INFRARED AND DIFFERENTIAL-SCANNING-CALORIMETRY STUDY OF THE ROOM-TEMPERATURE CUBIC PHASE OF RBC60

We present differential-scanning-calorimetry curves and infrared spect ra taken during the temperature cycling of stoichiometrically pure RbC 60 powder. In addition to the three known phases (high-temperature fcc , ortho-I, and ortho-II), we see evidence for an intermediate structur e around 300 K, observed previously by ESR and x-ray diffraction. This phase is obtained both by quenching from high temperature and by warm ing up the ortho-II structure. Its infrared spectrum is identical to t hat of the high-temperature fcc phase, indicating a structure containi ng C-60(-) monomers.