Precursor effects, broken local symmetry, and coexistence of order-disorder and displacive dynamics in perovskite ferroelectrics

The dynamical properties of ferroelectric perovskites have typically been classified as being either of order-disorder or displacive type. Here we show that this simple scheme is not generally applicable but that the generic and intrinsic properties of this material class always combine both aspects. Important in this respect is the fact that different length and time scales are associated with the two properties which makes it difficult to observe them with a single experiment. In addition, it is shown that independent of the transition temperature of the respective system precursor dynamics exist which set in approximately 75 K above the actual phase-transition temperature. This theoretical result is supported by measurements of elastic and dielectric coefficients.