Structural, elastic and electronic properties of transition metal carbides TMC (TM=Ti, Zr, Hf and Ta) from first-principles calculations

The structural, elastic and electronic properties of TiC, ZrC, HfC and TaC have been investigated by first-principles calculations using the plane-wave pseudopotential method. Different exchange-correlation functionals regarding the local density approximation and the PBE, RPBE and PW91 forms of generalized gradient approximation are taken into account. The NaCl-type cubic structures of TMC (TM=Ti, Zr, Hf and Ta) are optimized and confirmed to be mechanically stable. The elastic properties such as the elastic constants, bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio of TMC are investigated, and the performances of LDA and GGA are discussed. The electronic density of state, electron charge density and Mulliken population analysis have been explored to discuss the electronic properties and bonding behaviors of TMC. The present calculation results compare satisfactorily with the experimental data and previous theoretical calculations.     

Structural,elastic, electronic and thermal properties of M2SbP (M = Ti,Zr and Hf)

The structural, elastic, electronic and thermal properties of M₂SbP (M = Ti, Zr and Hf) were studied by means of a pseudo-potential plane-wave method based on the density functional theory within both the local density approximation and the generalised gradient approximation. The optimised zero-pressure geometrical parameters, i.e. the two unit cell lengths (a, c) and the internal coordinate (z), were in good agreement with available experimental and theoretical data. The effect of high pressure, up to 20 GPa, on the lattice constants shows that the contractions along the a-axis were higher than along c-axis. The anisotropic independent elastic constants were calculated using the static finite strain technique. Numerical estimations of the bulk modulus, shear modulus, Young's modulus, Poisson's ratio, average sound velocity and Debye temperature for ideal polycrystalline M₂SbP aggregates were performed in the framework of the Voigt–Reuss–Hill approximation. The calculated band structures show that all studied materials are electrical conductors. Analysis of the atomic site projected densities showed that the bonding is of covalent–ionic nature with the presence of metallic character. The density of states at the Fermi level is dictated by the transition metal d–d bands; the Sb element has little effect. Thermal effects on some macroscopic properties of M₂SbP were predicted using the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. The variations of the volume expansion coefficient, heat capacity and Debye temperature with pressure and temperature in the ranges 0–50 GPa and 0–2000 K were obtained successfully.     

Al_3X(X=Zr、Ti、Ce、Er)化合物的第一性原理研究

基于密度泛函理论的第一性原理计算方法,对Al_3X(X=Zr、Ti、Ce、Er)化合物生成焓、结合能、态密度、力学性能进行了计算.生成焓、结合能的计算结果表明:Al_3X(X=Zr、Ti、Ce、Er)化合物中,Al_3Zr最易形成,Al_3Ce则最难形成,计算所得的结合能均为负值,化合物具有热力学稳定性,其中Al_3Zr的热力学稳定性最好,Al_3Er则最差.态密度结果分析:Al_3X(X=Zr、Ti、Ce、Er)化合物在费米能级处态密度不为0,表现出金属特性,通过赝能隙大小判断出它们的共价性具有如下关系:Al_3ZrAl_3TiAl_3CeAl_3Er,这与Al-3s、3p电子分别与Zr-4d,Ti-3d,Ce-5d、4f,Er-4f轨道电子具有较为明显的轨道杂化作用有关.力学性能计算结果表明:Al_3Ti、Al_3Zr抵抗体积变形、剪切变形的能力、刚度较Al_3Er、Al_3Ce大.由于Al_3Zr、Al_3Ti、Al_3Ce、Al_3Er具有一定共价性,共价键为方向性键,不利于塑性变形,因此它们均表现为脆性材料. Al_3Zr、Al_3Ti、Al_3Ce、Al_3Er本征维氏硬度大小排序为:Al_3TiAl_3ZrAl_3ErAl_3Ce.     

Anisotropic elastic properties of MB (M = Cr,Mo, W) monoborides: a first-principles investigation

First principles calculations were performed to systematically investigate structure properties, phase stability and mechanical properties of MB (M = Cr, Mo, W) monoborides in orthorhombic and tetragonal structures. The results of equilibrium structures are in good agreement with other available theoretical and experimental data. The elastic properties, including bulk modulus B, shear modulus G, Young’s modulus E and Poisson’s ratio ν were calculated by the Voigt-Reuss-Hill approximation. All considered monoborides are mechanically stable. The results of elastic anisotropies show that elastic anisotropy of orthorhombic structure is larger than that of tetragonal structure. Moreover, the minimum thermal conductivities were also estimated using the Cahill’s model, and the results indicate that the minimum thermal conductivities show a dependence on directions.     

Electronic and magnetic properties of silicene nanoflakes by first-principles calculations

The geometric, electronic, and magnetic properties of silicene nanoflakes (SiNFs) and corresponding two-dimensional (2D) framework assembled by SiNFs are studied by first-principles calculations. We find that the hexagonal SiNFs exhibit semiconducting behavior, while the triangular SiNFs is magnetic. Although the triangular SiNFs linked directly is antiferromagnetic, the system linked with an odd-number Si chains can exhibit ferromagnetic (FM) behavior, which is ascribed to anti-parallel spin rule on Si atoms, consistent with the Lieb–Mattis criterion. More interestingly, the 2D framework composed of triangular SiNFs linked by a Si atom shows a half-metallic character with an integer magnetic moment. These results provide a better understanding for silicene-based nanoflakes, and expect to pave an avenue to assemble FM silicon materials in spintronics.     

The structural stability, elastic constants and electronic structure of Al-Sr intermetallics by first-principles calculations

The lattice constants, enthalpies of formation, elastic constants and electronic structures of Al-Sr intermetallics have been calculated by first-principles method within generalized gradient approximation. The calculated lattice constants and enthalpies of formation are in good agreement with experimental and other theoretical results. The polycrystalline bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio are also estimated from the calculated single crystalline elastic constants. The total and partial electronic densities of state for the intermetallics were obtained, and the results indicated that Al₂Sr-oI is more stable than Al₂Sr-cF. Finally, longitudinal, transverse and average sound velocities and Debye temperature are estimated.     

Investigations on structural,elastic, thermodynamic and electronic properties of TiN,Ti2N and Ti3N2 under high pressure by first-principles

The lattice parameters, cell volume, elastic constants, bulk modulus, shear modulus, Young's modulus and Poisson's ratio are calculated at zero pressure, and their values are in excellent agreement with the available data, for TiN, Ti₂N and Ti₃N₂. By using the elastic stability criteria, it is shown that the three structures are all stable. The brittle/ductile behaviors are assessed in the pressures from 0 GPa to 50 GPa. Our calculations present that the performances for TiN, Ti₂N and Ti₃N₂ become from brittle to ductile with pressure rise. The Debye temperature rises as pressure increase. With increasing N content, the enhancement of covalent interactions and decline of metallicity lead to the increase of the micro-hardness. Their constant volume heat capacities increase rapidly in the lower temperature, at a given pressure. At higher temperature, the heat capacities are close to the Dulong–Petit limit, and the heat capacities of TiN and Ti₂N are larger than that of c-BN. The thermal expansion coefficients of titanium nitrides are slightly larger than that of c-BN. The band structure and the total Density of States (DOS) are calculated at 0 GPa and 50 GPa. The results show that TiN and Ti₂N present metallic character. Ti₃N₂ present semiconducting character. The band structures have some discrepancies between 0 GPa and 50 GPa. The extent of energy dispersion increases slightly at 50 GPa, which means that the itinerant character of electrons becomes stronger at 50 GPa. The main bonding peaks of TiN, Ti₂N and Ti₃N₂ locate in the range from −10 to 10 eV, which originate from the contribution of valance electron numbers of Ti s, Ti p, Ti d, N s and N p orbits. We can also find that the pressure makes that the total DOS decrease at the Fermi level for Ti₂N. The bonding behavior of N–Ti compounds is a combination of covalent and ionic nature. As N content increases, valence band broadens, valence electron concentration increases, and covalent interactions become stronger. This is reflected in shortening of Ti–N bonds.     

Prediction of novel ultra-incompressibility compounds TM2B (TM=Mo,W, Re and Os) by first-principles calculations

Abstract

Several novel ultra-incompressibility compounds TM₂B (TM = Mo, W, Re and Os) have been predicted by means of the first-principles calculations. Those novel compounds were assumed to have a ReB₂-type structure [P6₃/mmc space group (No.194, Z = 2), atomic sites: TM 4f (2/3, 1/3, z), B 2c (1/3, 2/3, 1/4)]. We calculated the mechanical properties of the TM₂B, and the results reveal that they exhibit brittle behaviour and mechanically stable. The hardness values are 23.8 GPa, 23.3 GPa, 26.6 GPa and 26.3 GPa for Mo₂B, W₂B, Re₂B and Os₂B, respectively, which suggests that they are hard materials. Additionally, we found that the anisotropy of Re₂B is weaker than the others. Finally, the Mo₂B has the highest Debye temperature (905.8 K), while Os₂B has the lowest Debye temperature (615.5 K). We hoped that our results can help to offer a theoretical data for future experimental work and application of TM₂B.     

Structural and magnetic properties of Ti12M clusters (M=Sc to Zn)

The geometries, electronic, and magnetic properties of the 3d atom doped icosahedron (ICO) Ti₁₂M (M=Sc to Zn), where a dopant atom replaces either the centra l(Ti₁₂M^c) or surface (Ti₁₂M^s) Ti atom in ICO Ti₁₃ cluster, have been systematically investigated by using the density functional theory. The structures of all the optimized Ti₁₂M^c and Ti₁₂M^s clusters are distorted ICO. Sc, Ni, Cu, and Zn atoms prefer to displace surface Ti atom, V, Cr, Mn, and Fe atoms prefer to displace central Ti atom. The position of impurity atom depends on the strength of the interaction between the central atom and the surface atoms. As compared to the pure Ti₁₃ cluster, Ti₁₂M^c and Ti₁₂M^s (M=V, Fe, Co, and Ni) clusters are more stable, Ti₁₂M^c and Ti₁₂M^s (M=Sc, Cr, Mn, Cu, and Zn) are less stable. Both Ti₁₂Ni^s and Ti₁₂Ni^c are magic clusters, which originate from their electronic as well as geometric closed shells. Because the exchange interaction prevails over the crystal field in Ti₁₂M clusters, the valence electrons fill molecular orbitals in terms of Hund’s rule of maximum spin.     

First-principles calculations of structural stability,elastic, dynamical and thermodynamic properties of SiGe,SiSn, GeSn

A first-principles calculations, based on the norm-conserving pseudopotentials and the density functional theory (DFT) and the density functional perturbation theory (DFPT) as implemented in the ABINIT code, have been performed to investigate the structural stability, elastic, lattice dynamic and thermodynamic properties of the ordered SiGe, SiSn and GeSn cubic alloy in zinc-blende (B3) structure. The calculated lattice parameters and bulk modulus agree with the previous results. The second-order elastic constants have been calculated and other related quantities such as the Young’s modulus, shear modulus, anisotropy factor are also estimated. We also obtain the data of lattice dynamics and the temperature dependent properties currently lacking for SiGe, SiSn and GeSn. Findings are also presented for the temperature-dependent behaviors of some thermodynamic properties such as the internal energy, Helmholtz free energy, entropy and heat capacity.     

Structural transitions of NaAlH4 under high pressure by first-principles calculations

First-principles calculations have been performed on NaAlH₄ using the generalized gradient approximation pseudopotential method. The predicted β-NaAlH₄ (α-LiAlH₄-type) structure is energetically more favorable than α-NaAlH₄ for pressures over 15.9 GPa, which is apparently correlated with the experimental transition pressure 14 GPa. This transition is identified as first-order in nature with volume contractions of 1.8%. There is no pressure-induced softening behavior from our calculated phonon dispersion curves near the phase transition pressure. Based on the Mulliken population analysis, the β-NaAlH₄ structure is expected to be the most promising candidate for hydrogen storage.     

Structural stabilities, electronic, elastic and optical properties of SrTe under pressure: A first-principles study

The structural, electronic, elastic and optical properties as well as phase transition under pressure of SrTe have been systematically investigated by first-principles pesudopotential calculations. Five possible phases of SrTe have been considered. Our results show that SrTe undergoes a phase transition from NaCl-type (B1) to CsCl-type (B2) structure at 10.9 GPa with a volume collapse of 9.43%, and no further transition is found. We find that SrTe prefer h-MgO instead of wurtzite (B4) structure for its metastable phase because that the ionic compound prefers a high coordination. The elastic moduli, energy band structures, real and imaginary parts of the dielectric functions have been calculated for all considered phases, and we find that a smaller energy gap yields a larger high-frequency dielectric constant. Our calculated results are discussed and compared with the available experimental and theoretical data.     

Structural,elastic and mechanical properties of orthorhombic SrHfO3 under pressure from first-principles calculations

Structural, elastic and mechanical properties of orthorhombic SrHfO₃ under pressure have been investigated using the plane-wave ultrasoft pseudopotential technique based on the first-principles density functional theory. The calculated equilibrium lattice parameters and elastic constants of orthorhombic SrHfO₃ at zero pressure are in good agreement with the available experimental and calculational values. The lattice parameters, total enthalpy, elastic constants and mechanical stability of orthorhombic SrHfO₃ as a function of pressure were studied. With the increasing pressure, the lattice parameters and volume of orthorhombic SrHfO₃ decrease whereas the total enthalpy increases. Orthorhombic SrHfO₃ is mechanically stable with low pressure (<52.9 GPa) whereas that is mechanically instable with high pressure (>52.9 GPa). The bulk modulus, shear modulus, Young's modulus and mechanical anisotropy of orthorhombic SrHfO₃ as a function of pressure were analyzed. It is found that orthorhombic SrHfO₃ under pressure has larger bulk modulus, better ductility and less mechanical anisotropy than orthorhombic SrHfO₃ at 0 GPa.     

Narrow band in the intermetallic compounds MNiSn (M=Ti,Zr, Hf)

The presence of a narrow band below conduction band of nonmagnetic compounds MNiSn (M=Ti, Zr, Hf) is assumed to explain their low temperature properties such as the heat capacity, IR optics, electronics transport. A computation of the Seebeck coefficient supplies support for this assumption.     

双金属掺杂M2@Si20(M=Ti, Zr, Hf)团簇结构和性质的理论研究

采用密度泛函理论(DFT)和相对论有效原子核势近似(RECP),对M2@Si20(M=Ti,Zr,Hf)团簇的几何结构和电子结构性质进行研究,发现内嵌的金属二聚体和十二面体硅笼构成了稳定的富勒烯结构。通过对团簇电子结构的分析,结果表明Si20团簇掺杂双金属后稳定性得到了提高。对团簇的电荷布局分析表明过渡金属原子(Ti, Zr, Hf)和硅笼之间发生了电荷反转。     

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.     

The elastic and bonding properties of the sulvanite compounds: A first-principles study by local and semi-local functionals

The elastic properties of the compound family of the sulvanite Cu₃TMX₄ (TM=V, Nb, Ta; X=S, Se) have been calculated using first-principles total energy calculations within the density functional theory along with the local density and the generalized gradient approximations. The calculated elastic properties are the bulk modulus (B), the elastic constants (c₁₁, c₁₂ and c₄₄), the Zener anisotropy factor (A), the isotropic shear modulus (G) and the Young modulus (E). By means of these quantities we have also computed other thermodynamic properties such as the average sound velocity (s) and the Debye temperature (Θ_D). The calculated values of the elastic properties led to the conclusion that these compounds are brittle and fragile. We have also calculated the electron localization function, which exhibits the bonding characteristics in these compounds, showing that the Cu-X bond as well as the TM-X bond have a covalent character, but there is also present some ionicity in these compounds due to the Cu-TMX₄ bond.     

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.      

Site preference and elastic properties of ternary alloying additions in B2 YAg alloys by first-principles calculations

First-principles calculations were preformed to study the site preference behavior and elastic properties of 3d (Ti–Cu) transition-metal elements in B2 ductility YAg alloy. In YAg, Ti is found to occupy the Y sublattice whereas V, Cr, Co, Fe, Ni and Cu tend to substitute for Ag sublattice. Due to the addition of 3d transition metals, the lattice parameters of YAg is decreased in the order: V<Cu<Cr<Ni<Co<Fe<Ti. The calculated elastic constants show that Cr, Fe, Co and Cu can improve the ductility of YAg alloy, and Fe is the most effective element to improve the ductility of YAg, while Ti, Ni and V alloying elements can reduce the ductility of YAg alloy, especially, V transforms ductile into brittle for YAg alloy. In addition, both V and Ni alloying elements can increase the hardness of YAg alloy, and Y₈Ag₇V is harder than Y₈Ag₇Ni.     

Structural,electronic and elastic properties of potassium hexatitanate crystal from first-principles calculations

The structural, electronic and elastic properties of potassium hexatitanate (K₂Ti₆O₁₃) whisker were investigated using first-principles calculations. The calculated cell parameters of K₂Ti₆O₁₃ including lattice constants and atomic positions are in good agreement with the experimental data. The obtained formation enthalpy (−61.1535 eV/atom) and cohesive energy (−137.4502 eV/atom) are both negative, showing its high structural stability. Further analysis of the electronic structures shows that the potassium hexatitanate is a wide-band semiconductor. Within K₂Ti₆O₁₃ crystal, the TiO bonding interactions are stronger than that of KO, while no apparent KTi bonding interactions can be observed. The structural stability of K₂Ti₆O₁₃ was closely associated with the covalent bond interactions between Ti (d) and O (p) orbits. Further calculations on elastic properties show that K₂Ti₆O₁₃ is a high stiffness and brittle material with small anisotropy in shear and compression.