Electronic and Lattice Modes of Graphite-CoCl 2
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C.W. LOWE*, C. NICOLINIt, AND G. DRESSELHAUS§ Massachusetts Institute of Technology, Cambridge,
MA 02139,
USA
ABSTRACT Reflectivity measurements on graphite-CoC1 2 intercalation compounds between 0.06 eV and 2.0 eV for stages 3 and 5 show sharp Drude edges and superimposed interband structure. Stage 3 shows structure near 0.4 eV and 0.55 eV while stage 5 shows structure between 0.12 eV and 0.55 eV. The observed number of interband transitions is equal to the stage index which is consistent with a recently proposed kz-axis zone-folding model of the electronic energy bands. In addition to the electronic optical transitions, features associated with lattice vibrations are observed in the spectra. In particular, the graphite-like IR mode near 1580 cm-1 is clearly resolved in the intercalation compounds. A reflectivity minimum and splitting of this vibrational mode is observed for both stages 3 and 5 of graphite-CoCl 2 . INTRODUCTION Optical reflectivity measurements represent an important test for the validity of electronic energy band calculations [1]. Energy band calculations for graphite intercalation compounds (GIC) have been carried out recently using techniques such as self-consistent full-zone calculations, full-zone firstprinciples band calculations [21, •', perturbation expansions [3-51 and LCAO phenomenological expansions [6,7]. The measured optical properties have been calculated by Blinowski et al. [3,41 using the K'• expansions. Though the other band schemes would be expected to more accurately represent the energy bands, no calculations of the optical properties have yet been carried out for the full zone energy band calculations. This paper presents the reflectivity and its subsequent Kramers-Kronig analysis to obtain the complex dielectric function for stages 3 and 5 of graphite-CoCl 2 . The analysis clearly shows a stage dependence in the peak structure and in the free carrier contribution to the dielectric function. Though a number of measurements of the optical properties for well staged acceptor compounds have been carried out (e.g., graphite-SbCl 5 [3,81, -AICI 3 [31 and -Br 2 [31), this is the first report on a magnetic metal chloride compound. Past attempts to describe the electronic properties of acceptor compounds have involved band models based on a finite number of layers [2-5]. However, recently a 3D-model based on kz-axis folding of the Brillouin zone has been presented as a general calculational method for both donor and acceptor compounds [6,7]. The bands calculated with this model are shown to correctly predict the stage dependence of the observed transitions reported here. Phonon related structures in the reflectivity spectra of CoC1 2 -GIC near 1580 cm-1 are also observed. These structures are associated with the IR-active Elu graphite-like modes of the bounding and interior graphite layers. The observa*Department of Physics. $Center for Materials Science and Engineering. §Francis Bitter National Magnet Lab.
Mat.
Res.
Soc.
Symp. Proc. Vol.
sponsored by NSF.
20 (1983) ©Elsev
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