Pressure-induced pyrochlore-perovskite phase transformation in PLZST ceramics

The influence of applied pressure on the pyrochlore-perovskite phase transformation in the PLZST system was studied. A stoichiometric and homogeneous ferroelectric PLZST pyrochlore powder was prepared by the coprecipitation and freeze-drying method. In the absence of applied pressure, the phase transformation from pyrochlore to perovskite occurred at temperatures above 550°C, with 100% perovskite phase being obtained at 750°C. Because the molar volume of the pyrochlore is larger than that of perovskite phase by 8.5%, applied pressure is expected to accelerate the pyrochlore-perovskite phase transformation. When the pyrochlore powder was uniaxially cold-pressed, the temperature for initiation of the phase transformation was lowered, and the rate of transformation was increased. This effect is interpreted as an increased nucleation frequency due to the defects created at particle contacts. For evaluation of pressure effects during annealing at elevated temperature, the powder was hot-pressed at various temperatures and pressures in air. Hot-pressing was found to greatly enhance the phase transformation rate. This is attributed to the additional P · ΔV_molar driving force for the phase transformation provided by applied pressure.