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

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 BO 6 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.