Molecular Structure of the Fulleride Anions in Superconducting K₃C₆₀ and Insulating K₆C₆₀ Determined by Powder Neutron Diffraction

The structures of K₃C₆₀ and K₆C₆₀ are refined by Rietveld analysis of high-resolution timeof-flight powder neutron diffraction data. Bond length and angle constraints are not necessary to produce chemically sensible molecular structures for the fulleride anions. The results suggest loss of the distinction between single and double bonds on the fullerene molecule on reduction and are consistent with recent ab initio calculations. A simple chemical interpretation of the observed trends is given.