Structural, mechanical, electronic and thermal properties of the newly predicted NB2 from ab initio calculations

The density functional theory (DFT) is used to investigate the structural, elastic, electronic and thermal properties of the newly predicted superhard material tetragonal NB2 (t-NB2). The estimated values of the elastic constants of this material satisfy its mechanical stability criteria. The calculated large values of bulk modulus (B), shear modulus (G), Young's modulus (E), Vickers hardness (Hv), and small Pugh's modulus and Poisson's ratio identify this compound as a potential candidate for superhard materials with high brittleness. The bulk modulus, Debye temperature, specific heats, and volume thermal expansion coefficient are also obtained as a function of temperature and pressure for the first time through the quasiharmonic Debye model with phononic effects. Large values of Debye and melting temperatures suggest the strong microhardness of t-NB2. The analysis of electronic density of states and Mulliken populations emphasizes the strong covalent bonding of B-B and B-N atoms which can be attributed to the mechanism of superhardness of t-NB2.