Electronic Structure and Stability of Ordered Vacancy Phases of NbN
A recently reported metastable phase of NbN with a superconducting T c =16.4 K is characterized using full potential electronic structure methods. This new phase, which has Pm3m (cubic) symmetry, can be described as the B1 (rocksalt) structure with 25% ordered vacancies on each sublattice. We compare the equation of state and electronic spectrum of this Pm3m phase with its rocksalt counterpart [1] and with Nb 4 N 3 in the 14/mmm (tetragonal) phase, which allows the characterization of N vacancies without accompanying Nb vacancies. For Pm3m NbN, the calculated lattice constant is 5% smaller than reported and the energy is 1.00 eV/molecule higher than B1 NbN, suggesting that the newly reported phase is something other than a stoichiometric Pm3m phase of NbN. We report on the energy surface for tetragonal distortions of this phase, from which we evaluate its structural stability and obtain Poisson's ratio.
Superconducting Transition Temperatures of R. F. Sputtered NbN Films
NbN films are examined with respect to their electrical and superconducting properties. The films are deposited by r.f. reactive sputtering technique under argon gas atmosphere added with nitrogen. The films obtained have a wide range of resistivities and transition temperatures depending upon the partial nitrogen pressure in the argon atmosphere and the substrate temperature at which the films were deposited. The maximum transition temperature obtained is 17.3°K, higher by 0.9°K than the highest one reported so far.
The crystal structure of niobium monoxide
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