Negative/positive electrocaloric effect in antiferroelectric squaric acid

Abstract

Using the previously developed model we study the electrocaloric effect (ECE) in antiferroelectric crystals of squaric acid. At low temperatures, the polarization reorientation by the electric field applied along the crystallographic $a$ axis of these crystals is predicted to be a two-stage process, with one of the sublattice polarizations switched twice, by 90$^\circ$ each time. The $T$-$E$ landscapes of the model polarization and entropy are explored. The ECE, characterized by the introduced electric Gr\"uneisen parameter, is found to be negative in the antiferroelectric phase. In the intermediate ferrielectric phase with perpendicular sublattice polarizations it is positive at low fields, but becomes slightly negative below the transition to the ferroelectric phase. In the ferroelectric phase the ECE is positive at all temperatures and fields. Maximal EC temperature shift magnitude is predicted to be just less than -3 K at 200 kV/cm. The supercritical behavior of the Gr\"uneisen parameter in the crossover region between two bicritical end points is explored.