Structural Phase Transitions in the Ferroelectric Oxide SrBi 2 Ta 2 O 9

The structure of SrBi 2 Ta 2 O 9 between room temperature and 1100°C has been studied using high resolution powder neutron diffraction methods. At room temperature the structure is orthorhombic (space group A 2 1 am ) and this undergoes an apparently continuous transition to a paraelectric orthorhombic phase in space group Amam near 300°C in addition to a transition above 500°C to a tetragonal structure in space group I 4/ mmm . This sequence of transitions is consistent with a group theoretical analysis. The low temperature ferroelectric phase is characterised by both rotations of the TaO 6 and displacement of the Ta atoms from the center of these octahedra, the paralectric phase contains a single tilt of the TaO 6 octahedra while there are no rotations of the TaO 6 octahedra in the high temperature tetragonal structure.