Unit cell diamond network

The diamond structure, see Fig. 7.14 below, is a 3D network in which every atom is surrounded tetrahedrally by four neighbours. The eight atoms in the unit cell may be considered as forming two interpenetrating face-centred cubic networks. If the two networks are occupied by different atoms, the derivative cF8-ZnS (sphalerite) type structure is obtained. As a further derivative structure, the tI16-FeCuS2 type structure can be mentioned. These are all examples of a family of tetrahedral structures which have been described by Parthe (1964). 

Figure 3.1 shows a non-reconstructed ideal (lll)-(l x 1) surface of diamond with the three lattice vectors (211), (112) and (121) that form a triangular network with a unit mesh size of 0.252 x 0.252 nm" Shown also is the unit cell (shaded) with side lengths and ( /3/V ao- Superposed on this... 

VA Fig. 5.19 (a) A typical representation of the diamond lattice, (b) Reorientation of the network shown in (a) provides a representation that can be compared with the unit cell of zinc blende (Figure 5.18b) the atom labels correspond to those in diagram (a). This lattice is also adopted by Si, Ge and a-Sn. (c) The unit cell of P-cristobalite, Si02 colour code Si, purple O, red. 

Frequently, the crystal structure of ceramics is more complex than that of metals. Even an elementary ceramic, like diamond, does not crystallise in the cubic or hexagonal structure typical of metals. Because carbon in diamond is covalently bound with a valency of 4, each carbon atom has four nearest neighbours. A unit cell of the forming three-dimensional network is shown in figure 1.13. As can be seen, the structure of the diamond lattice is cubical, but it is not a Bravais lattice because it does not look the same from each atomic site. 

Diamond adopts a face-centered cubic unit cell, with each C tetrahedrally bonded to four others in an endless array. Throughout the crystal, strong single bonds make diamond the hardest natural substance known. Like most network covalent solids, diamond does not conduct electricity because the bonding electrons are localized. 

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