PHASE-TRANSITIONS OF CO-INTERCALATED C-60 CRYSTALS

We report the temperature dependence of the specific heat of (CO)(x)C- 60, and the temperature dependence of the lattice parameter, as determ ined by X-ray powder diffraction. A first-order phase transition and a glass transition are found to occur at temperatures below the corresp onding transition temperatures of pure C-60. The transition temperatur es depend on x. Specific-heat measurements on a sample with x approxim ate to 0.5 show the orientational ordering transition at T-c = 248.8 K and the glass transition at T-g = 83.9 K. Contrary to pure C-60, the temperature dependence of the lattice parameter of (CO)xC(60) (x appro ximate to 0.67) does not show an anomaly at the glass transition, whic h is explained by the already expanded lattice of(CO)(x)C-60. The ener gy difference between major and minor orientations is determined as 19 .1 meV.