INTERACTION OF OCTAHEDRAL BONDS IN TRANSITION-METAL CARBIDES AND NITRIDES

Local-density total energy calculations within the parameter-free self-consistent augmented-spherical-wave method are performed on ordered nonstoichiometric rock-salt-type structures of VC0.75 and VN0.75. Starting from the idea of two kinds of building units, M6X and M6square (M, X, and square are the transition metal, a nonmetal, and the vacancy, respectively), we investigate interactions of octahedral bonds formed by the octahedral arrangement of transition metals M6square. Using enlarged unit cells with the same composition we have found ordered structures with contacting corners of octahedra (corner-sharing structures) energetically favorable for both carbide and nitride as compared to the ordered structure of noncontacting octahedra. The interaction of the octahedral bonds in the corner-sharing structure of the carbide (E(st) = -4.9 mRy/V atom) does not seem strong enough to predominate in forming carbide superstructures since vacancies avoid fcc second-neighbor positions. On the other hand, significantly larger stabilization in nitride (E(st) = -13.2 mRy/V atom) agrees well with fcc second-neighbor positions of vacancies in nitride superstructures. (C) 1994 Academic Press, Inc.