The VO2 phase diagram shows the triple point where three solid phases exist in equilibrium at 65°C and zero stress. The olive region is monoclinic M1, the purple region is monoclinic M2, and the brown region is rutile. (Credit: Cobden; University of Washington.)
Lots of cool science involves expanding frontiers by exploring new materials that are free from the “tyranny of equilibrium,” thanks to new processing innovations. However, as a new paper in Nature (pdf) makes clear, equilibrium still has a lot to offer and can be a fascinating place.
In this case, the material is vanadium oxide—the material famous for possessing the fastest known phase transition, which occurs at about 10 times the speed of sound, or about 10 trillionths of a second. Researchers know that on heating, VO2 undergoes an insulator-to-metallic transition, usually called the “MIT,” or metal–insulator transition.
The optical properties and electrical conductivity of VO2 undergo rapid and large changes with the transition. The stunning speed of the transition, combined with its occurring at a very reasonable 68°C, has led to interest in the material for applications such as ultrafast optical and electrical switching, sensors, and more exotic uses such as ionic gating and ultrafast microscopy techniques.
In VO2, the MIT corresponds to a monoclinic-to-rutile transformation. However, several factors make the transition especially interesting and piqued the curiosity of University of Washington professor David Cobden. Similar to water, VO2 has a triple point where three solid phases exist in equilibrium. The material has two insulating monoclinic phases—M1 and M2—and a “metallic” rutile phase, R. There is little difference between the free energies of M1 and M2 near the MIT, so they compete against each other. (There is a triclinic insulating phase, too, but it is not present in the triple point, although its existence contributes to MIT theories.)
Cobden says in a press release, “If you don’t know the triple point, you don’t know the basic facts about this phase transition. You will never be able to make use of the transition unless you understand it better.”
However, VO2 is a challenging material. In bulk or film form it tends to be a complex soup of solid phases with compositional variations from oxygen vacancies and hydrogen doping, as well as inhomogeneities brought on by nonuniform strains.
Cobden’s team grew “nanobeams” of VO2 by physical vapor deposition and studied them using a special testing rig where they could observe the nanobeams in an optical microscope. The apparatus allowed them two variables, length and temperature. When temperature varies under constant length, the material undergoes a R–M2–M1 transition. However, when length changes under constant temperature, it undergoes a R–M1–M2 transition.
The three crystal structures differentiate most notably along the c-axis. Cobden explained in a phone interview that the nanobeams are high-quality single crystals that grew along the c-axis. “Nature has been very generous,” he says.
By varying length, the researchers actually vary stress (force/area), which Cobden compared to the role of pressure in the water P–T phase diagram. As the nanobeams “stretch,” the crystal structure aligns itself along its c-axis to accommodate the stress. Pulling on the sample encourages the R–M1–M2 transition, but there is a point—the triple point— where the three phases coexist in equilibrium balance. The lattice constant is respectively longer for R–M1–M2, also. Pulling on the nanobeam makes it longer, and the material compensates and reduces the elastic energy to zero by balancing the three phases in equilibrium.
Cobden says, “The key point is, the crystal structure of the three phases have three lattice constants along the nanowire. The unit cell gets a little longer and thermodynamically, the longest phase is favored because of the Clausius-Clapeyron equation.”
By systematically studying the phases present as functions of temperature and length (i.e., stress), the team determined that the triple point of VO2 corresponds to zero-stress and 65±0.1°C.
Also, VO2, like many transition metal compounds, is a strongly correlated material, meaning that its electronic structure is a mixture of free-electron (metallic) structure and ionic (insulator) structure, and electrons do not move independently. Cobden says in VO2 there is an “exotic nature to the behavior of the electrons. They must change collectively in a type of electron dance.”
The mechanism by which this happens in VO2 is not understood, but Cobden hopes his team’s “simple results will be a guiding influence for developing the theory.”
And then, armed with understanding of this material, researchers can embrace the tyranny of this equilibrium and exploit it to maximum benefit.
The paper is “Measurement of a solid-state triple point at the metal-insulator transition in VO2“, by Jae H. Park, Jim M. Coy, T. Serkan Kasirga, Chunming Huang, Zaiyao Fei, Scott Hunter, and David H. Cobden, Nature (DOI:10.1038/nature12425)