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 HfO are accessible only at substantially elevated temperatures (1720 and 2600 °C, respectively). Here, we demonstrate that the core-shell arrangement of VO and amorphous HfO promotes outwards oxygen diffusion along an electropositivity gradient and yields an epitaxially matched VO/HfO interface that allows for the unprecedented stabilization of the metastable cubic polymorph of HfO under ambient conditions. Free-standing cubic HfO, 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 HfVO. Variable temperature powder X-ray diffraction demonstrate that the prepared HfVO 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.