The Ne content and diffusion kinetics were monitored via changes in the lattice parameter versus time, following step changes in the applied Ne pressure, by means of in situ neutron powder diffraction techniques. In the case of the face-centered cubic phase, where the molecules were freely rotating, diffusion was slower at higher Ne pressures. In the case of the primitive cubic phase, where the molecules were orientationally ordered, the diffusion rate was lower and was essentially independent of pressure. The observations were explained in terms of 3 unequal competing effects. These were that increasing the external Ne pressure increased the driving force for diffusion, that compression of the lattice slowed diffusion with increasing pressure, but mainly that the host’s molecular dynamics permitted and enhanced diffusion via a thermally activated so-called paddle-wheel effect. The activation energy for such Ne-intercalated C60 reorientational hopping or rotation (13kJ/mol) was lower than that in pure C60 (22kJ/mol); thus suggesting that the Ne atoms acted as so-called roller-bearings.

B.Morosin, Z.Hu, J.D.Jorgensen, S.Short, J.E.Schirber, G.H.Kwei: Physical Review B, 1999, 59[9], 6051-7