Electric-field dependent freezing in relaxor ferroelectrics

The concept of polar nanoregions (PNRs) in relaxor ferroelectrics has recently been discussed in several numerical and theoretical studies for the special case of barium zirconate-titanate Ba(Zr sub(x)Ti sub(1-x))O sub(3) (BZT). Here we present a semiphenomenological model of relaxation in BZT and related systems in order to show that the correlation radius of a PNR is determined by the condition that the surrounding medium undergoes a local phase transformation into a correlated polar state. The model describes the growth and percolation of the PNRs on lowering the temperature or under the application of an electric field, leading to a generalized Vogel-Fulcher-type dielectric relaxation time. It is suggested that the above condition might be applied to discriminate between the relaxor state and the analogous dipolar glass state.