Phase formation in the bismuth-rich end of the Bi₂O₃-V₂O₅ system was investigated and a solid solution of sillenite-related phases found, Bi₁₂(Bi_1-x³⁺V_x⁵⁺)O_20+(x-0.5); 0.065<x<0.705. The solid solution limits were found to be far more extensive that those reported for any other sillenite-type solid solution¹.These materials are metastable and have unusual high temperature behaviour not observed in other sillenite phases. High temperature x-ray diffraction, differential thermal analysis and impedance spectroscopy have been used to characterise their high temperature properties. Structural studies of this family of bismuth vanadate sillenite materials have been carried out using powder neutron diffraction techniques to try to correlate the metastable nature of these materials to their structures. The structure of these materials is related to the sillenite structure (fig.1), but a new model was required to take into account the Bi³⁺, with its lone pair of electrons, partially occupying the tetrahedral site along with V⁵⁺ (fig.2) and the non-stoichiometric nature of the oxygen lattice. It is proposed that the disordered nature of the oxygen lattice is the key to their metastable nature.

[1] Valant et al, Chem. Mater., 14 (2002) 3471

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