A method for the preparation of high purity lead titanate zirconate solid solutions by carbonate–gel composite powder precipitation

Abstract

A novel wet chemical route for the preparation of perovskite titanates (ABO ₃ type, where A is a divalent cation and B is a tetravalent cation) such as PbTiO ₃ and its solid solutions, is described. The method involves the coprecipitation of the divalent cations (A) as fine particles of carbonates with hydrated gels of titania or zirconia (BO ₂ xH ₂ O, 12<x<130; B=Ti ⁴⁺ , Zr ⁴⁺ ) by the addition of ammonium carbonate. Such coprecipitation is possible because of the instability of the carbonates and oxycarbonates of Ti and Zr in aqueous media in comparison to the polymerised hydroxides, whereas lead carbonate is precipitated readily. The method gives rise to composite powders in which the submicron crystalline particles of the carbonates, crystalline to X-ray diffraction, are embedded within amorphous gels of BO ₂ xH ₂ O. The composite nature of the precipitate is confirmed by transmission electron microscopy. The precipitate is dried and calcined at elevated temperatures. Upon heating to 350–400 °C, the reaction between the carbonate and the amorphous dry gel proceeds via the formation of the intermediate PbOzTiO ₂ (z<0.09) solid solution which then converts to a defect pyrochlore phase (A ₂ B ₂ O _7-δ , δ=1). Above 450 °C, the latter converts to an isocompositional perovskite phase. The process is superior to ceramic methods because of the high purity, uniform chemical homogeneity and lower particle size of the final product.