DFT based comparative analysis of the physical properties of some binary transition metal carbides XC (X = Nb, Ta, Ti)

Abstract

Binary metallic carbides belong to technologically prominent class of compounds. Present work theoretically explores the structural, mechanical, optoelectronic, and some thermal properties of XC (X = Nb, Ta, Ti) compounds using the density functional methodology. A number of the results obtained are novel. The computed elastic constants and moduli disclose that the compounds possess moderate elastic anisotropy and reasonably good machinability (machinability index above 1.45 for all the compounds), mixed bonding characteristics, high Vickers hardness (in the range 19–25 GPa) with very high Debye temperatures (in the range 800–925 K) and brittle characteristics. The elastic/structural stability conditions are fulfilled for all the carbides under study. The Young's modulus and bulk modulus of TiC are lower than those of NbC and TaC. All the XC (X = Nb, Ta, Ti) compounds are hard and suitable for heavy duty structural applications. The electronic band structures show finite density of states at the Fermi level revealing metallic character of the XC (X = Nb, Ta, Ti) compounds. Mixed bonding features are evident from the charge density distribution maps of the XC (X = Nb, Ta, Ti) compounds. The lattice dynamical properties, such as the phonon dispersion curves and phonon density of states for XC (X = Nb, Ta, Ti) are investigated. Absence of negative phonon modes in the Brillouin zone indicates that the carbides under study are dynamically stable. The energy dependent optical constants are nearly isotropic. The optical absorption, reflectivity spectra, and the high value of the low energy index of refraction of XC (X = Nb, Ta, Ti) compounds reveal that they hold promise to be utilized in optoelectronics. The computed melting temperature, lattice thermal conductivity, Debye temperature, and minimum phonon thermal conductivity of the XC (X = Nb, Ta, Ti) compounds exhibit conformity with the computed elastic and bonding characteristics. Extremely high melting point (4399 K) of the compound TaC suggests that it is a very good candidate for applications at high-temperatures.