Ne intercalated C{sub 60}: Diffusion kinetics

{ital In situ} neutron powder-diffraction has been used to probe the Ne content and diffusion kinetics in C{sub 60} by monitoring the C{sub 60} lattice parameter vs time following step changes in applied Ne pressure. In the face-centered-cubic phase where the C{sub 60} molecules are freely rotating, diffusion is slower for higher Ne pressure. In the primitive cubic phase where the C{sub 60} molecules are orientationally ordered, the diffusion rate is slower and is essentially 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{sub 60} lattice slows diffusion with increasing pressure; and, the major one, (3) C{sub 60} molecular dynamics enables and enhances diffusion by a thermally activated {open_quotes}paddle wheel{close_quotes} effect. The activation energy for such Ne intercalated C{sub 60} reorientational hopping or rotation ({approximately}1500 K or 13 kJ/mole) is less than that in pure C{sub 60} ({approximately}2600 K or 22 kJ/mole), suggesting the Ne atoms act as {open_quotes}roller bearings.{close_quotes} {copyright} {ital 1999} {ital The American Physical Society}