Vanadium(IV) oxide
Vanadium(IV) oxide is the inorganic compound with the formula VO2. It is a dark blue solid. Vanadium(IV) oxide is amphoteric, dissolving in non-oxidising acids to give the blue vanadyl ion, [VO]2+ and in alkali to give the brown [V4O9]2− ion, or at high pH [VO4]4−.
Synthesis and structure
Following the method described by Berzelius, VO
2 is prepared by comproportionation of vanadium(III) oxide and vanadium(V) oxide:
At room temperature VO2 has a distorted rutile structure with shorter distances between pairs of V atoms indicating metal-metal bonding. Above 68 °C the structure changes to an undistorted rutile structure and the metal-metal bonds are broken causing an increase in electrical conductivity and magnetic susceptibility as the bonding electrons are "released". The origin of this insulator to metal transition remains controversial and is of interest in condensed matter physics.
Infrared reflectance
VO
2 expresses temperature-dependent reflective properties. When heated from room temperature to 80 °C, the material's thermal radiation rises normally until 74 °C, before suddenly appearing to drop around 20 °C. At room temperature VO
2 is almost transparent to infrared light. As its temperature rises it gradually changes to reflective. At intermediate temperatures it behaves as a highly absorbing dielectric.
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