Using quasi-elastic neutron scattering, the self-diffusion of alkali-metal atoms in stage-1 and stage-2 graphite intercalation compounds was studied (figures 10 to 12). For stage-1 compounds the diffusion proceeded via jumps to potential wells provided by the graphite substrate potential. The activation energies for diffusion in LiC6 and KC8 were 1.0 and 0.18eV, respectively, and agreed very well with theoretical values derived by DiVincenzo and Mele (1985) for the saddle-point energy between C-C bonds. The diffusion of alkali-metal atoms in stage-2 compounds was qualitatively very different from that in stage-1 compounds. The characteristics of the diffusive motion appeared to be intermediate between those of a free liquid and a lattice liquid. The activation energies were considerably lower and comparable with those of hydrogen in metals: 0.126, 0.063 and 0.077eV for K, Rb and Cs, respectively, in stage-2 compounds. Moreover, the stage-2 compounds exhibited a continuous melting transition, which extended over several hundred degrees Celsius. Over this temperature range, liquid-like diffusive motion and solid-like phonon excitations co-existed. It was argued that this behavior was characteristic of the melting of a two-dimensional structure on a periodic substrate.

Diffusion and Melting in Two Dimensions: a Quasielastic Neutron Scattering Study of Alkali Metals in Graphite. Zabel, H., Magerl, A., Rush, J.J., Misenheimer, M.E.: Physical Review B, 1989, 40[11], 7616-32

Figure 10

Diffusion of Cs in graphite (C24Cs)