Introduction to Structure
Contents
1. Introduction
2. Overview
3. Detailed reports
1. Introduction
The allotropic forms of crystalline carbon, diamond, and graphite occur in nature and can be synthesized. The crystalline form of diamond is face-centered cubic. Each atom is surrounded by four nearest neighbors at the corners of a regular tetrahedron. The smallest interatomic distance is 1.54 Å, which corresponds very closely to that found in aliphatic hydrocarbons. In fact, the diamond crystal represents the ultimate structure in a three-dimensional aliphatic carbon polymer. The crystal density of diamond (3.53 g/cm3) is somewhat higher than that of graphite (2.26 g/cm3). The three-dimensional system of strong C—C bonds produces the hardest material known.
In striking contrast to diamond, graphite is a soft electrically conducting material and, at ordinary temperatures and pressures, is thermodynamically slightly more stable. Graphites with a wide range of crystalline development can be manufactured. Those carbons in which the graphitic structure is not completely developed or in which the graphitic structure is limited to volumes on the order of a few thousand cubic angstroms are usually referred to as "amorphous carbons." Those in which the crystallites have been more highly ordered by heat-treatment at 2500 to 3000 °C are referred to as "graphites."
2. Overview
Graphite Crystal Structure
The two most important structural features that give graphite many of its unique properties are: (1) the system of strong chemical bonds forming large, sheets, of hexagonal rings in which the electrons are quite mobile and (2) the large spacing between carbon layer planes. The extended ring structure is a logical one if the formation of graphite is considered to proceed by the growth of larger and larger molecules of the condensed benzene-ring system. Within the planes the C—C bond lengths are all equal (1.42 A) and are close to the C—C bond length (1.39 A) found in benzene. The stacking of layer planes in the hexagonal structure is ABAB . . .; i.e., layers in which every atom has an atom directly above it are separated by one layer. The occurrence of extra lines on X-ray powder photographs suggests that about 5 per cent of the graphite has a second type of stacking, i.e., ABCABC . . . for which the unit cell is rhombohedral. The relation between the hexagonal and rhombohedral forms of graphite is similar to the relation between the hexagonal and cubic close packing of spheres. When related to a normal hexagonal c axis, the rhombohedral unit cell has a height of 3/2 c, giving rise to the extra X-ray lines.
3. Detailed Reports
