Imperfections in crystals

Very few crystals are perfect. Indeed, in many cases they are not required to be, since lattice imperfections and other defects can confer some important chemical and mechanical properties on crystalline materials. Surface defects can also greatly influence the process of crystal growth. There are three main types of lattice imperfection point (zero-dimensional, line (one-dimensional) and surface (two-dimensional). 

More complex point defects can occur in ionic crystals. For example, a cation can leave its site and become relocated interstitially near a neighbouring cation. This combination of defects (a cation vacancy and an interstitial cation) is called a Frenkel imperfection. A cation vacancy combined with an anion vacancy is called a Schottky imperfection. 

A foreign atom that occupies the site of a matrix atom is called a substitutional impurity. Many types of semiconductor crystals contain controlled quantities of substitutional impurities. Germanium crystals, for example, can be grown containing minute quantities of aluminium (p-type semiconductors) or phosphorus (n-type). 

The two main types of line defect which can play an important role in the model of crystal growth are the edge and screw dislocations. Both of these are responsible for slip or shearing in crystals. Large numbers of dislocations occur in most crystals they form readily during the growth process under the influence of surface and internal stresses. 

Screw dislocations give rise to a particular mode of growth in which the attachment of growth units to the face of the dislocation results in the development of a spiral growth pattern over the crystal face (see section 6.1.2). 

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Another imperfection in crystals is called "twinning". This usualty happens when enantiomorplis are present, or possible. A good example is quartz, i.e.-... 

Note 4 The degree of crystallinity can be determined by several experimental techniques among the most commonly used are (i) X-ray diffraction, (ii) calorimetry, (iii) density measurements, and (iv) infrared spectroscopy (IR). Imperfections in crystals are not easily distinguished from the amorphous phase. Also, the various techniques may be affected to different extents by imperfections and interfacial effects. Flence, some disagreement among the results of quantitative measurements of crystallinity by different methods is frequently encountered. 

Electron donors and acceptors for reversible redox systems must invariably exhibit at least two stable oxidation states, or the net result will be an irreversible chemical reaction. The donor or acceptor components of the redox system need not be confined to independent atoms, ions, or molecules but could even be imperfections in crystal lattices capable of functioning as electron traps. The well-known color centers in alkali halides are just such acceptor systems. 

Figure 8.23 (a) The transfer of imperfections in crystals via cleavage and subsequent growth,... 

Equation (12.61) is also true for residual stresses in metal. The latter kind of stress usually will have tensile and compressive stress fields associated with it. As far as the solubility of hydrogen is concerned, the effect of the tensile stress field (which increases solubility) overwhelms the counter-effect due to the compressive stress field (which tends only to decrease the already small solubility). Therefore, the larger the lattice strain or distortion, the larger the concentration of hydrogen (Fig. 12.74). All imperfections in crystals are regions of distortion or strain. Hence, absorbed hydrogen finds its way to, and concentrates at, such imperfections. 

To explain thermal motion and temperature effects, and the diffraction consequences of disorder and imperfections in crystals (see Chapter 13). 

Figure 2.10 Imperfection in crystals (a) perfect crystal (b) substitutional impurity (c) interstitial impurity ...

The imperfections in crystals discussed so far are called point defects because they involve a single unit of the crystal structure, that is an atom or molecule. Another type of imperfection is known as a line defect or dislocation. There are two types of dislocations known as edge dislocations and screw dislocations. 

Point imperfections in crystals can be divided into three main defect categories. They... 

Rosenberg, H. M. (1978) The Solid State. Oxford Clarendon Press Vainshtein, B. K., Chernov, A. A. and Shuvalov, L. A. (1984) (eds.) Modern Crystallography, vol. 1 Symmetry of Crystals, vol. 2 Structure of Crystals, vol. 3 Crystal Growth, vol. 4 Physical Properties of Crystals. Berlin Springer Van Bueren, H. G. (1960) Imperfections in Crystals. New York Interscience Wells, A. F. (1962) Structural Inorganic Chemistry, 3rd edn. Oxford Clarendon Press Wright, J. D. (1987) Molecular Crystals. Cambridge University Press... 

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