Low-temperature dynamics of matrix isolated methane molecules in fullerite C ₆₀ . The heat capacity, isotope effects

The heat capacity of the interstitial solid solution (CH ₄ )0.4C60 has been investigated in the temperature inter-val 1.4-120 K. The contribution of CH ₄ molecules to the heat capacity of the solution has been separated. The contributions of CH ₄ and CD ₄ molecules to the heat capacity of the solutions (CH ₄ ) _0.40 C ₆₀ and (CD ₄ ) _0.40 C ₆₀ have been compared. It is found that above 90 K the character of the rotational motion of CH ₄ and CD ₄ mole-cules changes from libration to hindered rotation. In the interval 14-35 K the heat capacities of CH ₄ and CD ₄ molecules are satisfactorily described by contributions of the translational and libration vibrations, as well as the tunnel rotation for the equilibrium distribution of the nuclear spin species. The isotope effect is due to main-ly, the difference in the frequencies of local translational and libration vibrations of molecules CH ₄ and CD ₄ . The contribution of the tunnel rotation of the CH ₄ and CD ₄ molecules to the heat capacity is dominant be-low 8 K. The isotopic effect is caused by the difference between both the conversion rates and the rotational spectra of the nuclear spin species of CH ₄ and CD ₄ molecules. The conversion rate of CH ₄ molecules is several times lower than that of CD ₄ ones. Weak features observed in the curves of temperature dependencies of the heat capacity of CH ₄ and CD ₄ molecules near 6 and 8 K, respectively, are most likely a manifestation of first-order polyamorphic phase transitions in the orientational glasses of these solutions.