Effect of<i>c</i>-axis dispersion on the optical properties of acceptor-type graphite intercalation compounds

Abstract

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 ${\ensuremath{\epsilon}}_{2}$(\ensuremath{\omega}) has been determined. It is found that a two-dimensional (2D) band-structure model, while describing qualitatively the spectral features below \ensuremath{\sim}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.