Phase-transition induced giant negative electrocaloric effect in a lead-free relaxor ferroelectric thin film

The electrocaloric (EC) effect has been widely investigated during the past decade due to the potential applications in commercial solid state refrigeration devices. The positive EC effect in lead-based materials has been significantly enhanced from 12 K to 40 K since it is observed in 2006, but the negative EC effect still stays at a very low level of about −10 K, which limits further enhancement of cooling efficiency, especially when we attempt to combine both negative and positive EC effect in one cooling cycle. Due to the toxicity of lead, lead-free materials are always sought after to replace lead-containing materials. In this study, a giant negative EC effect (maximum ΔT ∼ −42.5 K) comparable to the best positive EC effects reported so far is demonstrated for 0.5(Ba0.8Ca0.2)TiO3–0.5Bi(Mg0.5Ti0.5)O3 lead-free relaxor ferroelectric thin films prepared by using a sol–gel method. An electric-field induced structural phase transition (nanoscale tetragonal and orthorhombic to rhombohedral) along the out-of-plane [111] direction plays a key role in developing the giant negative EC effect. This breakthrough will pave the way for practical applications of next-generation refrigeration devices with high cooling efficiency in one cycle by utilizing and combining both the giant negative and positive EC effects.