Physical properties of tetragonal transition-metal borides Nb2MB2 (M = Mo, W, Re or Os) with a new superstructure

The mechanical and thermodynamic properties, chemical bonding characteristics and electronic structure of Nb2MB2 (M = Mo, W, Re or Os) with a new tetragonal U3Si2-type superstructure (space group P4/mnc, no. 128) were studied by means of density functional theory calculations. All Nb2MB2 structures studied were demonstrated to be thermodynamically and mechanically stable. The bulk, shear and Young's moduli, Poisson's ratio, Debye temperature and anisotropy factors were derived for ideal polycrystalline Nb2MB2 aggregates. Among these compounds, Nb2WB2 was found to have the highest shear modulus and hardness. The electronic densities of state and electronic localization function analysis revealed the metallicity and strong covalent B-B, Nb-B and M-B bonding in Nb2MB2. Moreover, these results reveal that the covalence between Nb 4d, M nd (n = 4 for Mo and 5 for W, Re and Os) and B 2p states is the cause of the relatively higher elastic modulus and hardness of the Nb-based compounds. Finally, thermodynamic properties, including the bulk modulus, heat capacity and thermal expansion coefficient of Nb2WB2 were obtained systematically under high temperature and pressure. (C) 2015 Elsevier B.V. All rights reserved.