The Structure of Diamond

There is considerable evidence in the thermoset literature that the fracture energy decreases with increasing crosslink density, consistent with the intuitive result that crosslinking inhibits flow. In the limit of very high crosslink density, where for example we approach the structure of diamond, fracture can occur on a single crystal plane such that... 

FIGURE 5.21 The structure of diamond, Each sphere represents the location of the center of a carbon atom. Each atom is at the center of a tetrahedron formed hy the sp1 hybrid covalent bonds to each of its four neighbors. 

The structures of diamond, silicon, germanium and tin are discussed in Chapter 12. 

FIGU RE 13.12 The structure of diamond showing four covalent bonds to each carbon atom. 

Two later sections (1.6.5 and 1.6.6) look at the crystalline structures of covalently bonded species. First, extended covalent arrays are investigated, such as the structure of diamond—one of the forms of elemental carbon—where each atom forms strong covalent bonds to the surrounding atoms, forming an infinite three-dimensional network of localized bonds throughout the crystal. Second, we look at molecular crystals, which are formed from small, individual, covalently-bonded molecules. These molecules are held together in the crystal by weak forces known collectively as van der Waals forces. These forces arise due to interactions between dipole moments in the molecules. Molecules that possess a permanent dipole can interact with one another (dipole-dipole interaction) and with ions (charge-dipole interaction). Molecules that do not possess a dipole also interact with each other because transient dipoles arise due to the movement of electrons, and these in turn induce dipoles in adjacent molecules. The net result is a weak attractive force known as the London dispersion force, which falls off very quickly with distance. 

Fig. 7.1 Unit cell of the structure of diamond (carbon). Note the tetrahedral (rp3) configuration about each atom. Cf. Fig. 4.2b. [From Ladd. M. F. C. Structure and Bonding in SoUd State Chemistry Ellis Horwood Chicester, 1979. Reproduced with permission.]...

The structure of diamond, a covalent crystal, is shown in Fig. 7.1. How is it related to tome cf the structures cf ionic compounds discussed in this chapter1 ... 

The classical valency concept of the tetrahedral carbon atom has been more than fully verified by its success in explaining the chemistry of countless thousands of organic compounds. The first direct physical confirmation of the tetrahedral distribution of carbon valency bonds, however, came with the elucidation of the structure of diamond by W. H. and W. L. Bragg (1913) using the newly discovered method of X-ray diffraction. 

The shapes of covalent molecules are determined by the number of valence electrons and orbitals available, giving H2, BF3, CH4, NH3, PH3, H20, HF, C1F, PF5, SF6, and IF7. Carbon has four valence electrons and four orbitals, so tetrahedral sp3 bonding dominates the chemistry of carbon. This matches the three-dimensional bonding in zinc blende (3 2PT), and it is the structure of diamond with carbon in P and T layers. 

The structure of diamond is like the zinc blende structure except that all of the atoms are carbon. Calculate the lattice parameter of diamond if the atomic diameter of carbon is 0.92 nm. 

The FAU framework structure is composed of sodalite cages, linked by 6-6 secondary building units (SBUs) like the carbon atom in the structure of diamond. The formed cube has an axis, a 24.3 A, and the framework produced by the union of these cubes contains windows, which lead to an approximately spherical cavity with a radius, R 6.9 A, known as the supercage or (1-cagc [108,109], These supercages have tetrahedral symmetry, and are opened through four 12-member ring (MR) windows, each with a diameter of d 1A A [108,109],... 

The structure of diamond consists of a continuous tetrahedral network of sp hybridized carbon atoms, thus creating an infinite array of chair cyclohexane rings (without the hydrogen atoms). The structure of graphite, on the other hand, consists of parallel sheets of sp hybridized carbon atoms, thus creating an infinite array of benzene rings. The parallel sheets are then held together by weak intermolecular interactions. 

How does the difference in the structures of diamond and graphite manifest itself in the physical properties of these substances ... 

FIGURE 7-6 The Structure of Diamond, (a) Subdivision of the unit cell, with atoms in alternating smaller cubes, (b) The tetrahedral coordination of carbon is shown for the four interior atoms. 

This is the basis of the structure of diamond, which is made up of carbon atoms in tetrahedral units covalently bonded together into what is, in effect, a huge molecule. 

Mohr, E. Die Baeyersche Spannungstheorie und die Struktur des Diamants. [Baeyer s tension theory and the structure of diamond.] Ckemisches Zentralblait 2, 1065 (1915). 

Relate the structures of diamond, graphite, and other allotropes of carbon to their properties. 

The canonical m.o.s of diamond are delocalized over the entire crystal. However, as we have seen in Chapter 8 for other systems, the occupied m.o.s can be the object of a unitary transformation leading to a set of equivalent and quasi-localized molecular orbitals . This is why the structure of diamond can (for some purposes) be described in terms of the overlap of sp hybrid orbitals, four for each C atom. As we have seen in Chapter 8, we must stress that such an alternative description cannot be used to infer information about electron energies. In particular, the localized bond description of the structure of diamond does not imply that all valence electrons have the same energy. This would be the case only if the sp -sp bonds were independent. It is because of residual interactions such as f and (3"... 

FIGURE 21.22 The structure of diamond. Each carbon atom has four nearest neighbors surrounding it at the corners of a tetrahedron. 

Silicon carbide, or carborundum, SiC, is one of the hardest substances known and is used as an abrasive. It has the structure of diamond with half of the carbons replaced by silicon. It is prepared industrially by reduction of sand (Si02) with carbon in an electric furnace. 

The diffraction of x-rays by crystals was discovered in 1912, and in 1913 the first determinations of the atomic arrangement in crystals were made by use of this technique by the British physicists W. H. Bragg and W. L. Bragg (father and son). Their work during this first year included the determination of the structure of diamond, as shown in the adjacent drawing. 

Another allotrope of elemental carbon is diamond. Besides being blinded by the brilliance of a cut diamond, you should know that diamond is the hardest natural substance. It s often used on the tips of cutting tools and drills. Can the structure of diamond explain its hardness Look at the model of diamond. Every carbon atom is attached to four other carbon atoms which, in turn, are each attached to four more carbon atoms. Diamond is one of the most organized of all substances. In fact, every diamond is one huge molecule of carbon atoms. This organization of covalently bonded carbons throughout diamond accounts for its hardness. If you tried to write... 

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].

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