Rhombohedrally-Distorted Orthorhombic Hf_0.5Zr_0.5O_2-x Stabilized by Cooperative Oxygen Vacancies and Lattice Distortions

Hafnia-based ferroelectrics have emerged as promising materials for next-generation nanoelectronics owing to their robust nanoscale properties and compatibility with metal-oxide-semiconductor technology. However, their metastable nature remains a key challenge for practical implementation. Utilizing scanning transmission electron microscopy, we investigated the atomic-scale mechanisms governing ferroelectric transitions and the metastability of polar phases in 10-nm-thick Hf_0.5Zr_0.5O₂ thin films. Our results demonstrate that oxygen vacancies, coupled with rhombohedral distortions of the cation lattice, facilitate ferroelectric phase transitions and enable robust polar switching through adaptive processes, including cell-by-cell oxygen displacement and domain-wall-mediated nucleation and growth. These findings underscore the pivotal role of oxygen vacancies and lattice distortions in stabilizing polar phases and provide detailed insights into the atomic structures and transition dynamics of polymorphic Hf_0.5Zr_0.5O_2-x, thereby advancing its potential for practical device applications.