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) C60 and polycrystalline C60 intercalated with He, Ne, Ar, and Kr has been investigated using high-resolution capacitance dilatometer. The investigation of the time dependence of the sample length variations on heating 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 C60 molecules. It is assumed that the reorientation is of quantum character. The inert gas impurities affect very strongly the reorientation of the C60 molecules especially at liquid helium temperatures. A temperature hysteresis of the thermal expansion coefficient of Kr- and He- C60 solutions has been revealed. The hysteresis is attributed to orientational polyamorphous transformation in these systems.Comment: 18 pages, 12 figure

PHOTOLUMINESCENCE AND STRUCTURE OF C60C_{60} INTERCALATED WITH HELIUM

Author Institution: Verkin Institute for Low Temperature Physics and Engineering; Department of Mechanical Engineering, Northwestern UniversityPowder x-ray diffractometry was employed to study infusion of He into $C_{60}$ fullerite. It has been shown that the intercalation at a pressure of 1 Bar is a two-stage process, the first stage being the saturation of the octahedral voids, virtually complete after 55 hr. Photoluminescence spectra were taken at 5 K from $C_{60}$ with completely saturated octahedral voids. Helium in the lattice voids is shown to reduce that part of the luminescent emission which is due to 0-0 transitions around 1.69 eV from the so-called deep traps, or according to existing notions, the covalently bound pairs of $C_{60}$ molecules. The effect of He intercalation on polymeric dimer formation is ascribed to the changes in the pentagon to hexagon configuration ratio caused by the intercalation-related increase of the lattice parameter and the formation of bound states of He atoms in the $C_{60}$ lattice voids