c-Axis conductivity and conductivity anisotropy in graphite intercalation compounds

Abstract

The stage (s) and temperature (4.2≤T≤295 K) dependence of the c-axis resistivity ρc and the conductivity anisotropy (ρc/ρa), taking into account all available literature data as well as those from the laboratory, are examined. For all intercalants, the ρc (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 Kρc 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.