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 determined by X-ray powder diffraction. A first-order phase transition and a glass transition are found to occur at temperatures below the corresponding transition temperatures of pure C 60 . The transition temperatures depend on x . Specific-heat measurements on a sample with x ≈ 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) x C 60 ( x ≈ 0.67) does not show an anomaly at the glass transition, which is explained by the already expanded lattice of (CO) x C 60 . The energy difference between major and minor orientations is determined as 19.1 meV.