Internal strain relaxation and the glassy behavior of La-modified lead zirconate titanate relaxors

Nonlinear internal friction measurements have been carried out on La-modified lead zirconate titanate relaxors with a Zr/Ti ratio of 65/35. Measurements have been made as a function of La content and dc bias. The elastic softening is believed to have a ferroelastic contribution due to the tilting of the oxygen octahedra, in addition to the expected electrostrictive effects. The temperatures of the maximum softening form a plausible extension to the rhombohedral-rhombohedral phase boundary [Proc. IEEE 61, 959 (1973)]. A frequency dependence of the softening has also been found which was interpreted as the existence of fluctuations in the orientations of the tilts. These fluctuations are proposed to arise in an attempt to partially relieve an inhomogeneity in the electrostrictive strains associated with the polarization fluctuations. The results are discussed in context with previously reported transmission electron microscopy work [C. Randall, Ph. D. dissertation, University of Essex (1987)]. Dielectric measurements have also been done as a function of La content and bias, and the freezing temperature of the polarization fluctuations determined as a function of composition. A revised phase diagram has been proposed using this data base which shows different phase fields having various combinations of freezing mechanisms.