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DF: Dielektrische Festkörper

DF 9: Phase Transitions

DF 9.2: Hauptvortrag

Donnerstag, 30. März 2006, 10:10–10:50, M{\"U}L Elch

Structural instabilities in Ferroelectric Aurivillius compounds — •J. Manuel PEREZ-MATO — Dept. Fisica Mat. Condensada, Fac. Ciencia y Tecnologia, Univ. del Pais Vasco, UPV/EHU, Apdo 644, 48080 Bilbao, Spain

The Aurivillius phases are layered bismuth oxides formed by the regular stacking of Bi2O2 slabs and perovskite-like blocks. Many are ferroelectric at room temperature and are of special technological interest. Apart from the polar distortive mode, other displacive modes at the Brillouin zone border are unstable and are frozen in the room temperature structure. The close interplay of these different instabilities situates these systems beyond the conventional framework for perovskite ferroelectrics. We present ab-initio calculations and thorough structural and symmetry analyses of two specific compounds, SrBi2Ta2O9 (SBT) [1], and Bi4Ti3O12 (BTO), which have respectively two and three layers of BO3 octhaedra within the perovskite blocks. The energy landscape around the parent tetragonal configuration has been explored analysing the possible existence of intermediate phases. The essential role of secondary hard modes for the stabilization of the ferroelectric phase is shown. Surprisingly, the rigid-like behaviour of the perovskite octahedra in BTO is the result of anharmonic couplings. Energy maps have been compared with empirical bond valence criteria with a striking agreement of both approaches [2].

[1]Perez-Mato et al., Phys. Rev. B (2004) 70, 214111

[2]Etxebarria et al., Phys. Rev. B (2005) 72, 174108