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Structure and Symmetry of Graphite

by Editor1 last modified March 30, 2008 - 11:03

Constantinos D. Zeinalipour-Yazdi University of California (San Diego), CA

From the introduction to:Structure and Symmetry of Graphite

Graphite is an allotropic form of carbon found both in nature and artificially produced by man. ” Graphite is used today as it was two centuries ago, as an art medium. Recently, it has found use in applications that make use of its crystal structure, such as the monochromator in X-ray radiation sources and as a calibration substrate for the piezoelectric crystals in STM heads.
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The ideal structure of graphite consists of layers composed of hexagonally arranged carbon atoms, bound together by weak van der Waals interactions. The carbon atoms are covalently bound to three neighboring carbon atoms, through sp2-sp2 axial hybrid orbital overlap.

Each isolated carbon atom has an electronic structure of the simplified form 1s22s22px12py1, where the plane of the layer is taken as the xy plane. The 1s2 core orbitals remain intact; essentially only the valence electrons undergo hybridization. For this reason, the 2s22px12py1 orbitals hybridize into three degenerate hybrid sp2 orbitals, leaving the lone 2pz electron available for parallel overlap and forming a rather diffuse electron distribution around the carbon rings (π-cloud). The layers are bound through weaker dispersion interactions (London forces) that are a consequence of the polarizable nature of the π-clouds. Ordered phases of graphite come in two different layer arrangements that result in two distinct crystal structures: the hexagonal and the rhombohedral. In both structures the carbon atoms within the graphitic layers are placed in a perfect honeycomb-like lattice with a distance of about 1.42 Å and an interlayer separation of 3.35 Å. The stacking sequence in hexagonal graphite is ABAB, whereas in rhombohedral graphite it is ABCA.

This review aims to provide an extensive list of scientific articles relevant to the structural elucidation of graphite. These span a time period of more than 100 years, starting with the macroscopic study of crystal geometries and ending with the microscopic resolution of atoms. The referenced articles in this review are presented in thematological order to emphasize the progress in structural analytical techniques and their capability to derive structural insights, information that was not previously
available.

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