First-principles calculations of physical properties and superconductivity of orthorhombic Mo2BC and Nb2BN

Abstract

Ab initio pseudopotential calculations have made for the structural, electronic, elastic, mechanical, and electron–phonon interaction properties of molybdenum borocarbide (${\mathrm{Mo}}_2$ BC) and niobium boronitride (${\mathrm{Nb}}_2$ BN) superconductors. Analysis of the structural and electronic properties reveals that the nature of bonding in both these compounds is a combination of covalent, ionic, and metallic. The near-Fermi electronic states in both compounds are occupied by the d states of transition metal atoms. The electronic density of states at the Fermi level in ${\mathrm{Mo}}_2$ BC is significantly higher than that in ${\mathrm{Nb}}_2$ BN. Lattice dynamical calculations verify their dynamical stability in the base-centered orthorhombic ${\mathrm{Mo}}_2$ BC-type crystal structure. We find that the total electron–phonon coupling constant is equal to 0.745 for ${\mathrm{Mo}}_2$ BC and 0.539 for ${\mathrm{Nb}}_2$ BN. The calculated superconducting transition temperature of 7.41 K for ${\mathrm{Mo}}_2$ BC and 3.50 K for ${\mathrm{Nb}}_2$ BN is comparable with their experimental values of 7.2 and 4.4 K, respectively.