Strain-tuned ferroelectric transitions in HfO2: role of $${X}_{2}^{-}$$ mode in ferroelectric instabilities

Abstract Recent studies have identified HfO2 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 Pca21 is still not fully understood. In this study, we aim to clarify the phase transition pathway by investigating the formation of the ferroelectric Pca21 phase via strain and symmetry-allowed phonon mode instability. Our results reveal that applying tensile strain regardless of direction induces an 𝑋2- 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 𝑋2- 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 𝑋2- 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.