ABSTRACT
I. INTRODUCTION Because of the unique nature of fluorine as an intercalate species, fluorineintercalated graphite intercalation compounds (GICs) have very special physical properties among all GICs. Whereas most intercalates exhibit ionic bonding to the graphene layers, fluorine in graphite has the possibility of forming either a purely ionic (the most common planar form of GICs) compound for relatively low fluorine concentrations [F], a purely covalent (with nonplanar sp3 bonding) compound for high [F] concentrated, or a material with mixed ionic and covalent character for intermediate [F] concentrations [1]. Thus for dilute [F] concentra tions, fluorine acts predominantly as an ionic acceptor, releasing holes into the graphene layers, as is the case for other acceptor GICs. However, for higher fluorine concentrations beyond stage 1, the C -F bond shifts to a more covalent character [2], which depresses the carrier concentration and displaces some of the carbon atoms slightly away from the highly planar structure of pristine graphite. As a result, instead of the usual increase in electrical conductivity a by an order of magnitude or more, typical of GICs as the intercalate concentra tion is increased, the peak in conductivity a max obtained for CXF compounds represents only a factor of 2 to 3 increase in a over that of the graphite host material. Furthermore, whereas in typical donor and acceptor GICs, a drops from its maximum value a max by about 30% at stage 1, a drops by several
orders of magnitude for fluorine concentrations above that corresponding to crmax in fluorine-GICs (see Fig. 1). In addition, unlike the situation for other GICs, where normally stage 1 denotes the maximum intercalate uptake [3], fluorine uptake continues beyond the concentration of the stage 1 compound, which is nominally QF.
