Transitions in the glassy ferroelectric PLZT (8.7/65/35)

Abstract

Transitions in glassy crystalline ferroelectrics are broad because of chemical disorder. Views differ on the length scale of the important compositional inhomogeneity-from macroscopic to fluctuations at the unit-cell level. A high-resolution X-ray scattering experiment performed on powdered material is reported. A small increase in the width of the (200) reflection occurs at about 640 K, the transition temperature for PLZT (0/65/35). At about 560 K the widths of the (220) and (321) reflections begin to increase with decreasing temperature. The root mean square displacements of atoms forming BO6 octahedra increase markedly with falling temperature, but atoms on A sites show the more usual decrease in vibrational amplitude. These findings are interpreted as the onset of local polarisation at 640 K in lanthanum-deficient regions which at 560 K becomes longer in range, producing a small rhombohedral distortion over many unit cells. Below room temperature, these regions become connected, causing a transition to overall rhombohedral symmetry.