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

Abstract

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 Pb0.97La0.02(Zr0.64Sn0.25Ti0.11)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.