Abstract
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 local structure of crystals undergoing ferroelectric antiferroelectric and antiferrodistortive transitions is distorted in a disordered fashion 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 ferroelectricity which accounts quantitatively for the temperature dependence of the dielectric function the soft mode frequency, the imaginary part of the dielectric constant and the central peak in PbTiO3 and KNbO3.
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