Although the spinel phase in pure LiGbO, could not be quenched to room temperature, both the spinel and the primitive-cubic form can be prepared under equilibrium conditions and quenched to room temperature when MgGazOl is present in amounts of 40 mo% or more. AcknowledgmentsThe writer thanks J. Cooper, R. Wyland, and D. McClellan for the X-ray measurements. L. Funaro and L. Haydu prepared the samples.

The solid-state reactions occurring in the system Pb0-Ti02-Zr02 were investigated using constant heating rates up to 1000°C. DTA, dilatometric length changes, and XRD analysis were used for characterization. PbO and T i 0 2 reacted exothermally to form the product PbTi03 with a large volume expansion between 450' and 600'C. Formation of PbZrO, from PbO and Zr02 occurred endothermally with a large volume expansion between 700° and 800'C. The expansion was due to reaction topology, differential molar volumes of products and reactants, and the pellet microstructure. In the formation of PZT from ternary powder mixtures, PT formed between 450' and 6OO0C, followed by PZT formation at >7OO0C with no measurable amounts of PbZr03 formed as determined by XRD analysis. The analysis of the mechanisms indicates that the overall kinetics of homogeneous PZT solid-solution formation are determined by either the ionic transport within the perovskite lattice or the phase-boundary reactions leading to perovskite formation and not by the diffusion of Ti across PbO, which is relatively rapid.