Atomic-Scale Phase Transition and Polarization Switching Mechanism of Fluorite Oxide Ferroelectrics by In-Situ Scanning Transmission Electron Microscopy

The discovery of ferroelectricity in fluorite oxides, exemplified by Hf_xZr_1-xO₂ (HZO), presents exceptional potential for silicon integration and robust electric polarization down to the thickness of several unit cells. Nevertheless, the mechanisms of ferroelectric phase stabilization and polarization switching remain elusive, necessitating direct imaging of in-situ structural evolution [1,2].



Here, we investigate the atomic-scale O-M phase transition mechanism and polarization switching processes in ZrO₂ freestanding thin films using in-situ scanning transmission electron microscopy (STEM). We reveal that the phase transition from polar O phase to non-polar M phase occurs through a reversible shear deformation pathway, which can be protected by 90° ferroelectric-ferroelastic switching. Nevertheless, the accumulation of localized tensile strain can disrupt reversibility, leading to ferroelectric fatigue. Furthermore, our study reveals bidirectional transitions between antiferroelectric and ferroelectric orders through 180° switching, accompanied by several types of phase transitions. In summary, our work provides fundamental insights for the design of next-generation ferroelectric devices.