Pressure-induced pyrochlore-perovskite phase transformation in PLZST ceramics

The influence of applied pressure on the pyrochlore-perovskite phase transf ormation in the PLZST system was studied. A stoichiometric and homogeneous ferroelectric PLZST pyrochlore powder was prepared by the coprecipitation a nd freeze-drying method. In the absence of applied pressure, the phase tran sformation from pyrochlore to perovskite occurred at temperatures above 550 degreesC, with 100% perovskite phase being obtained at 750 degreesC. Becau se the molar volume of the pyrochlore is larger than that of perovskite pha se by 8.5%, applied pressure is expected to accelerate the pyrochlore-perov skite phase transformation. When the pyrochlore powder was uniaxially cold- pressed, the temperature for initiation of the phase transformation was low ered, and the rate of transformation was increased. This effect is interpre ted as an increased nucleation frequency due to the defects created at part icle contacts. For evaluation of pressure effects during annealing at eleva ted temperature, the powder was hot-pressed at various temperatures and pre ssures in air. Hot-pressing was found to greatly enhance the phase transfor mation rate. This is attributed to the additional P . DeltaV(molar) driving force for the phase transformation provided by applied pressure.