Low-temperature thermal expansion of pure and inert-gas-doped fullerite C60

The low-temperature (2-24 K) thermal expansion of pure (single-crystal and polycrystalline) C-60 and polycrystalline C-60 intercalated with He, Ne, Ar, and Kr is investigated using a high-resolution capacitance dilatometer. The investigation of the time dependence of the sample length variations DeltaL(t) on heating by DeltaT shows that the thermal expansion is determined by the sum of positive and negative contributions, which have different relaxation times. The negative thermal expansion usually prevails at helium temperatures. The positive expansion is connected with the phonon thermalization of the system. The negative expansion is caused by reorientation of the C-60 molecules. It is assumed that the reorientation is of a quantum character. The inert gas impurities affect the reorientation of the C-60 molecules very strongly, especially at liquid-helium temperatures. A temperature hysteresis of the thermal expansion coefficient of Kr- and He-C-60 solutions is revealed. The hysteresis is attributed to orientational polymorphous transformation in these systems. (C) 2003 American Institute of Physics.