Electro-Chemo-Mechanical Coupling in Hf_0.5Zr_0.5O₂ Ferroionic Heterostructures

Ferroelectricity in Hf_0.5Zr_0.5O₂ (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_0.67Sr_0.33MnO_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.