Thermoelectric Power of Graphite Acceptor Compounds

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KO SUGIHARA,* Materials Research Laboratory, Matsushita Electric Industrial Co., Ltd., Moriguchi, Osaka 570, JAPAN ABSTRACT Temperature variations of the thermopower (TEP) of acceptor graphite intercalation compounds (GIC) are very different from that of pristine graphite. At low temperatures the TEP increases monotonically with T, then levels off above 150 K. This behavior is ascribed to the phonon drag effect. In the region where the TEP is nearly constant, phonon relaxation is mainly controlled by the Rayleigh scattering due to point defects or impurities. This process leads to T-independent phonon drag TEP. The importance of Rayleigh scattering is due to the large cross section diameter of the Fermi surface in GIC. At low temperatures where the boundary scattering becomes important, the TEP is proportional to T3 . Detailed calculations are carried out by solving the phonon-carrier coupled Boltzmann equation. INTRODUCTION Recently, the Louvain group in collaboration with the MIT group investigated the thermal conductivity and TEP of FeC1 3 -graphite, CoCl 2 -graphite and a They also measured the c-axis potassium graphite intercalation compound [1-51. TEP and resistivity [4]. Similar measurements on TEP and thermal conductivity of SbCl 5 -GIC spanning the stages 2-10 have been performed by Elzinga et al. [6]. In this article we will mainly pay attention to the TEP in the acceptor compounds. Figure 1 indicates typical features of the TEP in acceptor compounds [2,51, and other acceptor compounds show similar characteristics [1-61. These curves indicate that the TEP in GIC exhibits a different T-dependence from that of pristine graphite and it is nearly stage independent [1-61, except for high stage compounds. Highly-crystalline graphite exhibits a sharp negative dip of TEP around 35K, and this anomaly was attributed to the phonon drag effect [7-101. In this article it will be shown that the behavior of the TEP in the acceptor GIC can be also ascribed to the phonon drag effect. Since the carrier density of GIC is one or two orders of magnitude larger than that of graphite [11], the cross sectional diameter of the Fermi surface of GIC is much larger. Accordingly, it is expected that the Rayleigh scattering plays an important role in the scattering processes of the phonons interacting with carriers because its relaxation rate i/t(q) is proportional to q 3 for a two dimensional phonon, where q denotes the phonon wave vector. The introduction of Rayleigh scattering leads to a nearly T-independent region in the TEP at intermediate temperatures. By solving the carrier-phonon coupled Boltzmann equation, we can derive an expression of the phonon drag TEP. By combining the diffusion term, which is *Present address: MA 02139, USA.

Center for Materials Science and Engineering, MIT Cambridge,

Hat. Res. Soc. Symrp. Proc. Vol. 20 (1983) ©Elsevier Science Publishing Co., Inc.

158 important at low temperatures, with the phonon drag term, the overall features of the TEP in the acceptor compounds is qualitatively explained. DIFFU