Effect ofc-axis dispersion on the optical properties of acceptor-type graphite intercalation compounds

The c-face optical reflectance spectra of stage-1 graphite-${\mathrm{SbCl}}_5$ and graphite-ICl and stage-2 graphite-${\mathrm{SbCl}}_5$ have been measured in the photon energy range 0.08 to 10 eV. From a Kramers-Kronig analysis of the spectra, the interband contribution to the imaginary part of the dielectric function ${\epsilon }_2$(ω) has been determined. It is found that a two-dimensional (2D) band-structure model, while describing qualitatively the spectral features below ∼3 eV, requires the use of an effective temperature much greater than the experimental temperature to explain the width of the onset for valence- to conduction-band absorption in the (stage-1 and -2) compounds. The width of a spectral feature associated with valence- to valence-band absorption in the stage-2 compounds is also not explained satisfactorily by the 2D model. It is shown that weak c-axis dispersion provides a natural explanation for the widths of both of these features.