  The binary fluorite oxide Hf0.5Zr0.5O2 tends to grab a significant amount of
notice due to the distinct and superior 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
investigations 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
one-unit-cell-thick perovskite La0.67Sr0.33MnO3 buffer layer. In collaboration
with scanning transmittance electron microscopy (STEM) based characterizations,
we show that monolayer reconstruction driven by interfacial redox reactions
plays 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 HZO 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.
