Electronic band structure, optical properties, and generalized susceptibility of NbO2

The electronic structure of the high-temperature rutile phase of NbO2 is studied by the linearized-augmented-plane-wave method. Potentials constructed by superposition of neutral-atom and ionic-charge densities are used to explore variability of the electronic band structure. A rigid-band scheme is shown to accurately describe optical absorption of the rutile phase of NbO2 stabilized by the addition of 20 at.% Ti as measured by Raccah et al. Differences between the band results for rutile NbO2 and the optical absorption measurements on the low-temperature body-centered tetragonal phase of NbO2 are attributed to band splittings induced by lattice distortion which occur at the phase transition. The static-electron response function (q) is calculated in the constant-matrix-elements approximation. In contrast to the case of isoelectronic VO2, no Fermi-surface nesting features are observed, and (q) is found to be structureless in the vicinity of the point P=(14, 14, 12) which has been associated with a possible soft-mode phonon instability responsible for the lattice transformation.