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Recently it has been shown that electrons donated by the graphite host to the FeC13 intercalate freeze onto selected Fe'+ ions to change them into Fe + ions at low temperatures. We find that this process occurs continuously beginning at approximately 100 K. Two possible explanations of the data are presented. We examine the possibility that the number of donated electrons which are associated with selected iron sites is temperature dependent, and that a free-bound transition occurs as the temperature is lowered. We also discuss an alternative explanation first suggested by Hohlwein et a/. in terms of a temperature-dependent hopping rate for the electrons which becomes slow compared with the nuclear Larmor frequency at low temperatures. Although our fits to the experirnental spectra appear to favor the first explanation, we find that the second one cannot be ruled out on the basis of Mossbauer spectroscopy alone. Finally, we also discuss the sample dependence of the number of Fe + sites observed at low temperatures. Our findings support the suggestion of Wertheim et al. that chlorine atoms surrounding iron vacancies when present in sufficient numbers in the intercalate may act as the primary acceptor site for the electrons donated by the graphite to the intercalate.

Using relaxation theory, we have completed the first fits to the broadened Mossbauer spectra observed about the magnetic transition temperature in stage-1 and -2 ferric chloride intercalated graphite compounds. We conclude that the major Fe3+ site undergoes a magnetic phase transition at 4.2 %0.5 K for the stage-1 compound and 2.0 21.5 K for the stage-2 compound. These results are significant in showing that the magnetic phase transition, at least in the stage-1 compound, is strongly influenced by three-dimensional interactions. In agreement with Hohlwein et al. but in disagreement with others, we find that thermally activated electrons, donated by the graphite lattice, are frozen onto 20 f3'lo of the Fe + sites to create Fe + sites at low temperature.

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