Optical dielectric function of high-stage potassium graphite intercalation compounds: Experiment and theory

Optical reflectance of the stage-2–4 graphite potassium intercalation compounds in the range 0.2–6 eV has been measured and the experimental dielectric functions have been derived from Kramers-Kronig analyses. The parameters of a two-dimensional tight-binding model Hamiltonian are obtained by comparison of the calculated interband dielectric function with the experimental data. Good agreement between experiment and the model is obtained. Narrow peaks associated with low-energy interband transitions between graphitic ${\pi }^{\mathrm{*}}$ conduction bands observed in the third- and fourth-stage compounds are found to occur at photon energies approximately equal to the difference in electrostatic potential energy between bounding and interior carbon layers. A simple k dependence of the nearest-neighbor carbon transfer integral parameter ${\gamma }_0$ is introduced to the model Hamiltonian to obtain quantitative agreement with ∼4.6 eV interband spectral feature associated with the M points in a graphitic hexagonal Brillouin zone. The c-axis charge distribution, density of states at the Fermi level, and the extremal Fermi cross-sectional areas are also calculated and compared with values obtained from NMR, specific-heat, and de Haas–van Alphen experiments.