The structural and electronic properties of fluorine-and bromine-intercalated graphite fibers and HOPG are summarized. In contrast to the bromine intercalate, which is purely ionic for any experimentally attainable intercalate concentration, fluorine has a dual ionic and covalent behavior in graphite. Furthermore, whereas bromine-intercalated graphite is ordered, fluorine-intercalated graphite is disordered. The stiff graphene planes are buckled and islands of various fluorine concentrations are formed. A thermodynamic model is proposed that accounts for the differences between fluorine-and bromine-intercalated graphite materials. The model describes the competition between ionically bonded and covalently bonded intercalate phases of fluorine in graphite. Covalent bonding is more favorable energetically, but an important nucleation barrier exists due to strain and to the destruction of the conjugation of the double bonds.

We have investigated the transport properties of high-fluorine-concentration fluorineintercalated vapor-grown graphite fibers (C2 9F and C3,0F). As previously reported, the transport properties of dilute fluorine-intercalated graphite fibers (C F with x )3. 6) exhibit a weak disorder regime with a strong carrier-carrier interaction correction. These corrections to the metallic properties of the C3 6F fibers emphasize the presence of intercalation-induced disorder, which is believed to be associated with the semi-ionic bonding of fluorine in carbon. With increasing fluorine concentration (x (3. 0), the fibers undergo a transition from a metallic to an insulating regime. The temperature dependence of the resistivity of the fibers in the insulating regime is no longer logarithmic at low temperature as is measured for less concentrated samples (x ) 3. 6), but rather fits a two-dimensional Mott law quite closely. From the temperature dependence of the resistivity and the nonlinear I-versus-V relationship at high electric fields, a localization length on the order of (= 1000 A is extracted. This very large localization length shows that the fibers with C2.9F and C3.0F stoichiometries lie close to the metal-insulator transition. For the C2 9F and the C3 OF fibers, the transverse magnetoresistance is negative at low fields, but becomes positive at higher fields, whereas the longitudinal magnetoresistance is positive for all fields. Both the transverse and longitudinal magnetoresistance saturate at high field and low temperature. These features are explained by the superposition of an orbital quantum-interference-induced negative magnetoresistance and a spin-polarization-induced positive magnetoresistance.

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A digitization of the TEM pictures of fluorine-intercalated graphite fibers has been used to carry out quantitative measurements of the defect structure of this material. Emphasis is given to both the computer analysis technique and to the characterization of the defects. The amount of intercalation-induced disorder increases with increasing fluorine concentration. The fast Fourier transform of the digitized TEM image exhibits two diffuse spots, corresponding to the c-axis repeat distance of the intercalation compound. The length and width of the spots are a measure of the out-of-plane and in-plane disorder present in the fibers. From the fast Fourier transform, the distribution of interlayer repeat distances and the fraction of unintercalated graphite regions throughout the material is obtained. By selecting a small range of repeat distances and carrying out an inverse fast Fourier transform, the spatial distribution of material with a given repeat distance is determined. Regions with the same repeat distance are found to form islands. This particular feature of fluorine graphite intercalation compounds, as well as the nature of the microscopic defects and the staging behavior of fluorine-intercalated graphite fibers, are discussed in connection with the dual covalent and ionic nature of the carbon-fluorine bond in fluorine-intercalated graphite.

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