Infrared and differential-scanning-calorimetry study of the room-temperature cubic phase of ${\mathrm{RbC}}_{60}$

We present differential-scanning-calorimetry curves and infrared spectra taken during the temperature cycling of stoichiometrically pure ${\mathrm{RbC}}_{60}$ powder. In addition to the three known phases (high-temperature fcc, ortho-I, and ortho-II), we see evidence for an intermediate structure around 300 K, observed previously by ESR and x-ray diffraction. This phase is obtained both by quenching from high temperature and by warming up the ortho-II structure. Its infrared spectrum is identical to that of the high-temperature fcc phase, indicating a structure containing ${\mathrm{C}}_{60}^{\mathrm{-}}$ monomers.