Abstract :
Relaxor behavior of diffuse phase transition and large frequency dispersion is found among Pb-containing complex perovskites. With slightly different compositions or structures, these same complex perovskites also exhibit normal ferroelectric (FE) or antiferroelectric (AFE) behavior. The AFE transitions involve a volume decrease and are enhanced by increasing the size mismatch between ferroelectrically active B″ cations and ferroelectrically inactive B′ cations. This is in contrast to the FE transitions which involve a volume increase and are enhanced by enlarging the space available for ferroelectrically active B″ cations. In relaxors, the size of B′ cations is always larger than that of B″ cations, so an increase in the size mismatch is accompanied by a parallel increase in the molar volume. Therefore, a competition between AFE and FE couplings is always present. Such AFE/FE competition has an analogy in magnetism: the competition between antiferromagnets and ferromagnets gives rise to a spin glass behavior in the intermediate random alloys between antiferromagnets and ferromagnets. Evidence in support of this concept is found in the structural characteristics, transformation volumes, pressure dependencies, transition/depolarization temperatures, and phase diagrams of relaxors ferroelectrics and related compounds.
