Phase-formation mechanism for hydrothermally synthesizing lanthanum-modified lead zirconate titanate powders

Hydrothermal crystallization behaviors of two precursors with chemical composition of stoichiometric lanthanum-modified lead zirconate titanate (PLZT, 9/63/35) are examined over the temperature range 160-260 degrees C with durations from 2 to 48 h. Both precursors are prepared by chemical precipitation methods: one is a coprecipitated gel of the stoichiometric PLZT composition and the other is a mixture of individually precipitated Pb, La, Zr and Ti gels. Hydrothermally treated powders are investigated using X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) techniques. Monophase PLZT precipitates can be obtained from the coprecipitated PLZT gel. The amount of PLZT increases with increasing temperature and time. When the reaction temperature is increased to higher than 240 degrees C, the earlier formed PLZT dissolves to form PbTiO3 (PT) and PbZrO3 (PZ). For the mixture of Pb, La, Zr and Ti gels, PT and PZ rather than PLZT precipitate even after hydrothermal treatment at 240 degrees C for 180 h. The phase formation of PLZT under hydrothermal conditions is related to the homogeneous mixing of cations in the precursor and the appropriate process conditions. The reaction can be rationalized on the basis of reaction kinetics.