Epitaxial stabilization versus interdiffusion: synthetic routes to metastable cubic HfO2 and HfV2O7 from the core-shell arrangement of precursors.
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Metastable materials that represent excursions from thermodynamic minima are characterized by distinctive structural motifs and electronic structure, which frequently underpins new function. The binary oxides of hafnium present a rich diversity of crystal structures and are of considerable technological importance given their high dielectric constants, refractory characteristics, radiation hardness, and anion conductivity; however, high-symmetry tetragonal and cubic polymorphs of HfO2 are accessible only at substantially elevated temperatures (1720 and 2600 C, respectively). Here, we demonstrate that the core-shell arrangement of VO2 and amorphous HfO2 promotes outwards oxygen diffusion along an electropositivity gradient and yields an epitaxially matched V2O3/HfO2 interface that allows for the unprecedented stabilization of the metastable cubic polymorph of HfO2 under ambient conditions. Free-standing cubic HfO2, otherwise accessible only above 2600 C, is stabilized by acid etching of the vanadium oxide core. In contrast, interdiffusion under oxidative conditions yields the negative thermal expansion material HfV2O7. Variable temperature powder X-ray diffraction demonstrate that the prepared HfV2O7 exhibits pronounced negative thermal expansion in the temperature range between 150 and 700 C. The results demonstrate the potential of using epitaxial crystallographic relationships to facilitate preferential nucleation of otherwise inaccessible metastable compounds.