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 NbN(x). The stability of the NaCl-, NiAs-, AsNi-, and CW (anti-WC)-type NbN phases, the substoichiometric Nb(8)N(7), Nb(4)N(3), and Nb(32)N(31) 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.