Diamond polytypes

The cubic structure is the dominant crystal structure in both natural and synthetic diamond since the staggered conformation is more stable than the eclipsed due to the slightly lower energy (0.1-0.2 eV per carbon atom). Diamond polytypes and carbyne phases form only during the homogeneous nucleation and growth of diamond powder,... 

In another experiment,a mixture of cubic and hexagonal diamond polytypes, 10-500 nm in diameter, were produced by MW assisted combustion of acetylene in oxygen. It was found that diamond could form only over a limited range of gas compositions and pressures C/0 of 0.56-1.4 in premixed flame, 0.2-0.9 in diffusion flame, and 50-500 torr. Larger particles were observed at lower reactor pressures (<150 torr) and higher C/0 atomic ratios (0.83-1). The effect of the pressure on the particle size was attributed to its... 

Recent investigations have revealed the existence of a series of diamond polytypes such as the 6-H hexagonal diamond. The structure and properties of these polytypes are reviewed in Ch. 11Also under investigation is a hypothetical phase of carbon based on a three-dimensional network but with sp bonds. This phase could be harder than diamond, at least in theory.1 A carbon phase diagram incorporating these new polytypes has yet to be devised. 

H Diamond. Recent investigations have revealed the existence of another intermediate diamond polytype known as 6H diamond.f l This material is believed to belong to a hypothetical series of diamond types with structures intermediate between hexagonal and cubic. The membersof this series are tentatively identified as 4H, 6H, 8H, 10H. The series would include hexagonal (2H) diamond on one end and cubic (3C) diamond on the other (the digit indicatesthe number of layers). The existence of 4H, 8H and 10H diamonds has yet to be confirmed. 

Bhargava et al. [27] provided evidence for the formation of a 6H diamond polytype by depositing similar films by hot filament and microwave-assisted CVD on a singlecrystal silicon substrate. They ascribed the appearance of hexagonal diamond to high compressive strain and attributed the findings of the above-referred investigators to the same cause. 

Hirai et al. [4], in research first reported in 1991, catalogued similar results from studies of shock-compressed graphite using a rapid cooling technique. They observed carbon diamond polytype reflections in their heated, shock-compressed carbon samples, but in addition they recorded the (200), (222) and (420) symmetry forbidden Bragg reflections of cubic diamond in their diffraction analysis. 

Fd-3m, space group 227, one of the highest symmetry space group patterns. There are, in fact, innumerable possible polytypic patterns within the diamond topology, several of these have been discussed recently by Wen et al. [17] in some detail, and all of them collectively possess the same Wells point symbol of 6, and the corresponding Schlafli symbol of (6, 4). It is only by their symmetry character, that the members of the diamond polytypic series can be distinguished from each other. Thus, the simplest cubic diamond polytype, known as 3C, is shown in Fig. 5. 

In addition to this cubic Laves phase, a variant with magnesium atoms arranged as in hexagonal diamond exists in the MgZn2 type, and further polytypes are known. 

The familiar diamond structure, with each atom covalently bonded in a perfect tetrahedral fashion to its four neighbors, is adopted not only by C but also by Si and Ge. Silicon can also adopt a wurtzite structure (see below), an example of a polytype (one of several crystal structures possible for a substance having an identical chemical composition but differing in the stacking of layers, and which may exist in a metastable state after its formation at some different temperature or pressure). 

Equiatomic tetrahedral structure types. (Carborundum structure types). To this group pertain the diamond-type structure, the wurtzite (h) and the sphalerite (c) types, and the large family of SiC polytypes (such as he, hcc, hccc, hcchc,. .. (hcc)5(hccc)(hcc)5hc. .. (hchcc)17(hcc)2,. .. (hcc)43hc...). 

Simple binary tetrahedral structures and polytypes (ZnS-sphalerite, cF8-ZnS and ZnS-wurtzMe, hP4-ZnO, structural types). The sphalerite- and wurtzite-type structures (together with C diamond) are well-known examples of the... 

Silicon carbide is covalently bonded with a structure similar to that of diamond. There are two basic structures. One is a cubic form, /i-SiC which transforms irreversibly at about 2000 °C to one of a large number of hexagonal polytypes, and the other is a rhombohedral form also with many polytypes. Both the hexagonal and rhombohedral forms are commonly referred to as a-SiC. 

The surface morphology of grains has been studied by secondary electron microscopy (SEM) (Hoppe et al., 1995). Such studies have been especially useful for pristine SiC grains that have not been subjected to any chemical treatment (Bernatowicz et al., 2003). Finally, the transmission electron microscope (TEM) played an important role in the discovery of presolar SiC (Bernatowicz et al., 1987) and internal TiC and other subgrains in graphite (Bernatowicz et al., 1991). It has also been successfully applied to the study of diamonds (Daulton et al., 1996) and of polytypes of SiC (Daulton et al., 2002, 2003). 

As shown in Figure 3.7b the carbyne layer deposited consists of small columnar crystallites about 1 pm long that are arranged more or less perpendicular to the substrate surface, i.e. (Ill) of diamond. These structures closely resemble those reported by Onuma et al. [17] and Kawai et al. [18], and attributed to chaoite, a carbyne polytype with an assumed chain length of = 11 carbon atoms [12]. 

Another material that can be grown in either the wur-tzite or zinc blende forms is SiC. The bonding here is mainly covalent (-88%) since both Si and C are group IV elements. SiC is special in that it is very difficult to produce in a single structure. It always has the chemical composition SiC, but tends to be a mixture of the two stacking sequences. The two structures are two of the polytypes of SiC. The cubic form of SiC is being produced as a diamond simulant known as moissanite. 

Silicon carbide exists in several modifications being polymorphic and polytypical and crystallizing in a diamond lattice, like silicon [13]. 

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