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P. BOOLCHAND, G. LEMON, W. BRESSER, D. MCDANIEL University of Cincinnati, Cincinnati, OH 45221, USA R.E. HEINZ, P.C. EKLUND University of Kentucky, Lexington, KY 40506, USA E. STUMPP, G. NIETFELD Technischen Universitat Clausthal, 3392 Clausthal Zellerfeld, West Germany ABSTRACT 151Eu M~ssbauer experiments on EuC1 -ALCL graphite flakes 3 3 and HOPG samples have been performed as a function of temperature in the range 4.2K < T < 300K and indicate the presence2 of a narrow line characteristic of Eu3+. No evidence of Eu + was found. This suggests that enhanced conductivity of graphite upon intercalation with rare earth chlorides derives from a transfer of charge to the co-intercalated C1. INTRODUCTION There is now abundant evidence for a charge transfer [1] in metal chloride graphite intercalation compounds (GIC). However,the chemical species or the nature of the sites which are formed by the charge transfer process can vary with the type of metal chloride involved. In graphite FeC1 3 [2] and graphite SbC1 5 [3] there seems to be a partial reduction of Fe 3 + to Fe + and Sb5+ to Sb + respectively, whereas in graphite A1C1 3 . 2 , the co-intercalated excess chlorine is considered to act as an electron acceptor. To investigate these chemical aspects of graphite rare earth trichlorides we have used 1 5 1 Eu M5ssbauer spectroscopy and this paper describes results of our experiments. SAMPLE PREPARATION AND CHARACTERIZATION Approximately 3 g of freshly prepared EuCL 3 .6H2 0 was placed in contact with 0.5 g graphite (either HOPG or flake) in a silica tube. After drying in an HC1 stream by slowly raising the temperature to 300°C, 50 mg of freshly prepared AMCL was sublimed in a C12 stream onto the sample. The tube was sealed with C?2 gas at 1 atmosphere and heated to 540°C for 3.5 days. Following the reaction, A1C1 3 was sublimed to the cool end of the tube. The GIC was then washed with dimethylformamide to remove excess EuC1 3 which had not reacted and finally washed with acetone and dried at 100 0 C. Further details of the preparation are available in reference [4]. Two samples of graphite EuCI 3 exhibiting a light bluish color were prepared, one using graphite flakes and the other using HOPG. X-ray examination revealed that the graphite flakes-EuC1 3 product was a mixed stage 2 and 3 material while the HOPG product was a pure stage 2 material. An x-ray (00) trace of the latter sample is shown in Figure 1, revealing a repeat distance Ic = 1 3. 2 3+O.O4 A. MOSSBAUER SPECTROSCOPY 151HEu Mssbauer spectra of the GIC were taken at different temperatures T in the range 4.2K < T < 300K using an exchange gas liquid helium dewar [4].

Mat. Res.

Soc. Symp.

Proc.

Vol. 20 (1983) §Elsevier Science Publishing Co.,

Inc.

394

EuCI 3 - HOPG Stage I[ Cu Ka(I.542 A)

I 0 o0

.,Caused

by the specimen support IO°

15"

20°

25°

5"

2e

Fig. 1, X-ray 00k reflections from stage II HOPG-EuC1 3 . 100 7- 95 0

0 0o o0

o

90 (30 Cf) 7U)

P- 9 W EuCI3 6H2 0

-_

-3

0

+3

+6

UELOCITY (iM/SEC) Fig. 2. 151Eu M~ssbauer spectra of indicated