Structural, elastic and electronic properties for M2XC （M--Ti and Cr, X--Ga and A1） phases from ab initio calculations

The first-principles study of the structural, elastic and electronic properties of the M 2 XC phases depending on the type of M transition metal (M are Ti and Cr) and on X (X are Ga and Al) was reported. The calculations are performed using the pseudo-potential plane-wave approach in both the local density and generalized gradient approximations. The elastic constants are calculated using the static finite strain technique. Features such as structural and elastic parameters, Debye temperature, sound velocities and their pressure dependence have been investigated. In agreement with experimental and previous theoretical findings, it is found that the compressibility along a and c axis depends on the valence electron concentration (VEC). Correlations revealing the governing role of the X and M elements on the machinability indices of the material have been examined. The electronic properties have been discussed in terms of chemical bonding showing that bonding is due to Md-Cp and Md-Xp hybridizations. M-C bonds are stiffer than M-X ones and Al-Ti (Cr-C) bonds are stiffer than those corresponding to Ti-C (Al-Cr). It is shown that the stiffness of the M-X and M-C bonds increases with increasing the number of VEC.     

First-principles calculations of structural,elastic, electronic and optical properties of XO (X=Ca,Sr and Ba) compounds under pressure effect

The structural, elastic, electronic and optical properties of XO (X= Ca, Sr and Ba) compounds were investigated by the density functional theory. A good agreement was found between our calculated results and the available theoretical and experimental data of the lattice constants. Young's modulus, Poisson ratio, bulk modulus, elastic constants and their pressure derivatives are also calculated. SrO and BaO compounds present a transition phase at 39.72 and 27.28 GPa. The SrO compound shows a change from direct band gap (Γ–Γ) to indirect band gap (Γ–X) at about 15 GPa. The top of the valence bands reflects the s electronic character for all structures. We investigate the effective mass of electrons as function of pressure at the Γ point for CaO, SrO and BaO compounds. Calculations of the optical spectra have been performed for the energy range 0–60 eV. The origin of the spectral peaks was interpreted based on the electronic structures. The enhancement of pressure increases the static dielectric function and refractive index of CaO, SrO and BaO.     

First-principles calculations on elastic,electronic and optical properties for the alkaline platinum hydrides A2PtH6 (A=K,Rb and Cs)

The alkaline platinum hydrides are considered the most promising as hydrogen storage materials. The alloying ability of crystal, elastic constants and related parameters, electronic and optical properties have been studied using pseudo-potential plane–wave method based on the density functional theory. The investigated compounds show a weaker resistance to compression along the principal a-axis and their resistance to shear deformation is lower than the resistance to the unidirectional compression. The band structure indicates that A₂PtH₆ (A=K, Rb and Cs) are X–X direct gap semiconductors. The effective electron mass at equilibrium has been predicted towards X–Γ, X–W and L–W directions. The strong hybridization between Pt-d and H-s states in the upper valence band translates the existence of covalent bonding character in these compounds. The static optical dielectric constant is inverse proportional to the fundamental gap.     

Theoretical prediction of the structural,elastic, electronic and thermal properties of the MAX phases X2SiC (X = Ti and Cr)

The structural, elastic, electronic and thermal properties of the MAX phases Ti₂SiC and Cr₂SiC are studied by means of the pseudo-potential plane wave method within GGA and LDA. The effect of pressure on the normalized lattice constants a/a₀ and c/c₀ and the internal parameter z is investigated. Our results of elastic constants, sound velocities and Debye temperature are predictions. The Ti₂SiC and Cr₂SiC compounds behave as ductile material and show a stronger anisotropy. The analysis of the band structure and density of states show that these compounds are electrical conductors, having a strong directional bonding between Ti and C and Cr and C atoms assured by the hybridization of Ti–d and Cr–d atom states with C–p atom states. The thermal effect on the primitive cell volume, bulk modulus, heat capacities C_V and C_P were predicted using the quasi-harmonic Debye model.     

Structural, electronic, optical and thermodynamic properties of SrxCa1−xO, BaxSr1−xO and BaxCa1−xO alloys

The structural, electronic, optical and thermodynamic properties of Sr_xCa_1−xO, Ba_xSr_1−xO and Ba_xCa_1−xO ternary alloys in NaCl phase were studied using pseudo-potential plane-wave method within the density functional theory. We modeled the alloys at some selected compositions with ordered structures described in terms of periodically repeated supercells. The dependence of the lattice parameters, band gaps, dielectric constants, refractive indices, Debye temperatures, mixing entropies and heat capacities on the composition x were analyzed for x = 0, 0.25, 0.50, 0.75 and 1. The lattice constant for Sr_xCa_1−xO and Ba_xSr_1−xO exhibits a marginal deviation from the Vegard's law, while the Ba_xCa_1−xO lattice constant exhibits an appreciable upward bowing. A strong deviation of the bulk modulus from linear concentration dependence was observed for the three alloys. The microscopic origins of the gap bowing were detailed and explained. The composition dependence of the dielectric constant and refractive index was studied using different models. The thermodynamic stability of these alloys was investigated by calculating the phase diagram. The thermal effect on some macroscopic properties was investigated using the quasi-harmonic Debye model. There is a good agreement between our results and the available experimental data for the binary compounds which may be a support for the results of the ternary alloys reported here for the first time.     

Structural, elastic and electronic properties for M_2 XC (M=Ti and Cr, X=Ga and Al) phases from ab initio calculations

The first-principles study of the structural, elastic and electronic properties of the M 2 XC phases depending on the type of M transition metal (M are Ti and Cr) and on X (X are Ga and Al) was reported. The calculations are performed using the pseudo-potential plane-wave approach in both the local density and generalized gradient approximations. The elastic constants are calculated using the static finite strain technique. Features such as structural and elastic parameters, Debye temperature, sound velocit...