Volumetric effects in electrocaloric response of ferroelectric thin films

We theoretically analyze the contribution of volumetric changes to the caloric response of ferroelectric thin films on misfitting substrates. A self-consistent formalism employing the appropriate thermodynamic relations accounting for the elastic boundary conditions is developed, allowing for the calculation of the volumetric contribution to the caloric temperature changes, similar to the elastocaloric effect. The latter stems from the strain response of a ferroelectric film to an applied electric field. We apply the formalism to tetragonal BaTiO 3 (BT) in film form constrained on a cubic substrate. It is found that, contrary to the electrocaloric response, the elastocaloric-like response is negative and precedes the transition temperature where the intrinsic electrocaloric response reaches its maximum. Such a behaviour is predicted to cause a reduction in the total caloric temperature change before the transition, revealing a competition between the two effects. Larger misfit strains reduce the elastocaloric-like contribution and the intrinsic electrocaloric contribution governs the caloric temperature change. Overall our findings point out the importance of the volumetric changes in the caloric response of ferroelectric thin films. We develop a formalism for the contribution of volumetric changes in ferroelectric films to their caloric response. Electric field induced strain can influence caloric response not only via the electrocaloric effect but also the elastic effect.