Silicon crystal structure diamond-like

Silicon will serve as the paradigmatic example of slip in covalent materials. Recall that Si adopts the diamond cubic crystal structure, and like in the case of fee materials, the relevant slip system in Si is associated with 111 planes and 110> slip directions. However, because of the fact that the diamond cubic structure is an fee lattice with a basis (or it may be thought of as two interpenetrating fee lattices), the geometric character of such slip is more complex just as we found that, in the case of intermetallics, the presence of more than one atom per unit cell enriches the sequence of possible slip mechanisms. 

Although the silicon atom has the same outer electronic structure as carbon its chemistry shows very little resemblance to that of carbon. It is true that elementary silicon has the same crystal structure as one of the forms of carbon (diamond) and that some of its simpler compounds have formulae like those of carbon compounds, but there is seldom much similarity in chemical or physical properties. Since it is more electro-positive than carbon it forms compounds with many metals which have typical alloy structures (see the silicides, p. 789) and some of these have the same structures as the corresponding borides. In fact, silicon in many ways resembles boron more closely than carbon, though the formulae of the compounds are usually quite different. Some of these resemblances are mentioned at the beginning of the next chapter. Silicides have few properties in common with carbides but many with borides, for example, the formation of extended networks of linked Si (B) atoms, though on the other hand few silicides are actually isostructural with borides because Si is appreciably larger than B and does not form some of the polyhedral complexes which are peculiar to boron and are one of the least understood features of boron chemistry. 

Silicon is a shiny, blue-gray, high-melting, brittle metalloid. It looks like a metal, but it is chemically more like a nonmetal. It is second only to oxygen in abundance in the earth s crust, about 87% of which is composed of silica (Si02) and its derivatives, the silicate minerals. The crust is 26% Si, compared with 49.5% O. Silicon does not occur free in nature. Pure silicon crystallizes with a diamond-type structure, but the Si atoms are less closely packed than C atoms. Its density is 2.4 g/cm compared with 3.51 g/cm for diamond. 

Carborundum like silicon and germanium has 4ATvalence electrons for a crystal of Adatoms. The tetrahedral diamond structure will be favoured because all 4AT electrons will then be in bonding orbitals and the energy is lower than in the higher coordination structure. 

In its crystalline state, germanium, similar to silicon, is a covalent solid that crystallizes into a diamond cubic lattice structure. Like for Si, both the (100) and (111)... 

The other complicated structures come at the ends of the groups in the periodic table, and as we have said they correspond to something more like homopolar bonds than metallic bonds. We have already commented on germanium and tin (the so-called gray modification of tin), which crystallize in the diamond structure, corresponding to the four homopolar bonds which they could form. They are of course very different from diamond in their properties, though silicon is between a... 

Subsequently, a peak in the RSL spectra, similar to the one observed by Krishnan, was not found in some crystals, such as silicon and germanium, which have the same type of structure as diamond and have even stronger anharmonicity than diamond. This encouraged Tubino and Birman (33) to improve the accuracy of the calculations of the structure of the phonon bands in crystals with a diamond-type structure. It was shown as a result of comprehensive investigations that the dispersion curve of the above-mentioned optical phonon in diamond has its highest maximum not at k = 0, but at k 0. The result of these calculations indicates that the peak experimentally observed in the RSL spectra of diamond falls within the region of the two-phonon continuum. It cannot correspond to a biphonon and is most likely related to features of the density of two-particle (dissociated) states. 

It is interesting to note in passing that many other tetrahedrally bonded crystals have a similar negative expansion coefficient at low temperatures, for example germanium (< 48 °K), silicon (materials like vitreous silica (Collins White, 1964). This behaviour is thus apparently a feature of this type of bonding structure, rather than being a peculiarity of ice itself. 

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