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.
Electronic band structure and bonding in Nb3O3.
The local-density approximation is used to investigate the electronic and structural properties of Nb 3 O 3 and the reasons for its unusual defect rocksalt structure with 25% ordered vacancies. A tight-binding Hamiltonian is fitted to the energy bands which are compared to those of the hypothetical rocksalt phase. This analysis indicates that the ions in the ordered vacancy structure are nearly neutral, the Madelung energy is small, and electronic band effects contribute decisively to its stability. The tight-binding results are consistent with the interpretation that this structure favors a special hybridization between Nb d orbitals and their direct interaction gives rise to strong and nearly coulent metal-metal bonds
Theoretical interpretation of the valence X-ray photoelectron spectrum of TiC
The valence X-ray photoelectron spectrum of TiC is interpreted in terms of the density of states and in terms of a simple model for transition matrix elements. The conceptual difficulties arising with both the initial and the final state are discussed.