13 C NMR Chemical Shifts of Oriented Cesium-Graphite Intercalation Compounds
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SHIFTS OF ORIENTED CESIUM-GRAPHITE INTERCALATION
COMPOUNDS
D.D. DOMINGUEZ, H.A. RESING, C.F. PORANSKI, JR., AND J.S. MURDAY Naval Research Laboratory, Code 6120, Washington, DC 20375, USA
ABSIRACT' 13
The C NMR lines of the stage I cesium graphite compound are broad (ca. 600 Hz) at all orientations, reflecting immobile Cs intercalation; in terms of an axial pattern the For principal values are 611 = 95 + 5 and &6 = 130 ± 6 ppm. third and fourth stage compounds the bounding layers give a relatively sharp (60-100 Hz) pair of lines at all orientations, thus demonstrating axially symmetric shift tensors: Su = 66 and 56 ppm; 6.L= 159 and 149 ppm, These well defined axial tensors reflect respectively. atomic motions in the incommensurate Cs layer; the pair of lines may arise from the two classes of carbons - those above carbons in a subsequent layer and those above hexagon centers. Lines of inner layers are sharpest for special orientations ' I 'o and c I A*.
INTRODUCTION Conard et al. [1-41 have given powder anisotropy patterns for various graphite intercalation compounds and interpreted these in terms of the A surprising aspect of these spectra is that electronic density of states. Such symmetry implies an axis of they are approximately axially symmetric. 3-fold or higher symmetry at the site of every carbon atom represented in 13 the C spectra of the host lattice; this site symmetry is clearly not allowed for any rigid array of intercalant molecules or atoms. The purpose of this report is to show that additional features are observable when highly oriented 1 For 3C NMR pyrolytic graphite (HOPG) is used rather than graphite powder. this uniaxial polycrystalline material is equivalent to a single crystal as long as the intercalation reaction does not destroy 3-fold site symmetry at the carbon atoms. EXPERIMENTAL Highly oriented pyrolytic graphite (HOPG) obtained from Union Carbide was cut into s-face rectangles approximately 6 =r x 10 mm using a diamond string amand saw. The initial thickness of the samples was in the range 0.35 - 0.75 the initial weight of the samples fell in the range 75 - 150 mg (± 0.1 mg). A cesium ampoule (1g, Alfa Products) was broken inside an evacuated pyrex The tube (diameter -10 mm) containing the HOPG sample to be intercalated. intercalation was carried out using the two-zone vapor transport method The HOPG and commonly used to prepare alkali metal intercalated graphite [5]. the cesium were maintained at constant temperatures for at least one week to insure the formation of well-staged samples. The compounds were characterized by their color, weight uptake and x-ray diffraction patterns (Cu K ). Analysis of the (0,0,k) lines of the latter showed the compounds to be well-staged, in complete agreement with the weight uptake.
Hat.
Res.
Soc.
Symp.
Proc.
Vol.
20 (1983) OElsevier Science Publishing Co.,
Inc.
364 1
For the 3C NMR study the samples were transferred in an inert atmosphere of a glove box to 10 mm OD tubes and aligned so that the graphite t axes were perpendicular to the
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