Band model for the electronic structure of KHgC8.

There is both theoretical and experimental evidence that the band structure of graphite intercalation compounds (GIC's) can be considered as a first approximation, as a superposition of graphitic and intercalant bands. Assuming such a rigid-band-type model is valid, we have obtained the band structure of second-stage potassium-mercury amalgam GIC, ${\mathrm{KHgC}}_{8}$, by superposing K-Hg-K sandwich bands with the two-dimensional graphite bands. The graphite bands have been zone folded into the smaller Brillouin zone of ${\mathrm{KHgC}}_{8}$. The self-consistent full potential linearized-augmented-plane-wave formalism for thin films has been used to calculate the sandwich band structure and density of states. The K 3p states lead to a dispersionless set of bands at -17.75 eV. Hybridization of K 4s states with Hg 6p states is very strong. From our band calculation we extract the partial density of states (DOS) due to the intercalant at the Fermi level of ${\mathrm{KHgC}}_{8}$. Comparison with low-temperature specific-heat measurements suggests a very small contribution to the DOS at the Fermi level from carbon layers. We also compare the model predictions with optical experiments.