High temperature solution phase diagram of lead zirconate titanate

Abstract

Lead zirconate titanate (PZT) of composition Pb(Zr_XTi_1−X)O₃ is a non-congruently melting material, so crystal growth must be done from a high temperature solution. An understanding of the high temperature solution phase diagram is necessary to make this possible. A variety of solvent systems and solvent properties were evaluated for the growth of PZT, and two innovative lead oxide-phosphate solvents were developed: PbO-PbLiPO₄ (PLP), and PbO-Pb₂P₂O₇ (lead pyrophosphate). PZT crystals were grown from these solvents with compositions near the desirable morphotropic phase boundary composition X = 0.52. Both PLP and Pb₂P₂O₇ form molecular complexes in the melt that participate minimally in the solution of PZT, which is dissolved by the free uncomplexed PbO acting as both a solvent and solute ingredient. The phosphates do favorably reduce the melting temperature, PbO evaporation, density, and position of the melting minimum. These solvents were used to determine liquidus and solidus curves over the range of interest in X, and to develop a universal solubility equation. Modeling was used to fit and extrapolate these PZT-solvent phase diagrams to other PbO-based solvents. These results explain prior art data and point to a congruently melting indifferent point at the lead titanate end of the phase diagram.