Bi₂WO₆, Bi₆WO₁₂ and Bi₁₂WO₂₁: Bi- W- Oxides

The Bi- W- Oxide pseudobinary phase diagram is a complex one:

Figure 1: WO3-Bi2 phase diagram following Speranskaya, Neorg. Mater. 6(1) 149 (1970)

Bi₂WO₆

Bi₂WO₆ has a tetragonal, layered structure and a direct semiconductor band gap of 2.6 eV. It has be used successfully as a photocatalyst for oxidation of organic compounds (Tang et al., Catalysis Letters, 92(1-2) 53 (2004)).

Bi₆WO₁₂

Bi₆WO₁₂ has a pseudo cubic structure. It is one composition within a continuous solid solution range of possible compositions from Bi_5.6WO_11.4 to Bi₇WO₁₅. The variation arises from the substitution of Bi³⁺ for W⁶⁺ upon a subset of cation sites balanced by fractional occupancy across the O^2- sites. Interestingly, the lattice parameters of these compounds do not vary linearly with their composition. Work undertaken at Cambridge has measured an indirect semiconducting band gap of 2.9 eV in powdered Bi₆WO₁₂.

Bi₁₂WO₂₁

Bi₁₂WO₂₁ is a sillenite- an impurity stabilised form of the cubic δ-Bi₂O₃.