Valence and core electronic excitations inKHgC4andKHgC8

Electron-energy-loss spectroscopy has been used to measure the electronic excitations of the stage-1 and -2 mercurographitides, ${\mathrm{KHgC}}_4$ and ${\mathrm{KHgC}}_8$, respectively. Results for these ternary compounds are compared to similar spectra of the state-1 and -2 binary intercalation compounds, ${\mathrm{KC}}_8$ and ${\mathrm{KC}}_{24}$. Plasmon peaks at 2.1 and 6.7 eV in the stage-1 compound, and 1.6 and 6.7 eV for the stage-2 compound are in good agreement with the optical results. From 1 to 6 eV, the presence of excess oscillator strength in the ternaries compared to the corresponding-stage binary compounds is attributed to interband excitations involving electrons in intercalant-derived bands. The measured Fermi-energy shifts upon intercalation also indicate that the charge transfer from K to graphite is incomplete in the KHg compounds and that some of the original $\mathrm{K}\mathrm{}\left(4s\right)\mathrm{}$ charge is retained in intercalant bands. In the region between 10 and 40 eV, fine structure is observed in the mercurographitide spectra as in the case of ${\mathrm{KC}}_8$, confirming the earlier assignment of these features to backfolded interband transitions associated with the $M{\mathrm{C}}_8$ in-plane structure. Comparison of the $\mathrm{C}\mathrm{}\left(1s\right)\mathrm{}$ and $\mathrm{K}\mathrm{}\left(2p\right)\mathrm{}$ core-level excitations with similar data from x-ray photoemission experiments shows that the $\mathrm{K}\mathrm{}\left(4s\right)\mathrm{}$ bands are unoccupied in all four compounds studied here. Based on the above evidence, a model is presented for the charge-transfer mechanism in the ternary compounds. This model is compared with the observed superconducting properties of these compounds, and emphasizes the importance of three-dimensional intercalant-derived bands in the superconductivity of thee materials.