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.
|Number of pages
|Published - Nov 28 2019
Bibliographical noteFunding Information:
We gratefully acknowledge partial support from the National Science Foundation under NSF grants DMR 1809866, DMR 1455154 (BSG), and OIA 1355438 (MPT). J. L. A. acknowledges support from a NASA Space Technology Research Fellowship under grant number 80NSSC17K0182. M. P. T. acknowledges support from NASA Kentucky under NASA award no: NNX15AK28A. The research was funded in part by award # A-1978-20190330 from the Welch Foundation. The TAMU Materials Characterization Facility (MCF) is acknowledged for work done with the SEM and the TAMU Microscopy Imaging Center (MIC) is acknowledged for work done with TEM. A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.
© 2019 The Royal Society of Chemistry.
ASJC Scopus subject areas
- Materials Science (all)