At temperatures below Tc=340 K, VO
2 has a monoclinic (space group P21/c) crystal structure. Above Tc, the structure is tetragonal, like rutile TiO
2. In the monoclinic phase, the V4+ ions form pairs along the c axis, leading to alternate short and long V-V distances of 2.65 Å and 3.12 Å. In comparison, in the rutile phase the V4+ ions are separated by a fixed distance of 2.96 Å. As a result, the number of V4+ ions in the crystallographic unit cell doubles from the rutile to the monoclinic phase.
The equilibrium morphology of rutile VO
2 particles is acicular, laterally confined by (110) surfaces, which are the most stable termination planes. The surface tends to be oxidized with respect to the stoichiometric composition, with the oxygen adsorbed on the (110) surface forming vanadyl species. The presence of V5+ ions at the surface of VO
2 films has been observed by x-ray photoelectron spectroscopy (XPS) measurements.
At the rutile to monoclinic transition temperature, VO
2 also exhibits a metal to semiconductor transition in its electronic structure: the rutile phase is metallic while the monoclinic phase is semiconducting. The optical band gap of VO2 in the low-temperature monoclinic phase is about 0.7 eV.
Metallic VO2 contradicts the Wiedemann-Franz Law that holds that the ratio of the electronic contribution of the thermal conductivity (κ) to the electrical conductivity (σ) of a metal is proportional to the temperature. The thermal conductivity that could be attributed to electron movement was 10% of the amount predicted by the Wiedemann-Franz Law. The reason for this appears to be the fluidic way that the electrons move through the material, reducing the typical random electron motion.
Potential applications include converting waste heat from engines and appliances into electricity, or window coverings that keep buildings cool. Thermal conductivity varied when VO2 was mixed with other materials. At a low temperature it could act as an insulator, while conducting heat at a higher temperature.