Ne intercalated C-60: Diffusion kinetics

In situ neutron powder-diffraction has been used to probe the: Ne content a nd diffusion kinetics in C-60 by monitoring the C-60 lattice parameter vs t ime following step changes in applied Ne pressure. In the face-centered-cub ic phase where the C-60 molecules are freely rotating, diffusion is slower for higher Ne pressure. In the primitive cubic phase where the C60 molecule s are orientationally ordered, the diffusion rate is slower and is essentia lly independent of pressure. These observations can be understood in terms of three unequal competing effects. (1) Increasing the external Ne pressure increases the driving force for diffusion; (2) compression of the C-60 lat tice slows diffusion with increasing pressure; and, the major one, (3) C-60 molecular dynamics enables and enhances diffusion by a thermally activated "paddle wheel'' effect. The activation energy for such Ne intercalated C-6 0 reorientational hopping or rotation (similar to 1500 K or 13 kJ/mole) is less than that in pure C-60 (similar to 2600 K or 22 kJ/mole), suggesting t he Ne atoms act as ''roller bearings.'' [S0163-1829(99)01309-0].