Berlin 2008 – wissenschaftliches Programm

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DF: Fachverband Dielektrische Festkörper

DF 3: Phase Transitions

DF 3.1: Vortrag

Montag, 25. Februar 2008, 10:40–11:00, EB 407

Temperature-driven structural transformations in relaxor-ferroelectric PbSc_0.5Ta_0.5O₃ and Pb_0.78Ba_0.22Sc_0.5Ta_0.5O₃ — Boriana Mihailova¹, •Bernd Maier¹, Carsten Paulmann¹, Thomas Malcherek¹, Jörg Ihringer², Martin Gospodinov³, Rainer Stosch⁴, Bernd Güttler⁴, and Ulrich Bismayer¹ — ¹Universität Hamburg, Hamburg, Germany — ²Universität Tübingen, Tübingen, Germany — ³Bulgarian Academy of Sciences, Sofia, Bulgaria — ⁴Physikalisch-Technische Bundesanstalt, Braunschweig, Germany

X-ray diffraction and Raman spectroscopic studies in a wide temperature range on perovskite-type relaxors PbSc_0.5Ta_0.5O₃ (PST) and Pb_0.78Ba_0.22Sc_0.5Ta_0.5O₃ (PBST) are presented. The temperature evolution of phonon anomalies and the pseudo-cubic unit cell parameter for both PST and PBST reveals the existence of a critical temperature T^* between the Burns temperature T_B and the temperature of the dielectric-permittivity maximum T_m. T^* is associated with coupling of initially nucleated polar sub-clusters and their aggregation into larger polar nanoclusters. The temperature range between T_B and T^* is characterized by coupling between adjacent off-centered BO₆ octahedra to form initial polar clusters, while the range between T* and T_m is characterized by coupling between off-centered B-cations from adjacent polar clusters. Off-centered Pb atoms exist even above T_B and their coherence length governs the coupling between polar regions comprising B-cation off-centered shifts and directs the formation of proper or relaxor ferroelectric state.