IN-SITU X-RAY-DIFFRACTION STUDY OF THE IMPURITY EFFECTS ON THE CUBIC-TO-TETRAGONAL PHASE-TRANSFORMATION IN PBTIO3

The impurity effects on the phase transformation in a single crystal o f PbTiO3 have been studied by in-situ x-ray diffraction technique. Two single crystals were employed for this study with one containing a no ticeable amount of impurities. For the pure sample, a hysteresis of th e structural parameters such as lattice constants with respect to temp erature for the cubic-to-tetragonal phase transformation is found. Suc h a hysteresis loop is typical of the martensitic phase transformation . Existence of the athermal regions has been reported for the first ti me. The athermal regions were found to be asymmetric between heating a nd cooling cycles. Furthermore, the lattice parameters in both atherma l regions show different behavior. These are caused by microstrain eff ects during growth of the product phase. In fact, within the athermal regions both the cubic and tetragonal phases are highly strained due t o coherency strain. The sample with impurities has shown the existence of embryonic tetragonal fluctuations at temperatures far above T-c. T he presence of impurities lowers T-c, and results in a wider hysteresi s. The athermal regions are narrower for the sample with impurities, s uggesting preferred heterogeneous nucleation at impurity defects.