Semiconductor-to-metal transition in WO3-x: Nature of the oxygen vacancy

Hybrid functional density functional theory calculations of the oxygen vacancy (VO) in monoclinic bulk WO3 provide a coherent rationalization for the strong dependence of WO3-x electronic properties on the VO concentration and of the semiconductor-to-metal transition, a phenomenon intimately connected to the electrochromic effect. Different VO centers containing W4+, W5+, and W6+ species are expected to coexist. Optical transition levels are 0.7-1.0 eV below the conduction band minimum, in agreement with experiments. The complex nature of V-O in WO3 can only be detected with methods that properly describe band gaps and polaronic distortions at defect sites.