  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.
