Revisit of the Vogel-Fulcher freezing in lead magnesium niobate relaxors

The Vogel-Fulcher (VF) equation omega = omega(0) exp[-E-a/k(B)(T-m-T-f)] was frequently used to describe the dielectric relaxation of relaxor ferroelectrics where omega is the probe frequency at the peak temperature T-m of either the real or imaginary parts of the dielectric constant. We revisited this relation in a typical relaxor lead magnesium niobate and found that the parameters obtained were not physically meaningful. Meaningful results can only be obtained by fitting the VF relation on the characteristic relaxation time tau(c), whose temperature dependence can be obtained from the Cole-Cole model. The freezing temperature we obtained is 230 K, below which tau(c) becomes temperature independent. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3494531]