High-temperature structural studies of PbBi2M2O9 (M = Nb and Ta)

A high-resolution synchrotron diffraction study of the temperature dependence of the structures of the two layered Bi oxides PbBi2Ta2O9 (PBT) and PbBi2Nb2O9 (PBN) is reported. The transition from the low-temperature orthorhombic ferroelectric structure to the high-temperature tetragonal paraelectric structure in PBN involves an intermediate orthorhombic paraelectric phase. We conclude that this has the same Amam structure as observed in SrBi2Ta2O9 and its Bi-rich analogue. Identifying the sequence of phase transitions in PBT is more challenging since above 200 degreesC the structure is metrically tetragonal within the resolution of the synchrotron diffraction measurements. By contrast, neutron diffraction measurements suggest that PBT remains orthorhombic to above the Curie temperature 430 degreesC. The possible identity of the intermediate space group is discussed.