Strain-Tuned Ferroelectric Transitions in HfO2: Role of X2- Mode in Ferroelectric Instabilities

Recent studies have identified HfO ₂ as a promising ferroelectric material for thin films, highlighting its potential as a state-of-the-art option for future ferroelectric applications. However, due to the complexity of the fabrication process, the underlying mechanism of the phase transition to Pca2 ₁ is still not fully understood. In this study, we aim to clarify the phase transition pathway by investigating the formation of the ferroelectric Pca2 ₁ phase via strain and symmetry-allowed phonon mode instability. Our results reveal that applying tensile strain regardless of direction induces an 𝑋 ₂ ^- mode displacement, initiating a primary phase transition and consequently inducing instability in the polar and antipolar modes. Furthermore, the coupling between the single distortion modes andthe 𝑋 ₂ ^- modelowers the trilinear mode barrier, ultimately triggering a secondary phase transition that leads to the O phase. These findings suggest that optimizing substrate conditions to enhance the displacement of the 𝑋 ₂ ^- mode is critical to achieve robust ferroelectric properties. Our insights into substrate engineering provide a foundation for the practical realization of ferroelectricity in device applications.