Vanadium Dioxide Does A Solid Triple Point

Phase diagrams show the conditions at which solid, liquid, and gas phases of a material may coexist or occur alone. Phase diagrams can also be drawn for solid-state materials to illustrate the intersections of different electronic, magnetic, or structural phases. By studying temperature- and stress-dependent transitions between metallic and insulating states in vanadium dioxide, a group led by David H. Cobden of the University of Washington, Seattle, has now identified the first triple point for a solid, at which three solid phases co­exist: a coincidence of the metallic and two insulating states of VO₂ at 65.0 °C and zero applied stress (Nature 2013, DOI: 10.1038/nature12425). The finding was made possible because the VO₂ nanowires used in the study are relatively easy to work with, Cobden says. He notes that the triple point is near room temperature, the material is relatively stable, and his team was able to distinguish the three phases optically. The finding will help researchers understand and exploit the mechanisms of VO₂ phase transitions, which are of particular interest for sensing and switching applications. It may also help explain the properties of other materials with metal-insulator transitions, such as high-temperature superconductors.