Crystalline ferroelectrics with glassy polarization behavior

We report measurements of the optic index of refraction as a function of temperature, n (T), in the crystalline ferroelectrics having the simple perovskite (ABO₃) structure. We show that these crystals possess a local, randomly oriented, nonreversible polarization below a temperature T₃ several hundred degrees above the ferroelectric phase-transition temperature T₂. Using a simple model, we account for this behavior and understand quantitatively the values of T₃. This model, we believe, contains the basic physical understanding of ferroelectrics with a diffuse phase transition.