Production of complex lead-containing niobates transparent ferroelectric ceramics

The sequence of phases formed during the solid state reaction in oxide mixtures has been studied by X-ray diffraction and differential thermal analysis (DTA) in the binary oxide system PbO-Nb2, O5 and the ternary oxide system PbO-Nb2O5-Sc2O3. The following compositions have been investigated: 3PbO : 2Nb2O5, 2PbO : Nb2O5, 5PbO : 2Nb2O5, 3PbO : Nb2O5, 4PbO : Nb2O5, and 3PbO : 2Nb2O5 : X, 2PbO : Nb2O5 : X, 5PbO : 2Nb2O5 : X, 3PbO : Nb2O5 : X, 4PbO: Nb2O5 : X, the molar ratio X of Sc2O3 being taken 0; 0.025; 0.05; 0.1; 0.3; 0.5; 0.8; 1.0; 1.2.It is found that the formation of both the binary and ternary compounds includes complicated multistage processes with several intermediate products during the reaction. In all cases, except 3PbO : 2Nb2O5, one or more additional phases less rich in lead oxide coexist with the stoichiometric phase. Obviously, this is due to evaporation of lead oxide above 800°C. Therefore PbSc1/2Nb1/2O3, produced from the stoichiometric proportion of oxides 4PbO : Nb2O5 : Sc2O3 by a solid state reaction contains 5-10%vol. of a cubic pyrochlore structure included in the main phase of the perovskite structure. This has also been observed in all cases of synthesis of complicated lead-containing niobates. To produce a single-phase material it is necessary to take 0.2 mol. ratio of Sc2O3 above the stoichiometric proportion according to the calculations suggested.