Diffusion of He atoms in fullerite

What is it about?

In situ X-ray powder diffraction has been utilized to study the He diffusion kinetics in C60 by monitoring the time dependence of the structural characteristics (lattice parameter, reflection intensities, and widths). Based on the time variations of the structural characteristics the intercalation is shown to occur in two stages: octahedral voids are filled during the faster stage 1, while during the much slower stage 2 tetrahedral voids are filled. To make experimental intensities consistent with the estimates made, we came to the conclusion that during the first third of stage 1 at least part of octahedral voids accommodate two helium atoms rather than one. The room-temperature diffusion coefficient of He over octahedral voids of C60 has been 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 is an order of magnitude longer than for octahedral filling, which indicates that the respective effective diffusivity is four orders of magnitude slower compared to infusion into octahedral voids. Thermodynamics of intercalation of He into C60 at room temperature has been constructed to show that the entropy factor is the main thermodynamic force in the case discussed. This consideration leads also to the conclusion that the internal pressure due to the kinetic energy of He atoms in octahedral voids is large enough to expand the C60 lattice to the extent observed in the experiment.

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