c-axis conductivity and thermoelectric power in graphite intercalation compounds

Abstract

The measurements of the $c$-axis conductivity ${\sigma }_c$ at room temperature for acceptor graphite intercalation compounds typically show very small values of ${\sigma }_c$, ranging from ∼${10}^{-1\mathrm{}}$ to ∼10 ${\left(\mathrm{\Omega }\mathrm{c}\mathrm{m}\right)}^{-1\mathrm{}}$. The formula ${\sigma }_c=\frac{N{e}^2{l}_c}{m_c{v}_c}$ leads to a mean-free path $l_c<<{10}^{-8\mathrm{}}$ cm. This implies that the $c$-axis conduction is not a band conduction but a hopping process. The model for the phonon-assisted and impurity-assisted hopping conduction is presented and the diffusion thermoelectric power along the $c$ direction is also calculated. Reasonable orders of magnitude are obtained for these transport coefficients. The phonon-assisted hopping conductivity increases with temperature, while the impurity-assisted hopping conduction is temperature independent at low temperatures and decreases with temperature in the region where the phonon scattering process is predominant. The diffusion thermoelectric power $S_c$ is proportional to temperature and takes the form which is essentially equivalent to the one in metallic conductors.