Carbon: allotropes

Pure carbon is available in a number of different forms (allotrope). The most common form of pure carbon is α-graphite. This is also the thermodynamically most stable form. Diamond is a second form of carbon but is much less common. Recently, another allotrope of carbon was characterized: the fullerenes. Whereas diamond and graphite are infinite lattices, fullerenes such as buckminsterfullerene, C60, is a discrete molecular species. Amorphous forms of carbon such as soot and lampblack are materials consisting of very small particles of graphite.

Most graphite is α-graphite and it possesses a layer structure in which each carbon is directly bound to three other carbon atoms at a distance of 141.5 pm. Delocalization in the bonding is evident since the C-C distances are equal and shorter than normal carbon-carbon single bonds (typcally 154 pm). The distance between the layers of carbon atoms is 335.4 pm. In most graphite (α-graphite), the layers of atoms are arranged in an ABABAB... repeat fashion but the β-form (rhombohedral) the stacking is ABCABCABC... although the carbon-carbon distances and the interlayer spacing remains the same as in the α-form. The enthalpy difference between α- and α-graphite is less than 1 kJ mol-1 (0.59 ± 0.17 kJ mol-1. Forms of the heavier elements corresponding to graphite are not known and the structures of silicon, germanium, and grey tin are related to the diamond structure (below).

graphite crystal structure
lonsdaleite diamond crystal structure
Atom arrangements in the two most common allotropes of carbon: α-graphite (top), and β-graphite (bottom).

Diamond is a slightly more compact structure, hence its density is greater than that of graphite. The appearance of diamond is well known and it is also one of the hardest materials known. Like graphite, it is relatively unreactive but does burn in air at 600-800°C. Each carbon atom is bound to four neighbours at a distance of 154.45 pm in a tetrahedral fashion and so each diamond crystal is a single giant lattice structure. In principle (and in practice!) graphite may be converted into diamond by the application of heat and pressure. The unit cell of diamond is cubic with a = 356.68 pm. Nearly all diamonds posses this structure but a very small percentage show a hexagonal structure related to wurtzite and these are called lonsdaleite.

diamond crystal structure

lonsdaleite diamond crystal structure
Crystal strucutres of diamond (top) and the lonsdaleite form of diamond (bottom).

Recently another allotrope of carbon was characterized. Whereas diamond and graphite are infinite lattices, buckminsterfullerene, C60, is a discrete molecular species. The buckminsterfullerene molecule is a net of 12 pentagons and 20 hexagons folded into a sphere. The effect is very similar to the patchwork of 12 pentagonal and 20 hexagonal pieces of leather that sewn together make up an association football (soccer ball). The name buckminsterfullerene (or buckyball was coined because of the relationship between the structure of C60 and R. Buckminster Fuller's geodesic dome designs. Buckminsterfullerene is now commercially available and has also been identified in interstellar space and soot.

C60 is known in interstellar space
C60, Buckminsterfullerene

Other fullerenes (closed carbon cages) such as C60 and C84 are known as well, and inded available commercially. The smallest fullerene possible is the dodecahedral C20, consisting of 12 pentagons and no hexagons at all. Nanotubes are related to fullerenes. They are tubes giving the appearance of rolled graphite, although they are made from graphite. They are open ended while fullerenes are closed structures.


C70, (top) and along a nanotube (bottom).

One interesting feature of fullerenes is their ability to enclose atoms such as potassium and other alkali metals to make endohedral structures denoted as K@C60.

K@C60, an example of an endohedral fullerene.

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carbon atomic number
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