Polymorphism and metastability in NbN: Structural predictions from first principles.

We use ab initio pseudopotential total energy calculations with a plane wave basis set to investigate the structural energetics of various phases of polymorphic NbN. Particular attention is given to its recently discovered superconducting phase with a $T_c$ of 16.4 K, reported to have the NbO structure type. Results of total energy calculations show that it is in fact energetically unfavorable for NbN to form in this cubic structure, and its predicted theoretical lattice constant is significantly smaller than the experimental value. Various approaches to the identification of an alternative structure are discussed. In preliminary investigations, we have found two new structures that are energetically more favorable than the NbO structure.