Roadmap of phase transitions in hafnia-based superlattice films

Hafnia-based ferroelectrics hold significant promise for next-generation non-volatile memory. However, their functional properties are critically limited by uncontrollable phase transitions due to the poorly understood atomistic mechanisms driving specific transformations. Here, using single-crystalline HfO₂-based superlattice films as the prototype system, we propose an asynchronous sublattice distortion mechanism underlying the complex phase transitions in HfO₂-based materials. Aberration-corrected transmission electron microscopy reveals that sublattice preferential distortion behaviors trigger various phase transitions among orthorhombic, tetragonal and monoclinic phases, processes governed by the direction of orthorhombic phase. Critically, the complex lattice distortion pathways underlying the orthorhombic-to-monoclinic transition are elucidated, revealing their fundamental dependence on the monoclinic projection direction. Furthermore, polar-antipolar transition within the orthorhombic phase requires only oxygen sub-lattice dipole-order reversal, enabling polarization flipping. This work systematically clarifies the core mechanisms of structural phase transitions in HfO₂-based films, resolving previous controversies and providing a guidance for designing high-performance HfO₂-based electronic devices.