Optical Properties of Alkali Metal Binary and Ternary Graphite Intercalation Compounds.

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 function varepsilon(omega) has been derived from a Kramers-Kronig analysis. The parameters of a stage-dependent 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 are obtained. The narrow peaks associated with low energy interband transitions between graphitic pi* conduction bands observed in the third and fourth stage K-GICs are found to occur at photon energies approximately equal to the difference of the electrostatic potential energy between bounding and interior graphite 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 experimentally derived dielectric function in the energy range above 4 eV. The results of a comparative optical reflectance study at room temperature of the stage 1 and stage 2 CsBi _{rm x}- and KHg -graphite intercalation compounds (GICs) in the energy range 0.2-10 eV are also presented. A Kramers-Kronig analysis is carried out to determine the dielectric function varepsilon(omega) = varepsilon _{1}(omega) + ivarepsilon _{2}(omega) and the intra - and inter-band contributions to varepsilon( omega) are identified. The dielectric function for the respective compounds are analyzed in terms of a schematic density of states model involving contributions from two-dimensional graphite pi band(s) and intercalate bands. Separate contributions from the pi and intercalate electrons to the free carrier plasma frequency are determined via the identification of the carbon pito pi* interband absorption threshold E _{rm T} ~ 2E_{rm F'} where E_{rm F} is the Fermi energy measured with respect to the approximate mirror plane in the carbon pi band structure. The analysis of the optical data indicates that for CsBi_{rm x} -GICs the electrical transport properties should be dominated by the light carbon pi band electrons in contrast to the KHg-GICs, where the analysis indicates they should strongly depend on free carriers in both the pi* and the intercalate band(s). In the case of KHg-GICs, our interpretation of the experimental results are consistent with significant occupation of electronic states with Hg(6s) character. Our results suggest that nature of the superconductivity in the KHg- and CsBi _{rm x}-GICs should be quite different.