LOCAL-STRUCTURE AND THE PHASE-TRANSITIONS OF BATIO3

We have measured and analyzed the Extended X-ray Absorption Fine Struc ture (EXAFS) of BaTiO3 at the barium k edge and the X-ray Absorption N ear Edge Structure (XANES) at the titanium K edge. Our structural data show that the sequence of phase transitions in this material as the t emperature increases is explained by a disordering of domains wherein the local structural environment remains approximately rhombohedrally distorted at all temperatures around both metal sites. As the temperat ure is raised, the long range correlations between these local distort ions change, resulting in the observed sequence of phase transitions. Our measurements confirm the model of eight-site disorder used to expl ain the phase diagram of BaTiO3. We show that EXAFS and XANES are sens itive probes of both the magnitude and direction of the local structur al distortions which accompany ferroelectricity and therefore are sens itive probes of the microscopic mechanism of ferroelectricity.