Bipolar electrocaloric effect in Pb1-xGdx(Mg1+x/3Nb2-x/3)O3 ceramic: relaxor with super-dipolar glass state

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.