AN X-RAY DIFFRACTION STUDY OF PMN-PT CERAMICS IN THE REGION OF THE RELAXOR TRANSITION

Electrostrictive ceramics in the solid solution (1-x)PMN-xPT can be used to fabricate high strain actuators for smart structure applications. For example, multi-layer actuators fabricated from 0.9PMN-0.1PT have exhibited over twice the displacement capability of similar actuators made from a soft PZT having d33=580 pC/N (Waechter et al., 2000). To assist in the development of other compositions, the relaxor region of the phase diagram of the PMN-PT system has been investigated by high temperature X-ray diffraction using ceramics with compositions x = 0.00, 0.08 and 0.20. Lattice parameter measurements show that the thermal expansion coefficients of the samples increase by an order of magnitude at a well defined temperature Tα. Diffraction profile studies show that Tα can be correlated with the onset of the cubic-pseudocubic transformation, with its associated microregions of polarization. The profile studies also indicate that the onset of the low temperature stable rhombohedral phase can be correlated with the temperature at which remanent polarization ceases to exist. A tentative phase diagram for the relaxor region of the PMN-PT ceramic system is developed on the basis of these findings.