Phase stability and mechanical properties of niobium nitrides

First-principles pseudopotential calculations were performed to investigate the structural stability of various phases of niobium nitrides NbNₓ. The stability of the NaCl-, NiAs-, AsNi-, and CW (anti--WC)-type NbN phases, the substoichiometric Nb₈N₇, Nb₄N₃, and Nb₃₂N₃₁ compounds are analyzed on the basis of the results of electronic structure and phonon calculations. The behavior of these structures under uniaxial tensile strain was investigated. The electronic origin of the soft phonon modes and the mechanical properties of niobium nitrides are discussed.