THE SPONTANEOUS RELAXOR TO NORMAL FERROELECTRIC TRANSFORMATION IN LA-MODIFIED LEAD-ZIRCONATE-TITANATE

Tetragonal La-modified lead zirconate titanate (PLZT) ceramics (Zr/Ti = 40/60) with La-contents of 8, 12, and 15 at.% have been studied usin g dielectric spectroscopy, X-ray diffraction, and hot-stage transmissi on electron microscopy (TEM). Increasing La-content drives the lead zi rconate titanate from near conventional ferroelectric to typical relax or-type behaviours. The PLZT composition with 12 at.% La possessed a u nique transformation. Dielectric constant measurements revealed a spon taneous switching between relaxor and normal ferroelectric behaviours at a temperature significantly below that of the dielectric maximum. P ronounced X-ray line broadening was observed above the switching tempe rature indicating the presence of local cubic-tetragonal structural tr ansformations, while line splitting was observed below the switching t emperature indicating the formation of a macroscopic tetragonal struct ure. Neither the higher or lower La-content compositions exhibited an analogous spontaneous switching. In addition, the temperature dependen t micro structure evolution was investigated for the various compositi ons by TEM. A common sequence of domain transformations was identified between a polar nanodomain state and a long-range ferroelectric state . The extent to which this pattern was completed was found to be stron gly dependent on La-content. Evidence of the spontaneous relaxor to no rmal ferroelectric transformation, which is structurally a polar nanod omain to normal micron-sized ferroelectric domain transformation, was found for the composition with 12 at.% La.