Crystalline solids diamond-type structur

Elemental silicon has a diamond-type structure. Crystalline silicon is a gray metallic-looking solid that melts at 1410 °C. The element is a semiconductor, as we saw in Chapters 7 and 12, and is used to make solar cells and transistors for computer chips. To be used as a semiconductor, it must be extremely pure, possessing less than 10 (1 ppb) impurities. One method of purification is to treat the element with CI2 to form SiCl4, a volatile liquid that is purified by fractional distillation and then converted back to elemental silicon by reduction with H2 ... 

Examples of three types of crystalline solids. Oniy part of the structure is shown in each case. The structures continue in three dimensions with the same patterns, (a) An atomic soiid. Each sphere represents a carbon atom in diamond, (b) An ionic soiid. The spheres represent aiternating Na+ and Cl ions in soiid sodium chioride. (c) A moiecuiar soiid. Each unit of three spheres represents an H2O moiecuie in ice. The dashed lines show the hydrogen bonding among the poiar water moiecuies. 

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 same principles that are valid for the surface of crystalline substances hold for the surface of amorphous solids. Crystals can be of the purely ionic type, e.g., NaF, or of the purely covalent type, e.g., diamond. Most substances, however, are somewhere in between these extremes [even in lithium fluoride, a slight tendency towards bond formation between cations and anions has been shown by precise determinations of the electron density distribution (/)]. Mostly, amorphous solids are found with predominantly covalent bonds. As with liquids, there is usually some close-range ordering of the atoms similar to the ordering in the corresponding crystalline structures. Obviously, this is caused by the tendency of the atoms to retain their normal electron configuration, such as the sp hybridization of silicon in silica. Here, too, transitions from crystalline to amorphous do occur. The microcrystalline forms of carbon which are structurally descended from graphite are an example. 

A phase transition occurs when a pure component changes from one phase to another. Table 6.1 lists the different types of phase transitions, most of which should already be familiar to you. There are also phase transitions between different solid forms of a chemical component, which is a characteristic called polymorphism. For example, elemental carbon exists as graphite or diamond, and the conditions for phase transitions between the two forms are well known. Solid H2O can actually exist as at least six structurally different solids, depending on the temperature and pressure. We say that water has at least six polymorphs. (In application to elements, we use the word allotrope instead of polymorph. Graphite and diamond are two allotropes of the element carbon.) In mineral form, calcium carbonate exists either as aragonite or calcite, depending on the crystalline form of the solid. 

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