NATURE OF THE FERROELECTRIC PHASE-TRANSITION IN PBTIO3

We have measured quantitatively the temperature dependence of the loca l distortions of PbTiO3 crystals below and above the structural fenoel ectric phase transition, using x-ray absorption fine-structure (XAFS) measurements. Two probe atoms, Pb and Ti, were used. The results were analyzed by fitting parametrized theoretical XAFS spectra to the exper imental results. These measurements provide quantitative ''distortion parameters'' defined as the difference between the distance of the pro be to its nearest neighbor measured in the actual structure and that i n a centrosymmetric structure with the same unit cell dimensions. At l ow temperatures the Pb-edge spectra were fit using four shells and inc luded single, double, and triple scattering configurations. The high-t emperature fits included only two shells. The Ti-edge spectra were fit ted with the first shell only. The Pb and Ti distortions vary relative ly Little with temperature below the transition and decrease faster ne ar the transition temperature. Above the transition temperature, the P b and Ti distortions at the peaks of their distortion distribution fun ctions (DDF's) are both more than 70% of the corresponding low-tempera ture values. These results show that an essential element of order-dis order is present even in this nominally pure ferroelectric crystal whi ch displays a soft mode and a dielectric constant typical of displaciv e-type ferroelectrics. The presence of the local distortions suggests that the displacements should have at least two correlation length sca les, one associated with the the local distortions, the other with the order parameter.