Giant Negative Electrocaloric Effect over an Ultra-Wide Temperature Region in Relaxation Frozen State Ferroelectrics

A giant negative electrocaloric effect with the electrocaloric temperature change range from -27.3 to -9.8 K over an ultra-wide temperature region of 260 K (-180 to 80 °C) is obtained in a relaxation frozen state ferroelectric film of Bi₅Ti₃AlO₁₅. A strategy is proposed to extend the temperature region of the electrocaloric effect by gradually slowing and locally freezing the relaxation dynamics. The slow and frozen nanodomains below freezing temperature T_f of ∼80 °C do not yield to the applied electric field but are noncolinear to the field, which results in an inverse configuration entropy change and negative electrocaloric effect over an ultra-wide temperature region in frozen relaxor state ferroelectric films. A semiphenomenological model based on modified Ginzburg-Landau-Devonshire theory is presented to reveal the evolution of the slowing and freezing of the nanodomains. The breakthroughs in the operating temperature region and cooling temperature change make relaxation frozen state ferroelectrics excellent electrocaloric refrigeration materials.