Electrocaloric effect in relaxor ferroelectrics

A theoretical model for the electrocaloric effect (ECE) in relaxor ferroelectrics is presented. By solving a self-consistent relation for the ECE temperature change T and minimizing numerically the mean field free energy for relaxors, the field and temperature dependence of T is calculated. The corresponding harmonic Landau coefficient a = a (T), which differs from the ferroelectric case by always being positive, is derived from the spherical random bond-random field model, and the fourth-order coefficient b is treated as a phenomenological parameter, which can be either positive or negative. For b < 0, a line of field-induced first-order relaxor-to-ferroelectric phase transitions exists in relaxors, which terminates at a liquid-vapor type critical point E_CP, T_CP. The critical behavior close to E _CP, T_CP is analyzed. It is shown that near the first-order phase transition a temperature or field interval or gap formally appears, where T cannot be found. However, domain formation in the coexistence range should restore the continuous behavior of the ECE observed in real systems. Finally, it is shown that the ECE responsivity R 1 = T / E reaches a maximum near the critical point, in agreement with recent experiments.