Resistivity and E.S.R. studies of graphite HOPG/fluorine intercalation compounds

We present conductivity and e.s.r. studies of graphite HOPG/fluorine intercalation compounds. The in-plane resistivity, ρ{variant}_a, was measured as a function of fluorine concentration and temperature using a contactless technique. The c-axis resistivity, ρ{variant}_c, was measured as a function of temperature and hydrostatic pressure using the four-contact technique. The main features of our results can be summarized as follows: 1. (1) The conductivity along the planes increases initially with fluorine concentration and reaches a maximum conductivity of σ_a/σ₀ ≅ 11 (σ₀ is the in-plane conductivity of HOPG). Above a concentration of x = 0.18 in C_{1 - x}F_x the conductivity, σ_a, dramatically decreases. 2. (2) ρ{variant}_a is 'metallic' for concentrations x < 0.2 and can be fitted to the equation: ρ{variant}_a = A + BT + CT². For x = 0.25, the in-plane resistivity is significantly larger and temperature independent. 3. (3) The c-axis resistivity, ρ{variant}_c, versus temperature exhibits a clear maximum for x < 0.2 but is anomalously large and almost temperature independent for x = 0.25. We suggest that carrier localization due to structural deformation and consequent changes in the band structure are responsible for the limiting conductivity. However, a percolation mechanism and domain-wall formation might also play an important role. The maximum in c-axis resistivity is attributed to a cross-over between two mechanisms of conduction: 'tunneling' or 'conducting path' mechanisms at low temperatures, but a hopping mechanism at high temperatures. The activation energy for hopping is extracted, as well as its pressure dependence. Finally, we provide a critical analysis of previous interpretations of carrier spin resonance in GICs. We have critically checked the theory of Dyson by measuring the A/B ratio, ρ{variant}_a and ρ{variant}_c versus temperatures on the same samples. We demonstrate that extracting bulk resistivities from the e.s.r. lineshape may not be justified.