Thermal and structural studies of phase transitions in layered perovskite SrBi2Ta2O9

Specific heat and X-ray diffraction (XRD) in Bi-layered perovskite Sr0.85Bi2.1Ta2O9 were measured. Two anomalies in C-p were observed at 670 K (T-c) and 510 K (T*) in Bi-rich Sr0.85Bi2.1Ta2O9, although no anomalies in epsilon and P-s have been reported around T*. The temperature dependence of XRD patterns shows that the crystal structure of Sr0.85Bi2.1Ta2O9 (orthorhombic, A2(1)am at room temperature) transforms to a tetragonal phase (I4/mmm) above T-c, passing through a probable intermediate orthorhombic phase at T*. SrBi2Ta2O9 remains orthorhombic above T-c (608K) and becomes tetragonal at about 900 K, Stoichiometric SrBi2Ta2O9 and Bi-rich Sr0.85Bi2.1Ta2O9 show a fundamentally different sequence of phase transitions at high temperatures, although their crystal structures are isostructural in the ferroelectric phase.