Electrocaloric effect in relaxor ferroelectrics

TL;DR

This paper models the electrocaloric effect in relaxor ferroelectrics using a thermodynamic framework, revealing how the effect peaks near critical points and differs from normal ferroelectrics.

Contribution

It introduces a Landau-type free energy model with a modified dielectric response for relaxors, predicting the conditions for maximum electrocaloric effect.

Findings

Maximum ECE occurs near the critical point in relaxors.

The Landau coefficient a(T) varies with temperature in relaxors.

The upper bound of ECE temperature change is estimated.

Abstract

The electrocaloric effect (ECE) in normal and relaxor ferroelectrics is investigated in the framework of a thermodynamic approach based on the Maxwell relation and a Landau-type free energy model. The static dielectric response of relaxors is described by the spherical random bond--random field model, yielding the first Landau coefficient a=a(T), which differs from the usual expression for ferroelectrics. The fourth-order coefficient b is treated as a phenomenological parameter, which is either positive or negative due to the anisotropy of the stress-mediated coupling between the polar nanoregions. When b<0, the maximum ECE in a relaxor is predicted near the critical point in the temperature-field phase diagram, whereas in a ferroelectric it occurs at the first order phase transition. The theoretical upper bound on the ECE temperature change is estimated from the values of saturated polarization, effective Curie constant, and specific heat of the material.