Diamond, hexagonal

Yagi T and Utsumi W 1993 Direot oonversion of graphite into hexagonal diamond under high pressure New Funct. Mater. C 99... 

Crystal Structure. Diamonds prepared by the direct conversion of well-crystallized graphite, at pressures of about 13 GPa (130 kbar), show certain unusual reflections in the x-ray diffraction patterns (25). They could be explained by assuming a hexagonal diamond stmcture (related to wurtzite) with a = 0.252 and c = 0.412 nm, space group P63 /mmc — Dgj with four atoms per unit cell. The calculated density would be 3.51 g/cm, the same as for ordinary cubic diamond, and the distances between nearest neighbor carbon atoms would be the same in both hexagonal and cubic diamond, 0.154 nm. 

Figure 7 shows the crystal stmctures of graphite, ordinary (cubic) diamond, and hexagonal diamond. The layers of carbon atoms He in flat sheets in graphite, but in diamond the sheets are more wrinkled and He closer together. Taken separately, the sheets are similar, but they may be stacked in various lateral positions and stiU have bonding between them. 

Fig. 7. Crystal structures of graphite, ordinary cubic diamond, and hexagonal diamond A, B, and C are the lateral positions.

The eommonest erystalline forms of earbon, cubie diamond and hexagonal graphite, are elassical examples of allotropy that are found in every chemistry textbook. Both diamond and graphite also exist in two minor crystallographie forms hexagonal diamond and rhombohedral graphite. To these must be added earbynes and Fullerenes, both of which are crystalline earbon forms. Fullerenes are sometimes referred to as the third allotrope of carbon. However, sinee Fullerenes were diseovered more recently than earbynes, they are... 

By substituting alternately the carbon atoms in cubic diamond by zinc and sulfur atoms, one obtains the structure of zinc blende (sphalerite). By the corresponding substitution in hexagonal diamond, the wurtzite structure results. As long as atoms of one element are allowed to be bonded only to atoms of the other element, binary compounds can only have a 1 1 composition. For the four bonds per atom an average of four electrons per atom are needed this condition is fulfilled if the total number of valence electrons is four times the number of atoms. Possible element combinations and examples are given in Table 12.1. 

In addition, silicon adopts a number of metastable structures that can be obtained, depending on pressure, by rapid release of the pressure from Si-II, Si-XII is formed, and from this Si-III upon heating, Si-III transforms to the hexagonal diamond structure (Si-IV). Si-III has a peculiar structure with a distorted tetrahedral coordination of its atoms. The atoms are arranged to interconnected right- and left-handed helices (Fig. 12.7). The structure being cubic, the helices run in the directions a, b as well as c. Si-VIII and Si-IX... 

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. 

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). 

Figure 36, P-T phase and reaction diagram of carbon as results from Refs. 509 and 510. Solid lines represent equilibrium phase boundaries. The dashed line is the threshold for conversion of hexagonal diamond and both hexagonal and rhombohedral graphite into cubic diamond.

Structure tP4 (CuAu) is ordered with respect to an underlying face-centred cubic lattice, so that it takes the Jensen symbol 12/12. The CuAu lattice does show, however, a small tetragonal distortion since the ordering of the copper and gold atoms on alternate (100) layers breaks the cubic symmetry. Zinc blende (cF8(ZnS)) and wurtzite (hP4(ZnS)) are ordered structures with respect to underlying cubic and hexagonal diamond lattices respectively. Since both lattices are four-fold tetrahedrally coordinated, differing only in... 

In hexagonal diamond (wurtzite structure) the wrinkled sheets are stacked in an ABABAB sequence, as shown in the Figure 7. Looking down on the stack from above, hexagonal holes can be seen formed by the six-membered carbon rings. The crystal has hexagonal symmetry about this axis, hence the name hexagonal diamond, or wurtzitic carbon. 

There is a literature (see, for example, [GrSh87a, Section 9.4], [BGOR99], [BCH02], and [BCH03]) about proper parabolic polycycles (polyhexes, polyamends, polyominoes for 6,3, 3,6), 4,4, respectively) the terms come from familiar terms hexagon, diamond, domino, where the last two correspond to the case p, ... 

The use of shock waves was imitated from nature, more specifically from meteorite craters. Among other things, unique forms of diamonds were found in there, i.e. hexagonal diamonds formed out of graphite. Furthermore the mineral stishovite was also found there. 

Cubic diamond Hexagonal diamond Diamond-like carbon... 

FIGURE 2.10 Construction of the cubic and hexagonal diamond structures based on tetrahedra of carbon atoms. 

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