Preparation and properties of pigmentary grade BiVO4 precipitated from aqueous solution

Bismuth vanadate, nominally BiVO4, is used as a yellow pigment in the ceramic and plastic industries. It can be prepared by heating stoichiometric quantities of Bi and V precursors but brighter colours are obtained by an aqueous preparation route. Depending on precipitation conditions, either a zircon- or scheelite-structured phase may be obtained. Thermal analysis, coupled with effluent gas analysis and X-ray diffraction of the heated zircon-structured solid, discloses complex phase transitions in the range 200–450 °C. Initially the zircon-structured phase transforms to a scheelite-structured phase which, on cooling, converts to a fergusonite-structured phase. The crystallographic changes during the heating cycle are accompanied by a two-stage weight loss, totalling ∼2.0–3.0 wt.%: evolved gas species are principally nitrogen oxides and water. The structural role of these volatiles is examined. The pigmentary colour achieved depends, in part, on the microstructure, stoichiometry and density of the precipitated product as well as the sequence of phase transformations with loss of volatiles, all of which influence the complex texture and defect structures of the calcined product.