Raman scattering study of acceptor-acceptor-type graphite bi-intercalation compounds

Stages 4 and 5 FeCl3-graphite intercalation compounds (GIC's) were prepared by a two-bulb method, and were used as host materials for the synthesis of graphite bi-intercalation compounds (GBC's), where the bi-intercalated species were ICl, IBr, and SbCl5. Various types of GBC's were obtained by changing the reaction temperatures, and the layer sequences were clarified by x-ray diffraction. Lattice dynamics of the resultant GBC's was investigated by Raman spectroscopy. Since the layer sequence of GBC's from a stage 4 FeCl3-GIC with one bi-intercalated layer is G(FeCl3)G(1)G(2)(I)G(3)G(4)(FeCl3)G, where G(n), (FeCl3), and (I) denote the nth graphite. FeCl3, and bi-intercalated layers, respectively, and all graphene layers are adjacent to an intercalate layer The GBC's give only one Raman-active E-2g((2)b) mode frequency in the Raman spectra although the intercalates are different. GBC's from stage 5 FeCl3-GIC with one bi-intercalated layer give the stacking sequence of G(FeCl3)G(1)G(2)(I)G(3)G(4)G(5)(FeCl3)G. Since two types of graphene layers, interior (G(4)) and bounding (G(1,2,3,5)) layers exist, two peaks identified as Raman-active E-2g((2)l) and E-2g((2)b) mode frequencies appear in Raman spectra. Both frequencies were affected by the bi-intercalated species. From the E-2g((2)l) frequencies, the degree of electron affinities of the bi-intercalated layers was evaluated to be in the order of IBr<ICl<SbCl5. For the GBC's with two bi-intercalated layers, their layer sequences were determined to be G(FeCl3)GG(I)G(I)GG(FeCl3)G or G(FeCl3)GG(I)GG(I)G(FeCl3)G. In this case, only bounding layers of graphite exist. However, two peaks were observed in the Raman spectra. The difference of the Raman-active E-2g((2)b) mode frequencies was due to the intercalate coupling effect.