LEAD TITANATE IS NOT A CLASSIC CASE OF A DISPLACIVE FERROELECTRIC PHASE-TRANSITION

Recent titanium K edge XAFS results show that the ferroelectric to par aelectric phase transition at the Curie temperature (T(c)), 763K, in l ead titanate is an order-disorder transition. This contradicts the cla ssic displacive model used to describe this transition. The tetragonal distortion of the perovskite unit cell introduces distinctive structu re into the near-edge. This structure shows a weak, monotonically decr easing temperature dependence in the range 10K to 900K, and remains in the near-edge even above T(c) where the displacive model would predic t its substantial decrease. The Ti EXAFS also indicates that the tetra gonal distortion measured by crystallographic techniques below T(c) ch anges smoothly as the material is heated above T(c). This continuous v ariation in the local structure around the titanium atom across T(c) s hows that the ferroelectric transition is order-disorder and not the c lassic displacive model.