Dynamics of 2-D alkali domains in graphite intercalation compounds

The two-dimensional diffusion of alkali atoms, the in-plane alkali-modes, and the critical softening of the layer shear elastic constant C₄₄ in stage 2 MC₂₄ (M=K, Rb, Cs) compounds can be understood in a unified manner within the frame of a structural model based on alkali-domains separated by discommensuration walls. New data yield evidence that with increasing temperature the diffusion of atoms close to the domain walls coexists with phonon-like excitations within the individual domains. As the domain walls become soft at higher temperatures the alkali-modes exhibit increased anharmonicity and a diffusivity which extends throughout the domains. In this model, the extreme softening of C₄₄ is due to the unlocking of domain walls above the critical temperature for discommensuration-domain formation.