Allotropes of silicon

The crystal structure of silicon is similar to that of diamond however, the Si—Si bonds (226 kJ/mol) are weaker than the C—C bonds (356 kJ/mol), and silicon is not nearly as hard. There is no graphitic allotrope of silicon. Crystalline silicon is a blue-gray, somewhat shiny, brittle element that certainly appears metallic however, it is classified as a nonmetal or metalloid because it is a semiconductor that is, at low temperatures it is an insulator. However, when heated sufficiently, its electrical conductivity increases markedly. Very pure silicon for transistors is produced by reducing silicon tetrachloride prepared by... 

Interestingly, zinc sulfide (p-ZnS) may also crystallize in a cubic lattice, which consists of a fee array of S , with Zn occupying 1/2 of the available tetrahedral sites. This structure is known as sphalerite or zincblende, and is shared with other compounds such as a-AgI, p-BN, CuBr, and p-CdS. When the same atom occupies both the fee and tetrahedral interstitials of the sphalerite structure, it is described as the diamond lattice, shared with elemental forms (allotropes) of silicon, germanium, and tin, as well as alloys thereof. Important semiconductors such as GaAs, p-SiC, and InSb also adopt the sphalerite crystal structure. 

Figure 5.14 Carbene-stabilized allotropes of silicon, germanium, and tin.

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