Electro‐Chemo‐Mechanical Coupling in Hf _0.5 Zr _0.5 O ₂ Ferroionic Heterostructures

ABSTRACT Ferroelectricity in Hf 0.5 Zr 0.5 O 2 (HZO) originates from a polymorphic landscape where the metastable orthorhombic phase competes with monoclinic and tetragonal forms, making functional properties highly sensitive to structural instability. Recent strategies have exploited ionic‐vacancy mechanisms, either through redox interactions with the environment or by employing ferroionic heterostructures, to enhance ferroelectric performance. Here, we embrace the ferroionic heterostructure approach and demonstrate that dynamic oxygen‐vacancy exchange at epitaxial junctions produces an active interplay between ferroelectric and ionic layers. Epitaxial heterostructures with La 0.67 Sr 0.33 MnO 3‐δ (LSMO), yttria‐stabilized ZrO 2‐δ (YSZ), and Gd‐doped CeO 2‐δ (CGO) reveal coupled electro‐chemo‐mechanical responses, including ferroelectric diode characteristics and subtle lattice distortions. Epitaxial fluorite‐fluorite interfaces act as vacancy‐exchange gates that bias polymorphism, enhance polarization, strengthen piezoelectric response, and suppress leakage, in contrast to the electronically dominated perovskite‐fluorite junctions. These findings show that ferroionic heterostructures host reciprocal vacancy‐driven dynamics, establishing them as a platform for defect‐programmable ferroelectricity and tunable functionality in hafnia‐based oxides.