Synthesis and characterisation of Bismuth orthovanadate
A wide range of lime green/yellow to mustard/orange bismuth orthovanadate samples were prepared via solid state and precipitation techniques. The materials were characterised using thermal analysis, X-ray diffraction, evolved gas analysis, reflectance infra-red spectroscopy, electron microscopy, colour measurement, wet chemical analysis and ac impedance spectroscopy. This product characterisation identified more complex materials than previously reported, with generally non-stoichiometric compositions established. The fergusonite-type polymorph of BiVO4 was prepared via solid state synthesis. This phase was found to be slightly V-rich in composition, of BiV1.025O4+x. It occurs as a line composition and melts congruently at 930°C ± 10°C. A reversible, non-quenchable phase transition from fergusonite to scheelite-type BiV1.025O4 + x was determined at 240-250°C. The zircon and scheelite-type polymorphs of BiVo4 were produced by controlled room temperature precipitation, at pH 1-2 and 3-7 respectively, whereas the fergusonite-type polymorph was obtained at pH 1-7 on precipitation from solutions at 95°C. All three precipitated polymorphs were found to contain a proportion of foreign ions, totalling 2.0-3.0, 2.7-3.6 and 0.3-0.9 weight% for the zircon, scheelite and fergusonite-type polymorphs respectively. These foreign ions consist of adsorbed water and structural nitrate, carbonate and hydroxyl constituents. Chemical analysis identified essentially Bi-rich compositions for the precipitated polymorphs. An irreversible phase transition from zircon to scheelite-type BiVO4 was determined at 500-520°C. BiVO4 is studied as a possible alternative yellow pigment for the toxic cadmium and chrome-containing traditional yellow colourants. The sample colours obtained were compared with those of the commercial CdS, PbCrO4 and BiVO4/Bi2MoO6 pigments. Polycrystalline, phase pure BiV1.025O4 + x ceramics, prepared by solid state synthesis were investigated using ac impedance spectroscopy. These materials were found to exhibit mixed oxide ion/electronic conduction at both low and high temperatures, ie. <400°C and >600°C. Oxide ion conductivity dominates the temperature region between 400 and 600°C, with an activation energy of 0.80 + 0.01eV. The bulk conductivity below 400°C was found to be metastable and influenced by the presence of water. The possibility of protonic conduction below 400OC is identified.