Large Electrostrain and High Optical Transmittance via Oxygen Octahedron Rotation Disordering in KNN Ceramics

Lead-free K_0.5Na_0.5NbO₃ (KNN)-based ceramics are promising candidates for multifunctional electronic materials. Nevertheless, the synergistic optimization of electrostrain and optical transmittance in KNN polycrystalline ceramics remains a critical challenge, owing to the structural trade-off between large strain and high transparency. Here, we employ chemical disorder engineering specifically through BaTiO₃ (BT) substitution to manipulate the BO₆ oxygen octahedron rotation disorder. BT incorporated into KNN gradually modulates the BO₆ rotation mode from local ordering to a random distribution, which refines the size of cubic-phase-enriched polar nanoregions, facilitating high transparency in visible-light range. The disordered BO₆ rotation essentially lowers the thermodynamic switching barriers for polarization and strengthens the defect-dipole pinning effect, thereby enhance the electrostrain synergistically. The optimal composition, i.e., KNN-0.03BT exhibits a large electrostrain of 1.2% and an optical transmittance of 60% in visible-light range. By overcoming the long-standing bottleneck in traditional polycrystalline ceramics, this work elucidates the core mechanism of atomic-scale disordering-enhanced polarization response and reduced optical loss, thereby advancing multifunctional material applications such as transparent sensing and flexible electronics.