KINETICS OF THE HYDROTHERMAL CRYSTALLIZATION OF THE PEROVSKITE LEAD TITANATE

The hydrothermal crystallization kinetics for the perovskite PbTiO3 have been investigated under autogenous conditions at temperatures in the range of 225-250-degrees-C and feedstock concentrations of 0.1-1.0 M. At these temperatures, crystalline perovskite particles were obtained in approximately 4-7 h. Transmission electron microscopy (TEM) of the product oxides showed nanometer sized crystallites with an elongated morphology. The crystallization kinetics were monitored using X-ray powder diffraction on samples extracted from the reaction mixture at various times. The crystallization rate data were analyzed according to a generalized solid-state kinetic treatment which, along with microstructural evidence, suggest that the PbTiO3 formation reaction proceeds via a dissolution-recrystallization mechanism. It is proposed that the precursor amorphous hydrous oxides of lead and titanium dissolve and recrystallize to form the perovskite phase. The relative rates of dissolution and recrystallization were found to be strongly temperature dependent within the range examined. At all temperatures, the recrystallization kinetics appeared to obey a zero-order rate law. An apparent activation energy of 7.2 kcal/mol was estimated for the hydrothermal PbTiO3 formation reaction.