Dimerization and charge order in hollandite K₂V₈O₁₆.

The metal-insulator transition occurring in hollandite K₂V₈O₁₆ has been studied by means of neutron and x-ray diffraction as well as by thermodynamic and electron-spin resonance measurements. The complete analysis of the crystal structure in the distorted phase allows us to identify dimerization as the main distortion element in insulating K₂V₈O₁₆. At low-temperature, half of the V chains are dimerized perfectly explaining the suppression of magnetic susceptibility due to the formation of spin singlets. The dimerization is accompanied by the segregation of charges into chains.