Pyrochlore-perovskite phase transformation in highly homogeneous (Pb,La)(Zr,Sn,Ti)O₃ powders

Control of undesirable pyrochlore is critical to the processing of lead perovskite dielectrics and ferroelectrics. In this work, a homogeneous and stoichiometric fine powder of the ferroelectric Pb_0.97La_0.02(Zr_0.64Sn_0.25Ti_0.11)O₃ (PLZST) has been prepared by a hydroxide coprecipitation and freeze-drying method. Through systematic variation of processing temperature and time, we have characterized the pyrochlore-to-perovskite crystallization process of the powder. Studies of the crystallization behavior of the precursor as a function of temperature by X-ray powder diffraction and transmission electron-microscopy showed that the pyrochlore phase forms from an amorphous precursor, initially at low temperatures around 500-550°C. Further heat treatment to 750°C resulted in development of the perovskite phase with no significant pyrochlore crystallite growth. At intermediate temperatures the precursor yielded a fine mixture of pyrochlore and perovskite phases. When the pyrochlore phase was heat-treated in air a slight weight increase was observed in the temperature range of 300- 700°C, which is attributed to oxygen absorption. The weight increase was not observed upon firing in argon atmosphere; instead, a weight loss occurred near 700°C, which was identified as being mostly due to CO₂ gas evolution. This implies that the pyrochlore phase is crystallographically and thermodynamically metastable. An apparent activation energy of 53.9 kcal mol^-1 was estimated for the pyrochlore- perovskite phase transformation.