Optical properties of stages 1 and 2 potassium mercurographitides

We have measured the reflectivity spectra of stages 1 and 2 graphite intercalated with KHg amalgam (KHgC₄ and KHgC₈) from 0.5–6eV, and compared them to the corresponding stages 1 and 2 binary compounds, KC₈ and KC₂₄. Plasma edges for the ternaries appear at about the same energies as in the binaries, but differences are observed in the reflectivity values at the minima. Kramers-Kroenig analysis yields thresholds for the onset of interband transitions at 1.3eV and 1.2eV for KHgC₄ and KHgC₈, compared to 2.7eV and 1.7eV for KC₈ and KC₂₄. The thresholds are explained in terms of interband transitions in the mercurographitides involving the Hg levels. A model for these transitions is presented based on the Mott metal-semiconductor transition in mercury. This model is shown to be consistent with the hypothesis that the mercury layers are negatively charged.