  A physical mechanism for the freezing of polar nanoregions (PNRs) in relaxor
ferroelectrics is presented. Assuming that the activation energy for the
reorientation of a cluster of PNRs scales with the mean volume of the cluster,
the characteristic relaxation time $\tau$ is found to diverge as the cluster
volume reaches the percolation limit. Applying the mean field theory of
continuum percolation, the familiar Vogel-Fulcher equation for the temperature
dependence of $\tau$ is derived.
