SURFACE PHONON-DISPERSION IN GRAPHITE AND IN A LANTHANUM GRAPHITE-INTERCALATION COMPOUND

Using high-resolution electron energy-loss spectroscopy the surface-ph onon dispersion of graphite has been determined in the TK direction ov er the whole energy range and the whole Brillouin zone. Born-von Karma n model calculations are used to describe the dispersion relations. An unexpected result is the splitting of the ZA and ZO mode at the K poi nt. Following a previously introduced procedure to form in situ rare-e arth graphite intercalation compounds (GIG), which for lanthanum resul ts in an intermediate carbide phase, we prepared this carbidic phase a nd the final GIG-like phase. The carbide shows five dispersionless fea tures that may be attributed to Einstein modes of graphite islands. Th e phonon dispersion of the final phase shows the same modes as graphit e shifted in energy: softening of the optical and stiffening of the ac oustical phonons occurs. This is described within a Born-von Karman mo del by weakening the nearest-neighbor interaction and strengthening th e second-nearest-neighbor interaction. The evolution of the phonon dis persion gives a first hint that the GIG-like phase may develop in two stages: first a monolayer graphene on top of the carbide and then the very thin GIG layer.