shows that bonding is due to M d-C p and M d-Tl p hybridizations. The M d-C p bonds are lower in energy and stiffer than M d-Tl p bonds. The elastic constants are calculated using the static finite strain technique. We derived the bulk and shear moduli, Young’s modulus and Poisson’s ratio for ideal polycrystalline M₂TlC aggregates. We estimated the Debye temperature of M₂TlC from the average sound velocity. This is the first quantitative theoretical prediction of the elastic properties of Ti₂TlC, Zr₂TlC, and Hf₂TlC compounds that requires experimental confirmation.