Polarization rotation in high-performance KNN-based piezoceramics revealed by an in situ electric field pair distribution function

Understanding the atomic-scale structural dynamics that enable ultrahigh piezoelectric responses in lead-free piezoceramics remains a central challenge in materials science. Here, we employ in situ electric field pair distribution function (PDF) analysis to elucidate the local structural origin of a KNN-based piezoceramic with a nominal composition of 0.964K_0.5Na_0.5Nb_0.965Sb_0.035O₃–0.03(Bi_0.5Na_0.5)_0.9(Ga_0.5Li_0.5)_0.1ZrO₃–0.006BiFeO₃ that has an exceptional piezoelectricity coefficient (d₃₃ > 500 pC/N). Combined Rietveld refinement, PDF fitting, and reverse Monte Carlo simulations revealed the coexistence of long-range tetragonal and orthorhombic phases with local c-type monoclinic symmetry. In situ electric field PDF analyses indicated a reversible polarization rotation between the <001>_PC and <110>_PC directions via a monoclinic plane, with a critical switching field of approximately 0.4 kV/mm. These findings establish polarization rotation, rather than abrupt phase transitions, as the governing mechanism for the enhanced piezoresponse, providing a structural design principle for next-generation lead-free piezoelectrics.