X-ray diffraction evidence for nonstoichiometric rubidium-C60 intercalation compounds

Powder X-ray diffraction at 300 K on equilibrated samples of several nominal compositions x in Rb(x)C60 is reported. In addition to fcc (x = 3, superconducting), body-centered tetragonal (x = 4), and bcc (x = 6) stoichiometric phases, direct evidence for a dilute fcc-doped phase, between 0 and 1, and for a substoichiometric bcc phase, x of about 5, is presented. In contrast, x = 3 and x = 4 appear to be line phases with nearly zero solubility of Rb vacancies and interstitials at 300 K. These results are summarized in a provisional binary phase diagram. (Author)