1316

Csp'=Csp'

C*-OC-C*

118.1 b

a, points to the relevant carbon-carbon bond; b. overall value a, points to the relevant carbon-carbon bond; b. overall value

2 Crystalline Forms of Carbon

The commonest crystalline forms of carbon, cubic diamond and hexagonal graphite, are classical examples of allotropy that arc found in every chemistry textbook. Both diamond and graphite also exist in two minor crystallographic forms: hexagonal diamond and rhombohedral graphite. To these must be added carbynes and Fullerenes, both of which are crystalline carbon forms. FulleTenes are sometimes referred to as the third allotrope of carbon. However, since Fullerenes were discovered more recently than carbynes, they are chronologically the fourth crystalline allotrope of carbon. Crystalline Fullerenes are now commercially-available chemicals and their crystal structures and properties have been extensively studied. By contrast, convenient methods for mass production of pure carbynes have not yet been discovered. Consequently, carbynes have not been as extensively characterised as other forms of carbon. The structures and chemical bonding of these crystalline forms of carbon are reviewed in this section.

2.1 Diamond

Diamond is an important commodity as a gemstone and as an industrial material and there are several excellent monographs on the science and technology of this material [3-5], Diamond is most frequently found in a cubic form in which each carbon atom is linked to four other carbon atoms by sp3 a bonds in a strain-free tetrahedral array, Fig. 2A. The crystal structure is zinc blende type and the C-C bond length is 154 pm. Diamond also exists in an hexagonal form (Lonsdaleite) with a wurtzite crystal structure and a C-C bond length of 152 pm. The crystal density of both types of diamond is 3.52 g-cm3.

Fig. 2. The crystal structures of: A, cubic diamond; B, hexagonal graphite

Natural diamonds used for jewellery and for industrial purposes have been mined for centuries. The principal diamond mining centres are in Zaire, Russia, The Republic of South Africa, and Botswana. Synthetic diamonds are made by dissolving graphite in metals and crystallising diamonds at high pressure (12-15 GPa) and temperatures in the range 1500-2000 K [6]; see section 3. More recently, polycrystalline diamond films have been made at low pressures by carbon deposition from hydrocarbon-containing gas mixtures that are rich in hydrogen [7]; see section 4.2.

Natural and synthetic diamonds contain various impurities. Nitrogen and boron are found as substitutional impurity atoms in the crystal lattice. Diamonds are classified as Types I and II with subtypes [5], Most natural diamonds are Type la containing up to 0.5% of nitrogen in small aggregates, since this concentration is considerably in excess of the solubility limit for nitrogen in the diamond lattice. Type lb diamonds are rare in nature, but most synthetic diamonds produced by the high pressure method are of this type. Type lb diamonds contain up to 500 ppm of substitutional nitrogen. Type Ha diamonds are very rare in nature and contain barely detectable amounts of nitrogen. Type lib diamonds are even rarer in nature and are p-type semi-conductors, since the nitrogen content is insufficient to compensate for the substitutional boron present. Significant quantities of hydrogen and oxygen are found in diamonds, especially at surfaces where they stabilise dangling bonds. Metallic inclusions are found in diamonds, typically aluminium in natural diamonds and nickel and iron in synthetic diamonds produced at high temperatures and pressures by the catalytic method.

2.2 Graphite

As a well-established allotrope of carbon the crystal structure of graphite is fully documented [8]. The graphite crystal was an early subject for application of X-ray diffraction [9]. Subsequent studies [e.g., 10, 11] confirmed the well-known hexagonal crystal structure of graphite. The basis of the crystal structure of graphite is the graphene plane or carbon layer plane, i.e., an extended hexagonal array of carbon atoms with sp2 a bonding and delocalised n bonding. The commonest crystal form of graphite is hexagonal and consists of a stack of layer planes in the stacking sequence ABABAB..., Fig. 2B.

The rhombohedral form of graphite with a stacking sequence ABCABC... is a minor component of well-crystallised graphites. The proportion of rhombohedral graphite can be increased substantially (typically from a few percent to ~ 20%) by deformation processes, such as grinding [12], Conversely, the proportion of rhombohedral graphite can be reduced by high temperature heat-treatment, showing that the hexagonal form is more stable. The density of both forms of graphite is 2.26 g-cm "3.

For both forms of graphite the in-plane C-C distance is 142 pm, i.e., intermediate between Csp3-Csp3 and Csp2=Csp2 bond lengths, 153 and 132 pm respectively, Table 1. Consideration of the resonance structures between carbon atoms in the plane show that each C-C bond in the carbon layer plane has about one third double bond character. Carbon layer planes (of various dimensions and with different degrees of perfection) are a very important microstructural element in most engineering carbons and graphites (see Section 6).

There is a large difference between the in-plane C-C distance, 142 pm, and the interlayer distance, 335 pm, in graphite that results from different types of chemical bonding. Within planes the C-C bonds are trigonal sp2 hybrid ex bonds with delocalised 71 bonds. The large interlayer spacing suggests that the contribution to interlayer bonding from 71 bond overlap is negligible. The usual assumption has been that interlayer potentials are of the van der Waals type and there have been many attempts to calculate interplanar properties starting from Lennard-Jones and Buckingham pair potentials. This work has been reviewed in detail by Kelly [8] who concluded that there is no entirely satisfactory treatment of interlayer forces in graphite. More recent evidence from scanning probe microscopical images of a graphite surface suggest that there may be some 71 orbital interaction between planes [13].

Natural graphites occur widely around the world, although the quality of the ores varies widely. High purity graphite ores with up to 100% carbon contents are mined in Sri Lanka; lower grade ores which must be concentrated are mined in Russia, China, Germany, Norway, Korea, Mexico and Austria. Ticonderoga in the USA has been used as a source of high quality natural graphite flakes for fundamental studies. Principal uses of natural graphites are in the foundry and steel industries and in the refractory and electrical industries.

Most synthetic graphites used for engineering applications are granular composites consisting of a filler (usually a coke) and a binder carbon formed from pitch. The graphitic order in most engineering grade synthetic graphites is less well-developed than in natural graphite; see section 6. Well-graphitised synthetic graphites are produced by hot-pressing pyrolytic graphite (HOPG grade); recently, well-graphitised carbons have been formed by heat-treatment of compacted polyimide films [15].

2.3 Carbynes

Carbynes are a form of carbon with chains of carbon atoms formed from conjugated C(sp')=C(sp') bonds (polyynes):

...- C^C - C=C -.. .or polycumulene ...C(sp2)=C(sp2) ... double bonds. From X-ray diffraction studies of short chain (C4-C8) polyynes [16] C=C bond lengths ranged from 119-121 pm while C-C bond lengths ranged from 132-138 pm, depending upon the local molecular environment, cf. Table 2.

In the late 1960s El Goresy and Donnay [17] discovered a new form of carbon which they called white carbon or Chaoite in a carbon-rich gneiss in the Ries meteorite crater in Bavaria. Chaoite has an hexagonal crystal structure and it was proposed that it consisted of polyyne or polycumulene carbon chains lying parallel to the hexagonal axis. At about the same time other carbyne forms with hexagonal structures were obtained in Russia [18, 19] by dehydropolymerisation of acetylene: a-carbyne and (3-carbyne and by Whittaker and his group in the USA [20-22] (Carbons VI, VIII, and IX). Lattice parameters for some of these carbyne forms are summarised in Table 3.

Table 3. Crystal structure data for some carbynes

Carbyne

Chaoite

a-carbyne

ß-carbyne

carbon VI

Carbolite I

Structure3

hex.

hex.

hex.

rhomb.

hex.

a0/pm

0 0

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