In situ X-ray powder diffraction was utilized to study the He diffusion kinetics in C60 by monitoring the time dependence of the structure characteristics (lattice parameter, reflection intensities and widths). Based on the time variations of the structure characteristics the intercalation was shown to occur in two stages: octahedral voids were filled during the faster stage 1, while during the much slower stage 2 tetrahedral voids were filled. To make experimental intensities consistent with the estimates made, it was concluded that, during the first third of stage-1, at least some of the octahedral voids accommodated two helium atoms rather than one. The room-temperature diffusion coefficient (about 7 x 10-14cm2/s) of He over octahedral voids of C60 was evaluated from the time dependence of the lattice parameter. Analysis of the degassing kinetics gives diffusion coefficients very close to those obtained from intercalation measurements. The characteristic time constant of the tetrahedral filling was an order of magnitude longer than for octahedral filling, which indicates that the respective effective diffusivity was four orders of magnitude slower compared to infusion into octahedral voids. Thermodynamics of intercalation of He into C60 at room temperature was constructed to show that the entropy factor was the main thermodynamic force in the case discussed. These consideration leads also to the conclusion that the internal pressure due to the kinetic energy of He atoms in octahedral voids was large enough to expand the C60 lattice to the extent observed in experiment.

Diffusion of He Atoms in Fullerite C60. Yagotintsev, K.A., Strzhemechny, M.A., Stetsenko, Y.E., Legchenkova, I.V., Prokhvatilov, A.I.: Physica B, 2006, 381[1-2], 224-32