Thermoelectric power of graphite intercalation compounds

Abstract

A model for the temperature variations of the thermoelectric power (TEP) of acceptor graphite intercalation compounds (GIC's) is presented. At low temperatures, the TEP in GIC's increases monotonically with $T$, then levels off above \ensuremath{\sim}200 K in striking contrast to that of pristine graphite. The diffusion contribution to the TEP is proportional to $T$ and its magnitude is small as compared with that of the observed data. This observed behavior is attributed to the phonon drag effect. In the temperature region where the TEP is weakly temperature dependent, phonon relaxation is mainly controlled by the Rayleigh scattering due to point defects. The resultant TEP, which is composed of the phonon drag and diffusion terms, leads to a nearly $T$-independent value. Since the cross-sectional diameter of the Fermi surface in GIC's is larger than that of pristine graphite, the relaxation rate of the Rayleigh scattering, which is given by $\frac{1}{{t}_{I}(q)}=f{q}^{3}$, becomes very large at high temperatures ($T>100$ K). At low temperatures, where the boundary scattering plays a dominant role, the TEP is proportional to ${T}^{3}$. Detailed calculations are carried out by solving the phonon-carrier-coupled Boltzmann equation.