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Monoclinic (K,Na)NbO3 Ferroelectric Phase in Epitaxial Films

Monoclinic phases, as intermediate phases at the morphotropic phase boundary, have been considered as the origin of high piezoelectricity in various piezoelectric systems. However, those monoclinic phases have seldom been experimentally observed at the phase boundary in lead‐free (K,Na)NbO3 (KNN)‐based materials. This work shows that KNN epitaxial thin films demonstrate a monoclinic MC phase stabilized by elastic strain at room temperature, which is identified by both synchrotron X‐ray reciprocal space mapping and piezoresponse force microscopy. It is revealed that the piezoelectricity is greatly enhanced at 170 °C due to the monoclinic MC–MA phase boundary. The discovery of monoclinic phases in this work may open an avenue for developing a new thermotropic phase boundary in KNN‐based materials. The results also provide a guide for the exploration of high piezoelectricity driven by monoclinic phases in lead‐free piezoelectric materials.