Anisotropic structure deformation in the VO2 metal-insulator transition.

X-ray absorption fine structure data of tungsten (VI)-doped vanadium dioxide in the insulating phase, and during the metal-insulator transition, are presented for the first time. Tungsten L(III)- and vanadium K-edge data suggest that significant expansion in the [110] and [110] directions occurs across the phase transition from low to high temperature. This distortion breaks the bonds between Peierls-paired vanadium ions, opening a band gap, and reveals the nature of the mechanism by which tungsten doping lowers the transition temperature and enthalpy.