Lonsdaleite

Diamonds also occur in meteorites, probably as a result of high pressures produced dynamically by impact (10,11). The shock or explosive mode of synthesis is a viable process for fine diamond powders of both the cubic and hexagonal (lonsdaleite) polymorphs (12) naturally or otherwise. Some diamonds in space appear to have formed by processes more closely related to the low pressure chemical vapor deposition processes described later (see... 

Fig. 1. Crystal structures of (a) cubic diamond and (b) lonsdaleite. A, B, and C indicate the stacking sequence of sheets of atoms.

Fig. 1. Carbon-phase diagram where A, solvent-cataly2ed diamond growth B—G, diamond formation direcdy from graphite C, graphite formation from diamond, D, approximate region where formation of Lonsdaleite occurs from weU-ordered graphite crystals (7,8). To convert GPa to atm, multiply by...

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 fom other carbon atoms by sp ct bonds in a strain-free tetrahedral array. Fig. 2A. The crystal stmcture 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-cm. ... 

Carbon ean exist in at least 6 erystalline forms in addition to the many newly prepared fullerenes deseribed in Seetion 8.2,4 a- and /3-graphite, diamond, Lonsdaleite (hexagonal... 

Figure 15.1 Three major allotropes of carbon (1 to r) diamond, lonsdaleite, and graphite.

Besides the well-known cubic form, a hexagonal form (lonsdaleite) is found in meteorites, and it is also obtained synthetically. The relation between these two forms of carbon, similar to that between the two forms (cubic and hexagonal) of ZnS (sphalerite and wurtzite) is discussed in Chapter 7. 

Several superstructures and defect superstructures based on sphalerite and on wurtzite have been described. The tI16-FeCuS2 (chalcopyrite) type structure (tetragonal, a = 525 pm, c = 1032 pm, c/a = 1.966), for instance, is a superstructure of sphalerite in which the two metals adopt ordered positions. The superstructure cell corresponds to two sphalerite cells stacked in the c direction. The cfla ratio is nearly 1. The oP16-BeSiN2 type structure is another example which similarly corresponds to the wurtzite-type structure. The degenerate structures of sphalerite and wurtzite (when, for instance, both Zn and S are replaced by C) correspond to the previously described cF8-diamond-type structure and, respectively, to the hP4-hexagonal diamond or lonsdaleite, which is very rare compared with the cubic, more common, gem diamond. The unit cell dimensions of lonsdaleite (prepared at 13 GPa and 1000°C) are a = 252 pm, c = 412 pm, c/a = 1.635 (compare with ZnS wurtzite). 

The necessary atomic mobility can be provided by heating to about 1500 K (area D in Fig. 1). Various departures from ideality make it difficult to prepare pure wurtzitic carbon, even when the best graphite is used. The products obtained so far always contain some ordinary diamond as well as remnant graphite, parts of which are compressed by the nearby diamond regions. Hence, many physical properties of wurtzitic carbon are not well-known. It has been found in the Canyon Diablo meteorite and in some shock-made diamond from DuPont, but not in regular synthetic industrial diamond. This form of carbon has been given the name lonsdaleite. 

Clarke, R. S., Appleman, D. E. Ross, D. R. 1981 An Antarctic meteorite contains preterrestrial impact-produced diamond and lonsdaleite. Nature, Lond. 214, 396-398. 

This form of carbon is almost invariably found with the cubic structure shown in Fig. 7-1. There is also a hexagonal form (lonsdaleite),5 found in certain meteorites and also available synthetically, in which the puckered layers are stacked in an ABAB- pattern instead of the ABCABC- pattern. The hexagonal form is probably unstable toward the cubic, since unlike the cubic, it contains some eclipsed bonds. 

The carbon atoms are covalently bonded through sp bonds forming tetrahedral cells (Fig. 2). A rare form of diamond, hexagonal diamond, called lonsdaleite is also possible (Fig. 3). Essentially, the difference in the structures is the type of hybridization, sp or sp, or the ratio of sp and sp bonds and the structure type. 

Figure 2.1 Relationship between the graphite (a), lonsdaleite (b), and diamond (c) structures c is the interlayer distance. (Reprinted from Ref. [23] with permission from Elsevier.)...

Carbynes are white solids made up of carbon atoms with sp hybridization. Two main possibilities [23] exist for this Hnear structure polyine (—C=C—C=C—) and polycumulene (=C=C=C=C=) theoretical predictions and experimental evidence also point to the existence of cycHc carbynes [10]. According to a classification based on the type of bond present (Table 2.1), and also according to chronology, carbynes (rather than fuUerenes) should be considered as the third aUotropic form of carbon. Our scarce knowledge of carbynes and doubts about whether they really exist in pure form [10, 15, 26] are factors that have contributed to this erroneous interpretation. Moreover, lonsdaleite and rhombohedral graphite should be considered as polytypes rather than new structures with a different equation of state [27]. Strictly speaking, therefore, they should not be regarded as true allotropes of carbon (Table 2.1). 

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