Studies of bismuth vanadate (Biâ†4Vâ†2Oâ†1â†1) and BiMeVOX (Me=W'6'+, Ta'5'+, Nb'5'+, Zr'4'+ and Ti'4'+) materials
The oxide ion conductor bismuth vanadate (Bi4V2O11) can be doped with various metal cations, Me2+ to Me6+; Me partially substitutes for vanadium to produce a new family of solid electrolytes known as BiMeVOX (Bismuth Metal Vanadium Oxides. Bi4V2O11 is polymorphic having three forms α, β and γ; the high temperature form (γ) has high oxygen ion conductivity of comparable magnitude to existing solid electrolytes such as YSZ; however, this occurs at high temperatures (> 560°C). Doping Bi4V2O11 with Me ions, leads to stabilisation of the γ-phase to room temperature, thus producing a material with promising practical applications. The parent material, Bi4V2O11, is a non-stoichiometric phase forming a limited solid solution given by Bi4+yV2-yO11-y; by virtue of the variable Bi:B ratio this leads to a range of compositions for the BiMeVOX materials and hence the overall formula Bi4-yV2-x-yMexO11-y-σ. Thus, there is scope for a range of γ-phase compositions. The aims of this study were to - (1) investigate the Bi4+yV2-yO11-y solid solution and formation of stoichiometric Bi4V2O11 (i.e. y = 0); (2) optimise synthesis conditions of Me-doped Bi4V2O11 (Me = W6+, Ta5+, Nb5+, Zr4+ and Ti4+); (3) determine the compositional extent of each solid solution formed by using a variety of diffraction, thermal and microscopic techniques; (4) investigate stabilisation, to room temperature, of the high temperature gamma (γ) phase, formed on doping Bi4V2O11; (5) elucidate possible substitution mechanisms and initiate a crystallographic study of gamma phase materials. Bi4V2O11 : It is shown that stoichiometric Bi4B2O11 does not form, via solid state reaction, without precipitation of BiVoO4 (scheelite) as secondary phase; a slight excess of Bi (y ≈ 0.02) is required to form single phase material in this system.