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 using dielectric spectroscopy, X-ray diffraction, and hot-stage transmission electron microscopy (TEM). Increasing La-content drives the lead zirconate titanate from near conventional ferroelectric to typical relaxor-type behaviours. The PLZT composition with 12 at.% La possessed a unique transformation. Dielectric constant measurements revealed a spontaneous switching between relaxor and normal ferroelectric behaviours at a temperature significantly below that of the dielectric maximum. Pronounced X-ray line broadening was observed above the switching temperature indicating the presence of local cubic-tetragonal structural transformations, while line splitting was observed below the switching temperature indicating the formation of a macroscopic tetragonal structure. Neither the higher or lower La-content compositions exhibited an analogous spontaneous switching. In addition, the temperature dependent micro structure evolution was investigated for the various compositions 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 strongly dependent on La-content. Evidence of the spontaneous relaxor to normal ferroelectric transformation, which is structurally a polar nanodomain to normal micron-sized ferroelectric domain transformation, was found for the composition with 12 at.% La.