Structural origin of relaxor perovskites

Relaxor behavior of a diffuse phase transition and large frequency dispersion requires a small and broadly distributed activation barrier for interstate transitions. This can best be realized in a relatively random environment that apparently is provided by many complex perovskites, including those containing Pb. On the other hand, while the presence of Pb with its lone-pair electron and large polarizability certainly enhances ferroelectric interactions and increases the bias for the requisite interstate transition in the relatively random environment, there is also the possibility of a strong coupling between Pb and ferroelectrically active B cations through oxygen bonding that can raise the activation barrier. To resolve this issue, EXAFS data are presented to provide evidence that Pb is actually attracted to the larger B' cations which are usually ferroelectrically inactive, leaving the smaller B '' cations which are ferroelectrically active relatively free of Pb-O-B '' coupling. This observation explains why many Pb-containing complex perovskites are relaxers. In addition, the recognition of the dominant role of Pb and the size preference in Pb-O-B bonding naturally leads to several important predictions regarding the type of ordering, the hierarchy of symmetry, and the effects of cation alloying. These predictions are compared with the experimental data of a large number of Pb- and Bi-containing complex perovskites and are found in broad confirmation.