Optical properties across the insulator to metal transitions in vanadium oxide compounds

We review the optical properties of three vanadium oxide compounds V2O3, VO2 and V3O5, belonging to the so-called Magneli phase. Their electrodynamics across a metal to insulator transition is investigated as a function of both temperature and pressure. We analyse thoroughly the optical results, with a special emphasis on the infrared spectral weight. This allows us to discuss the nature of the mechanisms driving the phase transitions in the three compounds, pointing out the role of electron-electron and electron-phonon interactions in the various cases. (topical review)