  The electrocaloric (EC) effect is calculated for Gd-substituted PMN (x = 0 to
0.1) relaxors from temperature dependent heat capacity and polarization
measurements, using a thermodynamic Maxwell equation. Polarization change with
temperature reveals an anomalous behaviour around the glass transition
temperature, which is in the vicinity of a crossover from positive to negative
EC effect, which is explained evoking the critical slowing-down of the polar
nano-domains dynamics to super-dipolar glass state upon cooling. The maximum
negative EC coefficient is observed to decrease from 0.4 K-mm/kV at 164 K to
0.1 K-mm/kV with x.
