A METHOD FOR THE PREPARATION OF HIGH-PURITY LEAD TITANATE ZIRCONATE SOLID-SOLUTIONS BY CARBONATE-GEL COMPOSITE POWDER PRECIPITATION

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 cat ion) such as PbTiO3 and its solid solutions, is described. The method involves the coprecipitation of the divalent cations (A) as fine parti cles of carbonates with hydrated gels of titania or zirconia (BO2 . xH (2)O, 12 < x < 130; B = Ti4+, Zr4+) by the addition of ammonium carbon ate. Such coprecipitation is possible because of the instability of th e carbonates and oxycarbonates of Ti and Zr in aqueous media in compar ison to the polymerised hydroxides, whereas lead carbonate is precipit ated readily. The method gives rise to composite powders in which the submicron crystalline particles of the carbonates, crystalline to X-ra y diffraction, are embedded within amorphous gels of BO2 . xH(2)O. The composite nature of the precipitate is confirmed by transmission elec tron microscopy. The precipitate is dried and calcined at elevated tem peratures. Upon heating to 350-400 degrees C, the reaction between the carbonate and the amorphous dry gel proceeds via the formation of the intermediate PbO . zTiO(2) (z < 0.09) solid solution which then conve rts to a defect pyrochlore phase (A(2)B(2)O(7-delta), delta = 1). Abov e 450 degrees 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 f inal product.