Charge transfer in rare earth-trichloride graphite intercalation compounds

Abstract

We report the results of 151Eu Mössbauer and optical reflectivity measurements on intercalation compounds formed either by the reaction of graphite with EuCl 3, or with EuCl 3: AlCl 3 mixtures, in the presence of Cl 2. Results on graphite flakes and highly-oriented-pyrolytic graphite (HOPG) indicate the presence of a single narrow Mössbauer line characteristic of Eu 3+, which we identify with intercalated EuCl 3. No evidence for Eu 2+ was found in the Mössbauer spectra. Since Eu 3+ is the most easily reduced of the rare earth ions, we conclude that no reduction of any rare earth trichlorides takes place under similar conditions of intercalation. The optical reflectance spectrum of the HOPGEuCl 3Cl 2 exhibits a sharp Drude edge at ∼ 1.2 eV consistent with metallic conduction and a moderate charge transfer. These results taken collectively suggest that the enhanced metallic conductivity in rare earth chloride intercalation compounds is associated with charge transfer from the graphite to the co-intercalated Cl 2 forming Cl - and is consistent with measured stoichiometries C n EuCl 3+δ, where δ≅0.2.