  Anomalous electrocaloric effect (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 transitions. Here, we use a
first-principle-based method to study the ECE in Nd-substituted BiFeO3 (BNFO)
perovskite solid solutions, which has the 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. We also find that the
negative ECE can reach a large magnitude in BNFO.
