Structure of single-phase superconducting K₃C₆₀

RECENT reports^1-3 of superconductivity in alkali-metal-doped compounds of the icosahedral C₆₀ (buckminsterfullerene) molecule have attracted great experimental and theoretical interest. Superconductivity was originally discovered in samples prepared from gas-solid reactions¹, which made it impossible to determine the composition or structure of the superconducting phase. Holczer et al.⁴ demonstrated that potassium-doped C₆₀ has only a single stable superconducting phase, K₃C₆₀, with a transition temperature of 19.3 K. Improvements have since resulted in the preparation of 100% bulk superconductors³. Because of the absence of impurity phases, we have been able to perform accurate Rietveld analysis of X-ray diffraction data from the superconducting phase. Here we report our results for the crystal structure of K₃C₆₀, determining that this superconducting compound has a face-centred cubic structure with a well defined stoichiometry. These results should open the way to rigorous description of the normal and superconducting properties of this compound.