Metal-nonmetal transition induced by reorientation of the fluorine molecules in stage-2 graphite-fluorine compounds

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Abstract
X-ray (001) diffraction spectra of fluorine-intercalated graphite compounds with a carbon-to-fluorine ratio between 6.4 and 2.5 are presented. Analysis of the peak intensities and positions indicates that two different phases are formed in this composition range. In the low- and high-concentration regions, we found homogeneous phases with c-axis repeat distances of 9.49 and 11.1 Å, respectively. In the former phase, the fluorine molecules lie almost parallel to the carbon layers and in the second phase they change their orientation and lie perpendicular to the layers. No continuous change between the two phases was found, although the two phases coexist in the midcomposition range. This observation explains the unique behavior of both the in-plane and c-axis dc conductivity and the optical reflectivity from the c face, reported for this material in the low- and the high-fluorine-concentration region. A metal-nonmetal first-order transition is driven by reorientation of the fluorine molecules between the host graphite layers.