Optical Reflectance Studies of Stage 1–6 Graphite-FeCℓ 3 Intercalation Compounds
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D. S. SMITH* AND P. C. EKLUND Department of Physics & Astronomy,
University of Kentucky,
ABSTRACT Besults of Optical Reflectance measurements on stage 1-6 graphite-FeCZ 3 intercalation compounds are reported. The frequency dependences of the dielectric constants( 1 ,6 2 ) are determined. The contributions from free carrier and interband absorption are separated to determine important material parameters.
INTRODUCTION In this paper we report the results of optical reflectivity studies in the photon energy range (0.2-2.5 eV) for stage 1-6 graphite-FeCk3 acceptor-type The stage index refers to the number of graphitic intercalation compounds. carbon layers separating next-neighbor iron chloride intercalate layers [1,2]. Although not as air-stable as graphite-SbCZ 5 , graphite-FeCZ3 samples can be exposed to laboratory air for several minutes without inducing noticeable changes in the optical properties of their c-faces. This limited air stability makes graphite-FeC1 3 an attractive system for many experimental studies. However, a significant offsetting factor is that well-staged samples are difficult to prepare. Recent electron energy band calculations [4,5.13] in acceptor-type graphite intercalation compounds (GAC's) predict hole pockets along the H-K-H axes in the corners of a graphitic, (hexagonal) Brillouin zone. These calculations report nearly linear band dispersion for small radial wavevectors kp perpendicular to the H-K-H axes. This type of dispersion was reported much earlier in a two-dimensional, tight-binding calculation for pristine graphite . Most of the calculations in GAC's ignore the coupling along the 8 axis and therefore cannot comment on the extent of the actual kz dispersion. The Fermi level in GAC's falls in the range 0.5 , EF : 2 eV below the valence band maximum, depending on the stage index and the degree of charge transfer between graphitic and intercalate layers. Optical absorption at energies E < 2 eV in GAC's therefore arises from both intraband and interband processes that can be separated in a straightforward manner using methods we have reported previously , or using a Kramers-Kronig analysis. In another paper at this conference  we report on the optical reflectance of stage 1-4 graphite-SbCZ5 in the energy range (0.1-10 eV) using a Kramers-Kronig analysis. In the present work on graphite-FeCk 3 we take the phenomenological approach  because the upper limit of our data is -2.5 eV. EXPERIMENTAL
The samples were prepared from highly oriented pyrolytic supplied by Union Carbide using a two-zone, vapor-transport FeCk 3 is extremely hygroscopic we synthesized FeCk 3 in situ ampoule by the reaction of Ck2 gas with high-purity Fe wire gas was dried by passage through P2 0 5 . Iron oxide on the Present address
graphite (HOPG) method. Because in the reaction . The Ck2 iron wire
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Hat. Res. Soc. Symp. Proc. Vol.
20 (1983) QElsevier Science Publishing Co.,
had been previously remove