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Environmentally friendly lead-free K1-xNaxNbO3 (KNN) ceramics possess electromechanical properties comparable to lead-based ferroelectric materials but cannot meet the needs of device miniaturization, and the corresponding thin films lack theoretical and experimental studies. To this end, we developed the nonlinear phenomenological theory for ferroelectric materials to study the effects of non-equiaxed misfit strain on the phase structure, electromechanical properties, and electrical response of K0.5Na0.5NbO3 epitaxial films. We constructed in-plane misfit strain ( u 1 − u 2 ) phase diagrams. The results show that K0.5Na0.5NbO3 epitaxial film under non-equiaxed in-plane strain can exhibit abundant phase structures, including orthorhombic a 1 c , a 2 c , and a 1 a 2 phases, tetragonal a 1 , a 2 , and c phases, and monoclinic r 12 phases. Moreover, in the vicinity of a 2 c − r 12 , a 1 c − c , and a 1 a 2 − a 2 phase boundaries, K0.5Na0.5NbO3 epitaxial films exhibit excellent dielectric constant ε 11 , while at a 2 c − r 12 and a 1 c − c phase boundaries, a significant piezoelectric coefficient d 15 is observed. It was also found that high permittivity ε 33 and piezoelectric coefficients d 33 exist near the a 2 c − a 2 , a 1 a 2 − r 12 , and a 1 c − a 1 phase boundaries due to the existence of polymorphic phase boundary (PPB) in the KNN system, which makes it easy to polarize near the phase boundaries, and the polarizability changes suddenly, leading to electromechanical enhancement. In addition, the results show that the K0.5Na0.5NbO3 thin films possess a large electrocaloric response at the phase boundary at the a 1 a 2 − r 12 and a 1 c − a 1 phase boundaries. The maximum adiabatic temperature change Δ T is about 3.62 K when the electric field change is 30 MV/m at room temperature, which is significantly enhanced compared with equiaxed strain. This study provides theoretical guidance for obtaining K1−xNaxNbO3 epitaxial thin films with excellent properties. [ABSTRACT FROM AUTHOR]

Environmentally friendly lead-free K1-xNaxNbO3 (KNN) ceramics possess electromechanical properties comparable to lead-based ferroelectric materials but cannot meet the needs of device miniaturization, and the corresponding thin films lack theoretical and experimental studies. To this end, we developed the nonlinear phenomenological theory for ferroelectric materials to study the effects of non-equiaxed misfit strain on the phase structure, electromechanical properties, and electrical response of K0.5Na0.5NbO3 epitaxial films. We constructed in-plane misfit strain (u1−u2) phase diagrams. The results show that K0.5Na0.5NbO3 epitaxial film under non-equiaxed in-plane strain can exhibit abundant phase structures, including orthorhombic a1c, a2c, and a1a2 phases, tetragonal a1, a2, and c phases, and monoclinic r12 phases. Moreover, in the vicinity of a2c−r12, a1c−c, and a1a2−a2 phase boundaries, K0.5Na0.5NbO3 epitaxial films exhibit excellent dielectric constant ε11, while at a2c−r12 and a1c−c phase boundaries, a significant piezoelectric coefficient d15 is observed. It was also found that high permittivity ε33 and piezoelectric coefficients d33 exist near the a2c−a2, a1a2−r12, and a1c−a1 phase boundaries due to the existence of polymorphic phase boundary (PPB) in the KNN system, which makes it easy to polarize near the phase boundaries, and the polarizability changes suddenly, leading to electromechanical enhancement. In addition, the results show that the K0.5Na0.5NbO3 thin films possess a large electrocaloric response at the phase boundary at the a1a2−r12 and a1c−a1 phase boundaries. The maximum adiabatic temperature change ΔT is about 3.62 K when the electric field change is 30 MV/m at room temperature, which is significantly enhanced compared with equiaxed strain. This study provides theoretical guidance for obtaining K1−xNaxNbO3 epitaxial thin films with excellent properties.