Pressure‐Driven Two‐Step Second‐Harmonic‐Generation Switching in BiOIO3

Materials with multi-stabilities controllable by external stimuli have potential for high-capacity information storage and switch devices. Herein, we report the observation of pressure-driven two-step second-harmonic-generation (SHG) switching in polar BiOIO₃ for the first time. Structure analyses reveal two pressure-induced phase transitions in BiOIO₃ from the ambient noncentrosymmetric phase (SHG-high) to an intermediate noncentrosymmetric phase (SHG-intermediate) and then to a centrosymmetric phase (SHG-off). The three-state SHG switching was inspected by in situ high-pressure powder SHG and polarization-dependent single-crystal SHG measurements. Local structure analyses based on the in situ Raman spectra and X-ray absorption spectra reveal that the SHG switching is caused by the step-wise suppression of lone-pair electrons on the [IO₃ ]^- units. The dramatic evolution of the functional units under compression also leads to subtle changes of the optical absorption edge of BiOIO₃ . Materials with switchable multi-stabilities provide a state-of-art platform for next-generation switch and information storage devices.