The metal-insulator transition occurring in hollandite ${\mathrm{K}}_2{\mathrm{V}}_8{\mathrm{O}}_{16}$ 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 ${\mathrm{K}}_2{\mathrm{V}}_8{\mathrm{O}}_{16}$. 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.

• Received 14 December 2010

DOI:https://doi.org/10.1103/PhysRevLett.107.027201