Low-temperature heat capacity of fullerite C60 doped with deuteromethane

The heat capacity C of fullerite doped with deuteromethane (CD)(C) has been investigated in the temperature interval 1.2-120 K. The contribution Δ of the CD molecules to the heat capacity C has been isolated. It is shown that at T ≈ 120 K the rotational motion of CD molecules in the octahedral voids of the C lattice is weakly hindered. When the temperature is lowered to 80 K, the rotational motion of the CD molecules changes from weakly hindered rotation to libration. In the range T = 1.2-30 K, Δ is described quite accurately by the sum of contributions from the translational and librational vibrations and tunneling rotation of CD molecules. The contribution of tunneling rotation to the heat capacity Δ(T) is dominant below 5 K. The effect of nuclear-spin conversion of the CD molecules on the heat capacity has been observed and the characteristic times for nuclear spin conversion between the lowest levels of the A- and T-species of CD4 molecules at T < 5 K have been estimated. A feature observed in Δ(T) near T = 5.5 K is most likely a manifestation of a first-order phase transition in the orientational glass form of the solution. © 2012 American Institute of Physics.