Phase transitions of CO-intercalated C₆₀ crystals

We report the temperature dependence of the specific heat of (CO)xC60, 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 C60. The transition temperatures depend on x. Specific-heat measurements on a sample with x ≈ 0.5 show the orientational ordering transition at Tc = 248.8 K and the glass transition at Tg = 83.9 K. Contrary to pure C60, the temperature dependence of the lattice parameter of (CO)xC60 (x ≈ 0.67) does not show an anomaly at the glass transition, which is explained by the already expanded lattice of (CO)xC60. The energy difference between major and minor orientations is determined as 19.1 meV