Defects and Amorphous Solids

Although real crystals display beautiful symmetries to the eye, they are not perfect. As a practical matter, it is impossible to rid a crystal of all impurities or to ensure that it contains perfect periodic ordering. So, we describe real crystals as perfect crystals with defects, and define means to characterize these defects. If so many defects are present that crystalline order is destroyed, we describe the material as an amorphous solid. 

Point defects in a pure crystalline substance include vacancies, in which atoms are missing from lattice sites, and interstitials, in which atoms are inserted in sites different from their normal sites. In real crystals, a small fraction of the normal atom sites remain unoccupied. Such vacancies are called Schottky defects, and their concentration depends on temperature  

FIGURE 21.28 Point imperfections in a iattice. The red Xs denote vacancies. 

If an alkali halide crystal such as NaCl is irradiated with x-rays, ultraviolet radiation, or high-energy electrons, some CP ions may lose an electron  

As Chapter 1 emphasizes, the law of definite proportions was one of the principal pieces of evidence that led to the acceptance of Dalton s atomic theory. It is now recognized that a great many solid-state binary compounds do not have fixed and unvarying compositions but exist over a range of compositions in a single phase. Thus, FeO (wiistite) has the composition range Feo.ssOi.oo to Feo.ssOi.oo and is never found with its nominal 1 1 composition. The compounds NiO and CuiS also deviate considerably from their nominal stoichiometries. 

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Up to thi.s point we have focused almost entirely on the bulk properties of covalent crystals. Any real crystal has surfaces, but for large systems, say 10 atoms long, there are of the order of 10 atoms within the interior and only some 10 on the surface, so that many properties arc dominated by the interior. On the other hand, processes such as electron diffraction or catalysis are dominated by surface effects, and it is important that we include some discussion of them. As with other topics, this will necessarily be a cursory view, indicating some central concepts. There is a regular journal, Surface Science, exclusively devoted to current developments in the subject. We shall also discuss briefly in this chapter some related concepts concerning crystalline defects and amorphous solids. 

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