Ferroelectric phase transitions in perovskites with off-center ion displacements

We present a model of ferroelectrics which takes into account the softening of a transverse optical mode, the existence of dynamical spontaneous off-center displacements and their interaction with each other. In this paper we confine ourselves to temperatures above T-c. The model is quantitatively compared with experimental results in two very different materials, PbTiO3 and KNbO3. The results show that the existence of off-center displacements and their interaction with the soft mode play a crucial role in explaining the properties of ferroelectric phase transitions in perovskite crystals and possibly other crystal families. (C) 1997 Published by Elsevier Science Ltd.