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 the face-centered cubic (fcc) (x = 3, superconducting), body-centered tetragonal (x = 4), and body-centered cubic (bcc) (x = 6) stoichiometric phases, direct evidence for a dilute fcc doped phase, 0 < x less-than-or-equal-to 1, and for a substoichiometric bcc phase, x approximately 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.