Large electrocaloric response via percolation of polar nanoregions

Abstract

A first-principle-based effective Hamiltonian model is developed to investigate electrocaloric effects (ECE) of ferroelectric $\mathrm{Pb}({\mathrm{Sc}}_{0.5}{\mathrm{Ta}}_{0.5})\mathrm{O}3$ (PST) possessing different degrees of chemical ordering between Sc and Ta cations. It is found that PST exhibits large electrocaloric effects when the electric field drives a paraelectric-to-ferroelectric phase transition isothermally above the Curie temperature. More precisely, for any specific temperature above the Curie temperature, the electrocaloric coefficient exhibits its maximum at a critical electric field that is precisely the threshold of percolation for which the polar nanoregions begin to propagate inside the whole sample, with dipoles being parallel to the field's direction. This percolation-induced maximal ECE occurs for all the possible degrees of chemical ordering, therefore making it a general and novel mechanism, based on which a strategy is further proposed to improve the ECE.