Epitaxial stabilization versus interdiffusion: synthetic routes to metastable cubic HfO₂ and HfV₂O₇ from the core-shell arrangement of precursors

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.