Order and Disorder in Ferroelectrics

Recent developments in the understanding of order-disorder and displacive type transitions are presented. It is shown that classical order-disorder type transitions like those in squaric acid and KDP contain in addition to the order disorder also a displacive component in the phase transition mechanism. Similarly classical displacive ferroelectrics like BaTiO3 which have been traditionally described by the condensation of the soft mode contain an order-disorder component as the Ti ion is dynamically disordered between different off-center sites already in the cubic phase. The comparison of the experimental and theoretical electric field gradient tensors at the Ti sites in the various phases of BaTiO3 allows for a quantitative determination of the mixed character of this phase transition.