Among the transition metal oxides, tungsten bronzes have attained considerable attention due to their chemical, electrical and optical properties which depend on the type of alkali-metal atom and its composition. The term tungsten bronze is applied to the non-stoichiometric compounds of general formula MxWO3 (0 > x > 1), where M is an alkali-metal atom. The common structural feature of these materials is the WO6 octahedra which are repeated along three directions through corner sharing. This structural arrangement provides various kinds of tunnels, which are at least partly occupied by alkali...