Electrical and thermal properties of fluorine-intercalated graphite fibers

We report measurements on the electrical- and thermal-transport properties of fluorine-intercalated vapor-grown carbon fibers ${\mathrm{C}}_{\mathit{x}}$F with the carbon-to-fluorine-atom ratio x between 4.1 and 5.8. The electrical resistivity is found to be very sensitive to the fluorine concentration. Indeed, while the resistivities of the ${\mathrm{C}}_{5.2}$F and the ${\mathrm{C}}_{5.8}$F compounds are well below that of the pristine graphite fibers, the tendency is reversed when x≤4.5 and large resistivity values are obtained. All ${\mathrm{C}}_{\mathit{x}}$F compounds exhibit a logarithmic resistance increase at low temperature, which can consistently be explained by weak-localization and carrier-carrier interaction effects for two-dimensional electron systems. Large negative magnetoresistance values have been measured on the ${\mathrm{C}}_{\mathit{x}}$F compound with x=4.1. At higher temperature, an anomalous resistance-versus-temperature behavior appears for each fluorine-intercalated vapor-grown carbon fiber, possibly resulting from a phase transition. Heat transport in ${\mathrm{C}}_{\mathit{x}}$F (with x=4.1) is almost entirely governed by lattice vibrations between 2.5 and 300 K while the thermoelectric power is positive, and its temperature dependence is similar to that previously observed in other acceptor graphite intercalation compounds. Our results indicate that the high resistivity of the ${\mathrm{C}}_{\mathit{x}}$F compounds (with x≤4.5) is due to strong defect scattering rather than a low free-charge-carrier density.