Ferroelectric domain structures and temperature-misfit strain phase diagrams of K_1-xNa_xNbO₃ thin films: A phase-field study

Potassium-sodium niobate K_1-xNa_xNbO₃ (KNN) is one of the most promising lead-free piezoelectric materials. While there have been many studies on the microstructures and properties of KNN ceramics, the phase transitions and ferroelectric domain structures of KNN thin films are not well understood. In this work, we employ three-dimensional (3D) phase-field simulations to obtain the ferroelectric domain structures of KNN (0 ≤ x ≤ 0.5) thin films under a range of temperatures (0 K to 1300 K) and equiaxial misfit strains (-1.5% to 1.5%), based on which we establish the misfit strain-temperature phase diagrams of KNbO₃ and K_0.5Na_0.5NbO₃ thin films. We identify a wide variety of complex domain structures with coexisting ferroelectric phases, implying enhanced dielectric and piezoelectric properties. We expect this work to provide guidance for the strain engineering of domain structures and properties of KNN thin films.