Rhombohedral graphite

Hexagonal graphite Rhombohedral graphite Turbostratic structure... 

The use of the term graphite instead of the more exact term hexagonal graphite may be tolerated in view of the minor importance of rhombohedral graphite, the other allotropic form. See graphite, rhombohedral graphite... 

Graphite exists in two forms alpha and beta. These have identical physical properties, except for their crystal structure. Naturally occurring graphites are reported to contain as much as 30% of the rhombohedral (beta) form, whereas synthetic materials contain only the alpha form. The hexagonal alpha type can be converted to the beta by mechanical treatment, and the beta form reverts to the alpha on heating it above lOOOoC. 

The sequence of sheets in graphite is also ABAB however, an examination of the atomic positions shows that they are not simply related to those in either kind of diamond. Thus the simple compression of graphite should not be expected to yield diamond. However, we11-crysta11i2ed graphite, in which the ABAB sequence extends for at least hundreds of layers, tends to form wurt2itic carbon. The rare rhombohedral form of graphite has an ABCABC sequence of sheets, but its scarcity has hindered its study as a source for diamond. 

The eommonest erystalline forms of earbon, cubie diamond and hexagonal graphite, are elassical examples of allotropy that are found in every chemistry textbook. Both diamond and graphite also exist in two minor crystallographie forms hexagonal diamond and rhombohedral graphite. To these must be added earbynes and Fullerenes, both of which are crystalline earbon forms. Fullerenes are sometimes referred to as the third allotrope of carbon. However, sinee Fullerenes were diseovered more recently than earbynes, they are... 

Figure 8-2 Structure of the a (hexagonal) and (rhombohedral) forms of graphite.

Ordinary hexagonal graphite has a structure that repeats the ABAB. .. alternation of layers rhombohedral graphite has the repetition ABCABC. . ., with the C layer displaced from the other two. Sketch a structure for rhombohedral graphite. 

Kohs W, Santner H.J., Hofer F., Schrottner H., Doninger J., Barsukov I., Albering J.H., Moller K.-C., Besenhard J.O., and Winter M. A study on electrolyte interactions with graphite anodes exhibiting structures with various amounts of rhombohedral phase. J. 

Figure 36, P-T phase and reaction diagram of carbon as results from Refs. 509 and 510. Solid lines represent equilibrium phase boundaries. The dashed line is the threshold for conversion of hexagonal diamond and both hexagonal and rhombohedral graphite into cubic diamond.

Figure 4.15. The 3IP (double P layer) structure of rhombohedral graphite.

IPac for 2IP and IP/,/ for 3D , to give a simple sequence for all layers. One position is in common for each pair of layers and for hexagonal graphite the C positions are used in all layers. The spacing between layers is much larger (nonbonding tt interaction) than within a layer so the unit cell is not cubic, but rhombohedral with two atoms per unit cell. 

FIGURE 2.3 Unit cells of hexagonal and rhombohedral graphite with their crystallographic parameters. 

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