SPONTANEOUS LOCAL DISTORTIONS AND STRUCTURAL PHASE-TRANSITIONS

Xray Absorption Fine Structure (XAFS) measurements of the local atomic structure of perovskite crystals undergoing various structural phase transitions are summarized and discussed. The results show that the lo cal structure of crystals undergoing ferroelectric antiferroelectric a nd antiferrodistortive transitions is distorted in a disordered fashio n far above the transition to the high symmetry phase. The size of the distortions is a large fraction of the distortion at temperatures far below T-c. Based on these results we propose a model of ferroelectric ity which accounts quantitatively for the temperature dependence of th e dielectric function the soft mode frequency, the imaginary part of t he dielectric constant and the central peak in PbTiO3 and KNbO3.