Vanadium dioxide

Vanadium dioxide [vanadium(IV) oxide, (VO₂)] is one of four relatively common oxides of vanadium. Others are

• vanadium monoxide [vanadium(II) oxide, VO];
• vanadium sesquioxide or trioxide [vanadium(III) oxide, V₂O₃]; and
• vanadium pentoxide [vanadium(V) oxide, V₂O₅].

In addition, several mixed vanadium oxides exist, many in nature. Examples are V₃O₇ (VO₂ + V₂O₅) and V₄O₇ (2VO₂ + V₂O₃). The vast range of vanadium oxides differ widely in terms of color, crystal structure, and chemical properties.

VO₂ itself is sometimes expressed as V₂O₄, V₄O₈, or other V_nO_2n formulas, depending on its crystalline symmetry. At temperatures less than 67 ºC, VO₂ has a monoclinic crystal structure (shown). When heated to more than 67 ºC, it transitions to a tetragonal structure. At the same temperature, crystalline VO₂ converts from an electrical insulator to a conductor.

But wait, there’s more: In 2013, a team led by Masaki Nakano at the RIKEN Center for Emergent Matter Science (Wako) and Tohoku University (both in Japan) built on the insulator–conductor phenomenon to affect light transmission through VO₂ glass. It had previously been shown that low-temperature (<30 ºC) VO₂ glass is transparent to infrared (IR) radiation, but at >60 ºC, it reflects IR light. Nakano et al. accomplished the same result by applying an external voltage to a thin film of VO₂. This discovery is the basis of new field-effect transistors.

And still more: In 2017, Olivier Delaire at the Oak Ridge National Laboratory (TN) and Duke University (Durham, NC); Junqiao Wu at the University of California, Berkeley, and Lawrence Berkeley National Laboratory; and colleagues worldwide showed that VO₂ violates the venerable (1853) Wiedemann–Franz law, which holds that the ratio of the electronic component of the thermal conductivity of a metal to its electrical conductivity is proportional to the metal’s temperature. At and below the insulator–conductor transition temperature, the electronic thermal conductivity is anomalously low, which, the authors state, “is a signature of the absence of quasiparticles in a strongly correlated electron fluid where heat and charge diffuse independently."

What will scientists discover next about VO₂?