Robust ferroelectricity in nanoscale fluorite oxide-based thin films enables promising applications in silicon-compatible non-volatile memories and logic devices. However, the polar orthorhombic (O) phase of fluorite oxides is a metastable phase that is prone to transforming into the ground-state non-polar monoclinic (M) phase, leading to macroscopic ferroelectric degradation. Here we investigate the reversibility of the O–M phase transition in ZrO₂ nanocrystals via in situ visualization of the martensitic transformation at the atomic scale. We reveal that the reversible shear deformation pathway from the O phase to the monoclinic-like (M′) state, a compressive-strained M phase, 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 M′–M strain relaxation and the loss of ferroelectricity. 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.

纳米级萤石氧化物基薄膜中的鲁棒铁电性使其在硅兼容的非易失性存储器和逻辑器件中具有广阔的应用前景。然而，萤石氧化物的极性斜方（O）相是亚稳态相，很容易转变为基态非极性单斜（M）相，导致宏观铁电退化。在这里，我们通过原子尺度马氏体转变的原位可视化研究了 ZrO ₂纳米晶体中 O-M 相变的可逆性。我们揭示了从 O 相到类单斜 (M′) 状态（压缩应变 M 相）的可逆剪切变形路径受到 90° 铁电-铁弹性转换的保护。然而，随着M'态逐渐积累局部应变，临界拉伸应变可以固定铁弹域，导致不可逆的M'-M应变弛豫和铁电性损失。这些发现证明了铁弹性转换在相变可逆性中的关键作用，并且还为稳定萤石氧化物薄膜中的亚稳态铁电相提供了拉伸应变阈值。