Ground-State Structure and Physical Properties of NB 2 Predicted from First Principles

Using the newly developed particle swarm optimization algorithm on crystal structural prediction, we predict a new class of boron nitride with stoichiometry of NB2 at ambient pressure, which belongs to the tetragonal Im2 space group. Then, its structure, elastic properties, electronic structure, and chemical bonding are investigated by first-principles calculations with the density functional theory. The phonon calculation and elastic constants confirm that the predicted NB2 is dynamically and mechanically stable, respectively. The large bulk modulus, large shear modulus, large Young's modulus, and small Poisson's ratio show that the Im2 NB2 should be a new superhard material with a calculated theoretical Vickers hardness value of 66 GPa. Further analysis on density of states and electron localization function demonstrate that the strong B–B and B–N covalent bonds are the main reason for its high hardness in Im2 NB2.