Rhombohedrally-distorted orthorhombic Hf_0.5Zr_0.5O_2-x stabilized by cooperative oxygen vacancies and lattice distortions

Hafnia-based ferroelectrics are promising for next-generation nanoelectronics, offering robust properties at nanoscale and compatibility with metal-oxide semiconductor technology. However, their metastable nature poses challenges for practical implementation. Utilizing scanning transmission electron microscopy, we investigated the atomic mechanisms underlying ferroelectric transitions and metastability of polar phases in 10 nm-thick Hf_0.5Zr_0.5O₂ thin films. Our results reveal that oxygen vacancies, coupled with rhombohedral distortions of cationic lattice, facilitate ferroelectric transitions and sustain robust polar switching via adaptive mechanisms, including cell-by-cell oxygen shifts and domain-wall-mediated nucleation and growth. These findings highlight the pivotal role of oxygen vacancies and lattice distortions in stabilizing polar phases, providing detailed insights into the atomic structures and transition dynamics of polymorphic Hf_0.5Zr_0.5O_2-x, advancing its potential for practical applications.