Ferroelectric Phase Transition and New Intermediate Phase in Bi-Layered Perovskite SrBi 2 Ta 2 O 9

Thermal and structural studies showed that SrBi2Ta2O9 undergoes ferroelectric-intermediate and intermediate-paraelectric phase transitions at 608 K (T-C) and 850 K (T*), respectively, while Bi-rich Sr0.85Bi2.1Ta2O9 at 510 K (T-o) and 670 K (T-C). On heating, SrBi2Ta2O9 still remains to be orthorhombic above T-C, and then changes to tetragonal above T*. The TaO6 is a distorted octaheron along the c-axis even in the paraelectric phase, and locates in antiparallel manner above T*. Below T-C, this crystal favors canted arrangements of TaO6, resulting in the net polarization along the a-axis. It was clarified that SrBi2Ta2O9 shows a canted weak ferroelectricity, analogous to weak ferromagnetism with Dzialoshinski-Moriya interaction.