Bulk and surface electron states in WO3and tungsten bronzes

Abstract

Calculations of the electronic structure of WO3 suggest that the detailed crystallographic arrangement determines rather sensitively the size of the semiconducting energy gap. This increases from 1.6 eV in the perovskite-like cubic approximation to the structure, as used in the only previous study, to 2.4 eV in the full monoclinic distorted structure. Valence-band widths and density-of-states peaks are consistent with experimental photoemission spectra. The extra electrons introduced by alkali-metal atoms in the bronzes NaxWO3 enter the conduction band of W t2g states, behaving rather like a rigid-band model. Surface states are predicted to be absent from the semiconducting energy gap of the defect-free (001) surface of WO3.