In-Situ Investigation on Reversible Polar-to-Nonpolar Phase Transition in Fluorite Oxide Ferroelectrics

Switchable spontaneous polarization of ferroelectrics enables stable storage of two reversible polarization states applicable to next-generation electronic devices. As exemplified by Hf_xZr_1-xO₂ (HZO), fluorite oxide thin films demonstrate great silicon compatibility and robust FE polarization down to the thickness of several unit cells, which is beneficial for silicon-compatible and scalable electronics. However, fluorite oxides exhibit various polymorphs and the desirable ferroelectric orthorhombic (O) phases is metastable. Therefore, stabilizing O phase instead of nonpolar ground-state monoclinic (M) phase in thin films remains a significant challenge [1] and understanding the mechanisms that govern phase transitions and FE switching at the atomic scale [2] is crucial for rational design of fluorite oxide devices.<br/><br/>In this study, we investigate the reversibility of O-M phase transition in ZrO₂ nanocrystals by <i>in-situ</i> visualization of the martensitic transformation at atomic scale. We reveal that the reversible shear deformation pathway from O phase to M state is protected by 90° ferroelectric-ferroelastic switching. Nevertheless, as the M state gradually accumulates localized strain, a critical tensile strain can pin the ferroelastic domain, resulting in an irreversible O-to-M transformation and the loss of ferroelectricity. Additionally, four-dimensional scanning transmission electron microscopy (4D-STEM) analysis shed light on the crystal relationship in the thin film on a larger scale [3]. These findings demonstrate the key role of ferroelastic switching in the reversibility of phase transition, and also provide a tensile-strain threshold for stabilizing the metastable ferroelectric phase in fluorite-oxide thin films.<br/><br/><b>References:</b><br/>1. X. Li, <i>et al</i>., <i>Nat</i><i>.</i><i> Mater</i><i>.</i>, 2024. DOI: 10.1038/s41563-024-01853-9.<br/>2. X. Li, <i>et al</i>., <i>Adv.</i><i> Mater</i><i>.</i>, <b>35</b>, 2207736, 2023.<br/>3. Shi C, <i>et al</i>., <i>Nat. Commun.</i>, <b>14</b>, 7168, 2023.