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DF: Fachverband Dielektrische Festkörper

DF 4: Internal Symposium “Order/Disorder versus/with Displacive Behaviour”

DF 4.7: Hauptvortrag

Dienstag, 27. März 2007, 12:20–13:00, H11

Order - disorder versus displacive behaviour of ferroelectric perovskites — •Robert Blinc — J. Stefan Institute, Ljubljana, Slovenia

Whereas the first microscopic theory of BaTiO3 was based on order-disorder behavior, later on BaTiO3 has been considered as a classiacal example of displacive soft mode transitions, which can be described by anharmonic lattice dynamics. Already more than twenty years ago, electron paramagnetic resonance (EPR) measurements performed on Mn4+, Cr3+, and Fe3+ doped BaTiO3 by Müller et. al. seriously questioned the pure displacive character of the phase transitions. Recently, the problem has been studied by quadrupole perturbed 47Ti and 49Ti NMR. It was clearly shown that the Ti sits off-center not only in the tetragonal but also in the cubic phase. This off-center scenario confirms theoretical studies which showed a combined displacive and order-disorder character of the transitions in BaTiO3. The model clearly shows the characteristics of a displacive transition, but with a simultaneous partial ordering of the Ti subsystem as an additional order-disorder feature. Similar results were obtained for SrTiO3 and 18O enriched SrTiO3. Here we present quadrupole perturbed 17O NMR data of both BaTiO3 and SrTiO3 which throw some new light on the role of the oxygen network as well as the A and B ions at the phase transitions in ABO3 perovskite lattices.