Diamond-type network

The zinc blende (ZnS) lattice a diamond-type network... 

This section draws attention to some of the common structure types adopted by semiconductors. The diamond-type network (often referred to an adamantine solid structure) is adopted by Si and Ge the addition of dopants occurs without structural change. Related to this network is the zinc blende lattice and among compounds adopting this structure are GaAs, InAs, GaP, ZnSe, ZnTe, CdS, CdSe,... 

An alternative description is that of a diamond-type network. Figure 6.20a gives a representation of the structure of diamond. Each C atom is tetrahedrally sited and the structure is very rigid. This structure type is also adopted by Si, Ge and ot-Sn (grey tin). Figure 6.20b (with atom... 

C is equal to unity when each nodule has six first neighbors, by analogy with a simple cubic lattice29. Similarly Cis of the order of 0.87,1.09,1.12 when the geometric functionality is of the order of 4,8,12, respectively these values originate from calculations carried out on diamond-type, centered, and face-centered cubic lattices, which exhibit precisely these geometric functionalities (or coordination indices). In any case, Cis a constant for a given network, and its value is never very far from unity. 

Several nonmetallic elements and metalloids have a network covalent structure. The most important of these is carbon, which has two different crystalline forms of the network covalent type. Both graphite and diamond have high melting points, above 3500°C. However, the bonding patterns in the two solids are quite different... 

The most typical example of a network solid is diamond. In diamond each carbon atom is covalently bonded to four other carbon atoms forming a tetrahedral shape. (The type of hybridization that corresponds to this tetrahedral structure is sp3) This structure is extremely strong and this makes diamond the hardest natural substance. 

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). 

The hardest network atomic solid—in fact, the hardest (currently) known material on the planet—is a type of carbon that forms diamonds. The covalently bonded arrangement of carbon atoms within diamonds forms naturally at intense temperatures and pressures inside the Earth. 

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