Structural phase transitions in the relaxor ferroelectric Pb2Bi4Ti5O18

The relaxor ferroelectric Pb2Bi4Ti5O18 has been studied by Rietveld refinement of powder neutron diffraction data collected at temperatures of 100, 250 and 400 °C. Our refinements are compatible with the ‘average’ crystal structure of Pb2Bi4Ti5O18 undergoing the phase transition sequence F2mm→I4mm→I4/mmm as a function of increasing temperature, with the latter phase being observed above the known ferroelectric Curie temperature, Tm, and the intermediate phase consistent with a previously observed dielectric anomaly around 207 °C. The results are, however, in conflict with both observation of a symmetry lowering (to space group B2eb) in the lowest temperature phase, observed by electron diffraction, and also with electrical property measurements, which suggest both a- and c-axis polarisation up to Tm. Nevertheless, these crystallographic results are consistent with the observation of relaxor behaviour in this material, and underline the importance of considering ‘long-range’ versus ‘local’ structural effects in relaxor materials.