The Graphite Crystal

Graphite makes an ideal substance to study by electron scattering because graphite crystals come in thin sheets. A graphite crystal consists of a series of planes of carbon atoms. Within one plane the atoms have the hexagonal structure shown in Figure (1), reminiscent of the tiles often seen on bathroom floors. The spacing between neighboring atoms in each hexagon is 1.42 A^o as indicated at the bottom of Figure (1).

Figure 1: The hexagonal array of atoms in one layer of a graphite crystal. Lines of atoms in this crystal act as crossed diffraction gratings.
The atoms within a plane are very tightly bound together. The hexagonal array forms a very strong framework. The planes themselves are stacked on top of each other at the considerable distance of 3.63 A^o as indicated in Figure (2). The forces between these planes are weak, allowing the planes to easily slide over each other. The result is that graphite is a slippery substance, making an excellent dry lubricant. In contrast, the strength within a plane makes graphite an excellent strengthening agent for epoxy. The resulting carbon filament epoxies, used for constructing racing boat hulls, light airplanes and stayless sailboat masts, is one of the strongest plastics available.

Figure 2: Edge view of the graphite crystal, showing the planes of atoms. The planes can easily slide over each other, making the substance slippery.