In situ TEM analysis of reversible non-180° domain switching in (K,Na)NbO₃ single crystals

Reversible non-180° domain switching is believed to be able to enhance the electromechanical performances of piezoelectrics. However, the slow improvement of strain performances of (K,Na)NbO₃ (KNN) is caused by an inadequate understanding of reversible non-180° domain switching. Here, with an electric field-available sample holder, we directly observe the reversible domain wall motion by in situ transmission electron microscopy and non-180° domain switching in a compositionally graded KNN single crystal which has an ultrahigh large-signal piezoelectric coefficient. The characteristics of domain wall motion under varying electric fields are investigated. The analysis of high-angle annular dark-field images with an atomic resolution indicates that local compositional fluctuation, strain fluctuation and polarization rotation exist in the domains. We propose that the restoring force derives from the built-in flexoelectric effect caused by compositional fluctuation at the atomic scale. The new mechanism paves the way for enhancing the piezoelectric properties and designing high performance piezoelectrics.