In situ X-ray microdiffraction studies inside individual VO_2 microcrystals

摘要

Synchrotron X-ray microdiffraction provides quantitative structural measurements with submicron spatial resolution, and hence enables investigations of how local microstructural inhomogeneities affect materials' properties. A combination of polychromatic and monochromatic X-ray microdiffraction was used to investigate domain formation, interface orientations and strain distributions inside individual vanadium dioxide (VO_2) microcrystals. Using in situ measurements near the VO_2 metal-insulator phase transition, it was found that the observed phase evolution is critically dependent on external strain. Substrate-induced strains or inhomogeneous sample heating can directly alter phase stability and affect the local domain orientations. In different clamped or freely suspended single-crystal samples, all the predicted twin laws for the M2 phase in VO_2 were observed, except one. When the rutile and M2 phases coexist, it was found that different interphase boundary orientations can be stabilized by sample size and by interfacial elastic strain. The large variations in phase sequences and domain orientations observed in relatively simple, small single crystals provide insight into the mechanisms responsible for the broad structural and electronic transitions observed in epitaxial VO_2 films.