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

Reversible non-180 degrees 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 degrees 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 degrees 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.