Structure versus relaxor properties in Aurivillius type compounds

Structure refinements in MIIBi4Ti4O15 (MII = Ca, Ba), Bi4−xBaxTi3−xNbxO12, (Na0.5Bi0.5)1−xBaxBi4Ti4O15 and Bi4Ti3O12–PbTiO3 systems are used to report structural features of relaxor Aurivillius phases. In compounds with relaxor-like behaviour, the average structure is almost undistorted, closed to the archetypal HT paraelectric phase, with a tolerance factor ≳0.996. The coordination number of Bi3+ in fluorite layers changes from {4 + 2} for ferroelectrics to {4} for relaxors. Transmission Electron Microscopy reveals some characteristic features of relaxors such as micro-twinning, shearing-type defects which attest from the existence of a compositional inhomogeneity and a disorder at a local scale.