SURFACE PHONON-DISPERSION IN GRAPHITE AND ITS MODIFICATION UNDER INTERACTION WITH LANTHANUM

Using high resolution electron energy loss spectroscopy (HREELS) the s urface phonon dispersion of graphite has been determined in the Gamma K direction over the whole energy range and the whole Brillouin zone. By depositing lanthanum and annealing we prepared a GIC-like phase whi ch grows on top of an intermediate carbide. The phonon dispersion of t his phase shows the same modes as graphite, but the optical ones are s oftened and the acoustical ones are stiffened. This is described withi n a Born-von Karman model. The evolution of the phonon dispersion give s a first hint that the GIG-like phase may develop in two stages: firs t a monolayer graphite on top of the carbide and then the very thin GI C layer.