The results of ab initio calculations indicated that the dispersion of the bands near to the gap region for various phases of WO3 (namely, ε-WO3, δ-WO3, γ-WO3, β-WO3, ortho-WO3, α-WO3, and hex-WO3) was rather close. The rapid increase in the absorption coefficient started at the lower energy range for α-WO3 and hex-WO3 than for the other phases in accordance with the calculated band gaps. An oxygen vacancy turned out to decrease the gap by 0.50eV and to shift the absorption coefficient to the lower energy range in the room temperature γ-WO3 phase. The changes caused by molybdenum and sulphur doping of γ-WO3 were also traced. Only sulphur-doped γ-WO3 was revealed to display the formation of the impurity band along with a sizable reduction in the gap and the shift in the absorption coefficient to the lower energy range.

Tungsten Oxides. I. Effects of Oxygen Vacancies and Doping on Electronic and Optical Properties of Different Phases of WO3. D.B.Migas, V.L.Shaposhnikov, V.N.Rodin, V.E.Borisenko: Journal of Applied Physics, 2010, 108[9], 093713