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17 August 1981
THERMOELECTRIC PROPERTIES OF A DILUTE GRAPHITE DONOR INTERCALATION COMPOUND J. HEREMANS
J.-P. ISSI and I. ZABALA-MARTINEZ
Laboratoire de Physico-Chimie et de Physique de l’Etat Solide, Université Catholique de Louvain, B-i348 Louvain-la-Neuve, Belgium
and M. SHAYEGAN and M.S. DRESSELHAUS Center for Materials Science and Engineering and Department of Electrical Engineering and Computer Science, MIT, Cambridge, MA 02139, USA Received 15 April 1981 Revised manuscript received 5 June1981 Data on the in-plane thermal conductivity and thermoelectric power of a stage-5 potassium donor graphite intercalation compound are reported in the temperat~i~re range 3
The first thermal conductivity measurements performed on a graphite intercalation compound have recently been reported , and were for the in-plane thermal conductivity of a graphite—FeC13 acceptor-
temperature range, and is larger in magnitude than that of pristine graphite. Blackman et a!.  measured the room temperature thermopower of potassium corn-
type compound in the temperature range 1.7 < T <300 K. It was shown that the main effect of intercalation is to decrease the lattice thermal conductivety relative to pristine graphite over the whole temperature range. However, in the liquid helium range, an increase in the total thermal conductivity was observed and attributed to the large enhancement of the electronic component due to the holes introduced by intercalation. Thermopower measurements were also carried out on a stage-2 graphite—FeC!3 intercalation compound in the temperature range 4.2 < 7’ < 300 K , where the stage refers to th’e number of graphite layers between consecutive intercalate layers. The measurements showed, as could be expected for an acceptor compound, that the thermopoweris positive over the whole
pounds as a function of stage, and these are the only thermopower data available for a donor compound. We here report the first thermal conductivity measurements on a donor compound, and thermopower data in the temperature range 3
A irant FNRS Supported by agrant from Gobierno Vasco, Departamento de Educacion, Universidades e Investigacion.
stage-S graphite—K . Characterization studies of these materials  indicated a slow desorption rate for stage n >4, which simplified our sample handling in
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2000 50CC 000I
For the acceptor compound, it was also shown that the room conductivity is mainly defectstemperature introduced thermal during the intercalation processdue . -
~ 20 ~50 3 F—
donor compound near room temperature is much lower than that of pristine graphite. This observation is attributed to the decrease of the lattice thermal conductivity through increased scattering of phonons by
to the lattice and electronic contributions of the graph-
ite compound layers. Using the the contributions procedure from for the phonons the FeCl3 compound, onerelative maysame roughly estimateas for donor and electrons. If we assume that the Wiedemann— Franz (WF) relation KeLOTU
0 20 50 I TemperatureT(K)
Fig. 1. Log—log plot of the thermal conductivity ofa stage-S
graphite—potassium intercalation compound (dotted curve) versus temperature in the range 3~T < 300 K. For comparison the thermal conductivity of pristine graphite (solid curve) and of a stage-2 Fed 3 intercalation compound (dashed curve) aie also presented.
the thermal conductivity and thermopower measurements. The sample was mounted in a special sample holder, which was thermally anchored to the heat sink ofa vanable temperature liquid helium cryostat. Experimental details about the sample mounting and experimental procedures for thermal conductivity and thermopower measurements are given elsewhere [1,2]. From the moment the sample was taken out of its vacuumtight glass bulb, where it was intercalated, it was handled in a controlled atmosphere, free from water vapor and oxygen, until it was mounted in the evacuated sample chamber of the ciyostat. The sample was cycled three times from room temperature to liquid helium temperatures and the low temperature data were reproducible within experimental error. Also, the stage of the sample was checked after the thermal conductivity and thermopower measurements were performed. In fig. 1 the temperature variation of the thermal conductivity of the stage-S potassium intercalation compound is compared to that of a FeCI3 stage-2 cornpound and pristine graphite. As is also the case for the acceptor compound, the thermal conductivity of the 388
17 August 1981
(1) and usingLorenz the room temperais applicable with2aK—2) free electron number (L0 = 2.45 X 108 conductivity V ture electrical value (a 2 X l0~f14 rn1) for a stage-5 K compound , we obtain a value for the electronic thermal conductivity ice 150 W m1 K—1 at T = 300 K, which is about one quarter of the total observed thermal conductivity. In the lowest temperature range (roughly 3
thermal conductivity of the donor compound is pro40
Temperature T (K)
Fig. 2. The thermopower versus temperature of a stage-S graphite—potassium intercalation compound in the range 3 ~ T < 300 K (dotted curve). For comparisonthe thermopower of pristine graphite (solid curve) and a stage-2 FeC13 intercalation compound (dashed line) are also presented.
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portional to the absolute temperature. lI$s behavior combined with the fact that we have measured in the same temperature range a constant electrical resistivity, indicates that we are observing pure electronic thermal conduction in the residual range. The higher value of K, at low temperatures obtained with the donor compound than with the acceptor compound is consistent with the fact that the latter has a much higher residual electrical resistivity. The WF law yields a value for the residual resistivity of the donor compound of 1.9 X 1O-g Q m, indicating that our sample has a residual resistivity comparable to the lowest values reported in the literature [ 8-101. Jn fig. 2 the temperature dependence of the thermopower for the donor sample is compared to that of a FeCl, stage-2 compound and that of pristine graphite. We do not observe in the intercalated material the humps which are apparent in pristine graphite. Instead the acceptor and donor compounds exhibit similar temperature dependences, but with opposite signs: a monotonic increase with temperature then a levelliig off at higher temperatures. Note the anomaly for the donor compound around 200 K. For comparison, the room temperature value for the thermopower obtained by Blackman et al.  for a stage-5 potassium compoundis-32X 10-6VK-1,whileformostofthe other stages it lies around -22 to -25 X 10e6 V K- l.
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The authors are indebted to Dr. G. Dresselhaus and Professor J.-P. Michenaud for valuable discussions and advice and to AFOSR contract ##F49620-81-C-0006 for partial support of this work. The authors are thankful to Dr. A. Moore of Union Carbide for kindly sup plying the HOPG material and to P. Coopmans for his skill in mounting the sample. References [l] J. Boxus, B. Poulaert, J.-P. Issi, H. Mazurek and MS. Dresselhaus, Solid State Commun. (1981), to be published.  J.-P. Issi, J. Boxus, B. Poulaert, H. Maimrek and M.S. Dresselhaus, J. Phys. Cl4 (1981) L307.  L.C.F. Blackman, J.F. Mathewsand A.R. Ubbelohde, Proc. R. Sot. 258A (1960) 339.  A. H&old. Physics and chemistry of materials with layered structures, Vol. 6, ed. F. L6vy (Reidel, Dordrecht, 1979) p. 323. [S] G. Dresselhaus, S.Y. Leung, M. Shayegan and T.C. Chieu, Synthetic Metals 2 (1980) 321.  C. Underhill, T. Krapchev and M.S.Dresselhaus, Synthetic Metals 2 (1980) 47.  E. &Rae, D. Billaud, J.F. Mareche and A. H&old, Physica 99B (1980) 489.  D. Gu&ard, GMT. Foley, M. Zanini and J.E. Fischer, Nuovo Cimento 38B (1977) 410.  D.G. Onn, GMT. Foley and J.E. Fischer, Phys. Rev. B19 (1979) 6474. [lo] H. Suematsu, K. Higuchi and S. Tanuma, J. Phys. Sot. Japan 48 (1980) 1541.