Carbon Voi 20. No 4. pp 297-301, 1982 Printed in Great Britain. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA SCANNING TRANSMISSION ELECTRON MICROSCOPY OF MULTIPHASES IN GRAPHITE-ALKALI METAL INTERCALATION COMPOUNDS H. MAZUREK,~$ M. S. DREssELHAUstff and C. DRESSELHAUSq Massachusetts Institute of Technology, C~bridge~ MA 02139,U.S.A. (Received 30 October 1981) Abstract-Structural and micro-analytical evidence is presented for the presence of multiphase regions in graphite- Rb intercalation compounds for stages n 2 2. The intercalate layers are composedof islands of alkali metal, ordered incommensuratelywith respect to the adjacent graphite layers and embedded in a background of disordered rubidium in the intercalate layer. The results confirm the non-integral stoichiometry of graphite alkali metal intercalation compounds for stages n 2 2. 1. INTRODUCTION The transmission electron microscope (TEM) is an espe- cially versatile instrument which can be used to in- vestigate both the real space (imaging) and reciprocal space (diffraction) structure of graphite intercalation compounds. The electron diffraction studies done with the TEM are in fact complementary to X-ray[l-31 and neutron[rl, 51 diffraction studies. Recent high resolution bright field images (with a resolution of 3 8, or better), formed by the transmitted electron beam, have shown evidence of possible Daumas-H~roId domains~6] in FeCl, intercalated graphite[7]. TEM has also been widely used to study the in-plane structure of graphite alkali metal intercalation compounds [8-l 11. In these studies the real space bright field micrographs give direct evidence for ordered alkali metal islands in the inter- calate layers for compounds with stages n 2 2. Because of this island structure, bulk X-ray and neu- tron diffraction measurements yield diffraction vectors and structure factors which can only be interpreted zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA in terms of averaged in-plane densities and in-plane inter- calant order[12-141. Thus the in-plane intercalate den- sity, as determined by analysis of integrated intensity X-ray profiIes~l5, 161,does not agree with the accepted integral stoichiometry, such as LX for alkali metal donor compounds with stage> 2. These X-ray measurements of the average in-plane intercalate density[l5, 161 are consistent with the results of the TEM studies which show that the in-plane island struc- ture consists of regions with different in-plane densities[9-I I]. Such multiphase structure is expected to give rise to non-integral average stoichiometries. The bright field TEM micrographs, obtained for the inter- calant in the alkali metal donor compounds for stage L 2, VZenter for Materials Science and Engineering. SPresent address, ARC0 Chemical Comoanv. Research and Engineering Center, Newtown Square, PA 1607j, U.S.A. PDepartment of Electrical Engineering and Computer Science. TFrancis Bitter National Magnet Laboratory, supported by NSF. show dark alkali-rich “island” regions on a lighter back- ground[lll]. Stereo micrographs clearly show that the islands are not only associated with the surface[14] but are distributed throughout the bulk. Although the electron diffraction superlattice patterns obtained by TEM yield valuable information on the structure of the multiphase system, the conventional TEM lacks the capability of giving quantitative infor- mation on the distribution of intercalant species throughout the graphite host. In addition to conventional di~raction and bright and dark field (vi& injra) real space imaging, the scanning transmission electron microscope (STEM} can provide in situ quantitative, micro-analytical X-ray fluorescence data, characteristic of the intercalant, from areas as small as 15 x IS A’. Similarly, electron energy loss spectroscopy may be performed in situ, providing complementary quantitative elemental analysis. The scanned beam minimizes radia- tion damage; such radiation damage is normally a problem with the use of transmission electron microscopy for the study of graphite intercalation compounds at ambient temperatures. At high beam currents, damage can be detected by the time variation of the image. The STEM enables one to carry out simuItaneous microstructural and quantitative chemical study of graphite intercalation compounds. The systematic struc- tural and micro-analytical study of various stages of graphite-rubidium intercalation compounds described in this paper was undertaken to study the stage dependence of the island formation and the intercalate concentration in the islands. 2. EXPERIMENTAL PROCEDURE The samples were prepared by intercalating Rb into highly oriented pyrolytic graphite (HOPG) using the conventional two-zone vapor transport technique~l71. After intercalation, the compounds were characterized for stage index and stage fidelity by their (001) X-ray diffractograms. The large bulk samples were transferred to an argon-filled glove box, where thin specimens were prepared and transferred to the sample holder of the