The relaxational properties of compositionally disordered ABO3 perovskites

Random lattice disorder produced by chemical substitution in ABO(3) perovskites can lead to the formation of dipolar impurities and defects that have a profound influence on the static and dynamic properties of these materials that are the prototypical soft ferroelectric (FE) mode systems. In these highly polarizable host lattices, dipolar entities form polar nanodomains whose size is determined by the dipolar correlation length, r(c), of the host and that exhibit dielectric relaxation in an applied ac field. In the very dilute limit (<0.1 at.%) each domain behaves as a, non-interacting dipolar entity with a single relaxation time. At higher concentrations of disorder, however, the domains can interact leading to more complex relaxational behaviour. Among the manifestations of such behaviour is the formation of a glass-like relaxor (R) state, or even an ordered FE state for a sufficiently high concentration of overlapping domains. After a brief discussion of the physics of random-site electric dipoles in dielectrics, this review begins with the simplest cases, namely the relaxational properties of substitutional impurities (e.g:, Mn, Fe and Ca) in the quantum paraelectrics KTaO3 and. SrTiO3. This is followed by discussions of the relaxational properties of Li- and Nb-doped KTaO3 and of the strong relaxors in the PbMg1/3Nb2/3O3 and La-substituted PbZr1-xTixO3 families. Some emphasis will be on the roles of pressure and applied do biasing electric fields in understanding the physics of these materials including the R-to-FE crossover.