Valence and core electronic excitations in KHgC4 and KHgC8

Abstract

Electron-energy-loss spectroscopy has been used to measure the electronic excitations of the stage-1 and -2 mercurographitides, KHgC4 and KHgC8, respectively. Results for these ternary compounds are compared to similar spectra of the state-1 and -2 binary intercalation compounds, KC8 and KC24. 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 K(4s) 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 KC8, confirming the earlier assignment of these features to backfolded interband transitions associated with the MC8 in-plane structure. Comparison of the C(1s) and K(2p) core-level excitations with similar data from x-ray photoemission experiments shows that the K(4s) 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. © 1984 The American Physical Society.