Stabilization of ferroelectric Hf_0.5Zr_0.5O₂ epitaxial films via monolayer reconstruction driven by valence-dependent interfacial redox reaction and intralayer electron transfer

The binary fluorite oxide Hf0.5Zr0.5O2 tends to grab a significant amount of notice due to the distinct and supe-rior ferroelectricity found in its metastable phase. Stabilizing the metastable ferroelectric phase and delineating the underlying growth mechanism, however, are still challenging. Recent discoveries of metastable ferroelectric Hf0.5Zr0.5O2 epitaxially grown on structurally dissimilar perovskite oxides have triggered intensive investiga-tions on the ferroelectricity in materials that are nonpolar in bulk form. Nonetheless, the growth mechanism for the unique fluorite-perovskite heterostructures has yet to be fully explored. Here, we show that the metastable ferroelectric Hf0.5Zr0.5O2 films can be stabilized even on a 1-unit-cell-thick perovskite La0.67Sr0.33MnO3 buffer layer. In collaboration with scanning transmittance electron microscopy (STEM)-based characterizations, we show that monolayer reconstructions driven by the valence-dependent interfacial redox reaction of Mn, along with intralayer electron transfers, play a vital role in the formation of a unique heterointerface between the two structurally dissimilar oxides, providing the template monolayer that facilitates the epitaxial growth of the metastable Hf0.5Zr0.5O2 films. Our findings offer significant insights into the stabilization mechanism of the ferroelectric Hf0.5Zr0.5O2, and this mechanism could be extended for exploring functional metastable phases of various metal oxides.