METASTABILITY OF NBN IN THE ORDERED VACANCY NBO PHASE

A metastable phase of NbN with superconducting T-c = 16.4 K was report ed recently by Treece and collaborators. The reported structure of the thin-film sample deviates from the rocksalt (B1) NbN structure with 2 5% ordered vacancies on each sublattice (space group Pm3m) and a latti ce constant of 4.442 Angstrom. Using full-potential electronic-structu re methods, we contrast the electronic structure with that of B1 NbN. The calculated energy, 1.00 eV/molecule higher than B1 NbN, and calcul ated lattice constant of 4.214 Angstrom indicate that the new phase mu st be something other than the ordered stoichiometric Pm3m phase.