Lonsdaleite structure

The C—C distance in lonsdaleite is equal to that in diamond [11]. One quarter of the Cg rings are in chair and three quarters are in boat conformation. The lonsdaleite structure is possibly idealized. An imperfect lonsdaleite could be formed by stacking... 

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

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. ... 

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). 

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). 

Figure 1.3.5 The structures of diamond dia (left) and lonsdaleite (hexagonal diamond)...

Figure 1.3.8 The structures of diamond (dia) (left) and lonsdaleite (hexagonal diamond, Ion) (right) showing in grey the natural tile with 10 vertices for dia (adamantane cage) with transitivity [1111] and the two natural tiles with 8 and 12 vertices for Ion with transitivity [1222].

Very small carbon crystals with the hexagonal structure of Fig. B.4 have been found in some meteorites, and this form of carbon is called lonsdaleite. 

Diamond has two basic crystal stmctures, one with a cubic symmetry (more common and stable) and the other with a hexagonal symmetry (rare but well established, found in nature as the mineral lonsdaleite). The close-packed layers, 111 for cubic and 100 for hexagonal, are identical. The cubic structure can be visualized as stacking of puckered planes of six-membered saturated carbon rmgs man EO EO sequence along (111) direction, referred to as 3C diamond (Fig. 1). All ofthe rings exhibit the chair... 

Lonsdaleite is made up of carbon atoms, just like diamonds, but the hexagonstructure makes it even heu der than the sparkling gem. 

At some point you may have heard that diamond is the hardest material known to humankind. While diamond is extremely hard, there are actually a few materials that are even harder still Years ago, some synthetically produced nanomaterials were discovered that are even harder than diamonds. Even more recently, two additional naturally occurring materials, both of which are even harder yet, were discovered. These materials are wurtzite boron nitride and a mineral called lonsdaleite. Wurtzite boron nitride has its atoms arranged in a very similar structure to the arrangement in diamond, but they are just different atoms (boron and nitrogen, rather than carbon). The other material, lonsdaleite, is actually also made from carbon atoms, but these are arranged dif-... 

Figure 2. Crystal structures of cubic diamond (left-hand side, space group Fd3m) and hexagonal lonsdaleite (space group P6i/mmc). The tetrahedral environments of the carbon atoms and the stacking sequences ABC, ABC and AB, AB, respectively, are indicated.

Yin [37], Biswas et al. [38], Fahy and Louie [39], Mailhiot and McMahan [40], Boercker [41], Crain et al. [42], Furthmuller et al. [43], and Teter [44] all investigated a dense carbon phase containing sp atoms only as in diamond, which is known from silicon and is called BC8. This and other interesting metastable structures with four-fold coordination such as ST12 [45], R8 [42], lonsdaleite, and H3 [44] have been investigated to establish theoretically the stability limit for diamond. 

Table 2. Calculated equilibrium volumes, bulk moduli, and cohesive energies of some postulated ultrahard materials. The calculated and/or experimental values of diamond, lonsdaleite, cubic BN, and PS13N4 are given for comparison. The experimental volume and density values are derived from crystal structure data. Ch. = all sp -bonded hexagonal carbon phase 126,73,74] C c, ..4 = all sp -bonded body-centered tetragonal carbon [26,72,76] BN, = all sp-bonded body centered...

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