The role of fluorine chemistry in studies of the synthetic metals: intercalated graphite and doped polyacetylene

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The degree of charge transfer involved in the intercalation of graphite has been the subject of controversy. Information on this process has been obtained from the intercalation of transition metal hexafluorides MF 6 (M = Mo,Tc,Re,Os,Ir) into highly oriented pyrolytic graphite (HOPG). These hexafluorides are of interest, because they comprise a unique group of intercalants differing mainly in their electronic configurations and oxidizing properties. The intercalation may generally be written as nC + MF 6 → C n +x (MF 6 −) x(MF 6) 1−x where x denotes the degree of charge transfer. In some cases x can be extracted from simultaneous ESR and susceptibility studies of the magnetic intercalate species. For OsF 6 such studies confirm earlier conclusions by Bartlett that x = 1. Similar measurements on graphite compounds with MoF 6 lead to x ⋍ 0.2. For HOPG/ReF 6, susceptibility measurements indicate x ∼ 0.35. Combining these results with data on TcF 6 and PtF 6 compounds show that the c-axis spacing of an occupied layer is a smooth function of the degree of charge transfer and the electron affinity of the MF 6. Only graphite compounds with IrF 6 deviate from these relations indicating that IrF 6 may have a lower electron affinity than previously estimated and/or that it is partially reduced to IrF 6 2− in the process of intercalation, i.e. x > 1. Results of doping polyacetylene with MF 6 parallel those with graphite, causing large increases in conductivity. The maximum conductivities attainable vary inversely with the electron affinity of the MF 6. Here, too, IrF 6 does not fit this generality, which may mean that IrF 6 undergoes a different reaction upon doping than the other MF 6 compounds.