Origin of negative electrocaloric effect in Pnma -type antiferroelectric perovskites

Electrocaloric materials of solid state hold great promise for next-generation high-efficiency cooling technology. Besides the normal electrocaloric effect (ECE) in ferroelectrics or relaxors, anomalous ECE with decreasing temperature upon application of an electric field is known to occur in antiferroelectrics (AFEs), and previous understanding refers to the field-induced canting of electric dipoles if there is no phase transition. Here we use a first-principles-based method to study the ECE in Nd-substituted ${\mathrm{BiFeO}}₃$ (BNFO) perovskite solid solutions, which has a Pnma-type AFE ground state. We demonstrate another scenario to achieve and explain anomalous ECE, emphasizing that explicit consideration of octahedral tiltings is indispensable for a correct understanding. This mechanism may be general for AFEs for which the antipolar mode is not the primary order parameter or for other commonly occurring Pnma-type perovskites, even without being AFEs. We also find that the negative ECE can reach a large magnitude in BNFO.