c-Axis conductivity and conductivity anisotropy in graphite intercalation compounds

The stage (s) and temperature (4.2≤T≤295 K) dependence of the c-axis resistivity ρ_c and the conductivity anisotropy (ρ_c/ρ_a), taking into account all available literature data as well as those from the laboratory, are examined. For all intercalants, the ρ_c (s, T) variations are highly similar. The temperature coefficient of resistivity is generally positive over the entire T range for the richest compounds; it slowly decreases in magnitude and changes sign as s increases. The sign change occurs for a compound between the second and fifth stage depending on the intercalant. At 4.2 Kρ_c is T independent. Except for a few stage 1 compounds, the anisotropy always rises upon decreasing T. An analysis of the existing c-axis conductivity models is given: these invoke phonon- and impurity-assisted hopping mechanisms, interlayer conducting paths, and stacking faults. Some unanswered questions are raised that may indicate directions of future research.