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

Abstract

The electronic structure of the high-temperature rutile phase of Nb${\mathrm{O}}_2$ 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 Nb${\mathrm{O}}_2$ stabilized by the addition of 20 at.% Ti as measured by Raccah et al. Differences between the band results for rutile Nb${\mathrm{O}}_2$ and the optical absorption measurements on the low-temperature body-centered tetragonal phase of Nb${\mathrm{O}}_2$ are attributed to band splittings induced by lattice distortion which occur at the phase transition. The static-electron response function $\chi \left(\overrightarrow{\mathrm{q}}\right)$ is calculated in the constant-matrix-elements approximation. In contrast to the case of isoelectronic V${\mathrm{O}}_2$, no Fermi-surface nesting features are observed, and $\chi \left(\overrightarrow{\mathrm{q}}\right)$ is found to be structureless in the vicinity of the point $P=(\frac{1}{4}, \frac{1}{4}, \frac{1}{2})$ which has been associated with a possible soft-mode phonon instability responsible for the lattice transformation.