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

Ferroelectric/antiferroelectric thin/thick films with large positive or negative electrocaloric (EC) effects could be very useful in designing commercial refrigeration devices. Here, a giant negative EC effect (maximum ΔT ∼ -42.5 K with ΔS ∼ -29.3 J K^-1 kg^-1) comparable to the best positive EC effects reported so far is demonstrated for 0.5(Ba_0.8Ca_0.2)TiO₃-0.5Bi(Mg_0.5Ti_0.5)O₃ (BCT-BMT) lead-free relaxor ferroelectric thin films prepared on Pt(111)/TiO_x/SiO₂/Si substrates 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 very 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 ingeniously utilizing and combining both the giant negative and positive EC effects. Moreover, a large energy density of 51.7 J cm^-3 with a high power density of 1.15 × 10¹⁰ W kg^-1 at room temperature is also achieved in the thin film, indicating that it is also an attractive multifunctional material for energy storage.