Phase transitions and electrocaloric effects of (111)-oriented K<sub>0.5</sub>Na<sub>0.5</sub>NbO<sub>3</sub> epitaxial films: effect of external stress and misfit strains

Lead-free K_1–<i>x</i>Na_<i>x</i>NbO₃ thin films, as a candidate for sensors and electromechanical and electrocaloric cooling devices, have increasingly received attention. However, for (111)-oriented films, the relation between phase transitions and electrocaloric effect is not clear. Here, we derive the thermodynamic potential of (111)-oriented thin film ferroelectrics K_1–<i>x</i>Na_<i>x</i>NbO₃ based on the 8^th order polynomial function, and then establish the temperature-misfit strain and out-of-plane stress-in-plane misfit strain phase diagrams and calculate electrocaloric (EC) entropy changes Δ<i>S</i> and temperature changes Δ<i>T</i>. This study focuses on mechanical and orientation controls of room-temperature EC effect of K_0.5Na_0.5NbO₃ films, which is critical for environmentally friendly electrocaloric refrigeration applications in practice. Under the stress-free and zero misfit strain conditions, the (111)-oriented K_0.5Na_0.5NbO₃ film in an electric field of 30 MV/m has a maximum EC Δ<i>T</i> of ～18 K near the rhombohedral ferroelectric-paraelectric phase transition temperature (about 673 K). However, an out-of-plane stress of about –6.7GPa can reduce the optimal operating temperature to room temperature where the K_0.5Na_0.5NbO₃ film has the EC Δ<i>T</i> of ～7.5 K under the action of applied electric field of 30 MV/m. The present work provides theoretical guidance for exploring the strain engineering and orientation engineering of K_1–<i>x</i>Na_<i>x</i>NbO₃-based thin films with optimized electrocaloric and electromechanical properties.