Lead titanate is not a classic case of a displacive ferroelectric phase transition

Recent titanium K edge XAFS results show that the ferroelectric to paraelectric phase transition at the Curie temperature (Tc), 763K, in lead titanate is an order-disorder transition. This contradicts the classic displacive model used to describe this transition. The tetragonal distortion of the perovskite unit cell introduces distinctive structure into the near-edge. This structure shows a weak, monotonically decreasing temperature dependence in the range 10K to 900K, and remains in the near-edge even above Tc where the displacive model would predict its substantial decrease. The Ti EXAFS also indicates that the tetragonal distortion measured by crystallographic techniques below Tc changes smoothly as the material is heated above Tc. This continuous variation in the local structure around the titanium atom across Tc shows that the ferroelectric transition is order-disorder and not the classic displacive model.