Lead Titanate is not a Classic Case of a Displacive Ferroelectric Phase Transition

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Abstract
Recent titanium K edge XAFS results show that the ferroelectric to paraelectric phase transition at the Curie temperature (T c ), 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 10 K to 900 K, and remains in the near-edge even above T c where the displacive model would predict its substantial decrease. The Ti EXAFS also indicates that the tetragonal distortion measured by crystallographic techniques below T c changes smoothly as the material is heated above T c . This continuous variation in the local structure around the titanium atom across T c shows that the ferroelectric transition is order-disorder and not the classic displacive model.

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