Freezing in relaxor ferroelectrics and dipolar glasses

A recently proposed semi-phenomenological model of freezing in relaxor ferroelectrics, based on the concept of polar nanoregions (PNRs) embedded in a polarizable medium, is reviewed. A generalized Landau-type free energy for the medium is discussed, where the medium polarization couples linearly to the PNR polarization. When the fourth-order Landau coefficient is negative (b < 0), the correlation radius r(c), which measures the PNR size, depends on the temperature T and the applied field E. As T is lowered or E increased, r(c) increases and the volume of a cluster of PNRs grows until the percolation limit is reached. This leads to a generalized expression for the Vogel-Fulcher (VF) relaxation time with a field-dependent VF freezing temperature T-0(E). The case b > 0, in which the percolation mechanism cannot be realized, is considered to be appropriate for dipolar glasses.