Regensburg 2007 – wissenschaftliches Programm

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DF: Fachverband Dielektrische Festkörper

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

DF 4.4: Vortrag

Dienstag, 27. März 2007, 11:20–11:40, H11

Is the observation of a soft mode sufficient to characterize a transition as displacive? The case of SrTi18O3 — •Annette Bussmann-Holder — Max-Planck-Institut für Festkörperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Deutschland

For many years it has been believed that ferroelectric phase transitions can be strictly classified as either being of displacive or of order / disorder type. First doubts about this clear cut distinction came from local probes like EPR and NMR, where evidence for order / disorder behaviour was obtained in systems exhibiting classical soft mode behaviour. A similar seemingly controversial situation is now also encountered in the isotope induced ferroelectric system SrTi18O3 where long wave length testing experiments reveal classical mode softening whereas NMR and nonlinear dielectric response support order / disorder dynamics. It is shown here that displacive and order / disorder dynamics coexist in SrTi18O3, however obeying different length and time scales. Far above the actual lattice instability self-induced polar nano-domains are formed which stem from optic-acoustic mode coupling and are characterized by order / disorder dynamics, whereas simultaneously a classical soft mode exists in the long wave length limit. The polar state is novel in this compound since an incomplete ferroelectric state forms below Tc.