Defects and Non-Stoichiometry

In a perfect crystal, all atoms would be on their correct lattice positions in the structure. This situation can only exist at the absolute zero of temperature, 0 K. Above 0 K, defects occur in the structure. These defects may be extended defects such as dislocations. The strength of a material depends very much on the presence (or absence) of extended defects, such as dislocations and grain boundaries, but the discussion of this type of phenomenon lies very much in the realm of materials science and will not be discussed in this book. Defects can also occur at isolated atomic positions these are known as point defects, and can be due to the presence of a foreign atom at a particular site or to a vacancy where normally one would expect an atom. Point defects can have significant effects on the chemical and physical properties of the solid. The beautiful colours of many gemstones are due to impurity atoms in the crystal structure. Ionic solids are able to conduct electricity by a mechanism which is due to the movement of fo/ 5 through vacant ion sites within the lattice. (This is in contrast to the electronic conductivity that we explored in the previous chapter, which depends on the movement of electrons.) 

Defects fall into two main categories intrinsic defects which are integral to the crystal in question—they do not change the overall composition and because of this are also known as stoichiometric defects and extrinsic defects which are created when a foreign atom is inserted into the lattice. 

Intrinsic defects fall into two categories Schottky defects which consist of vacancies in the lattice, and Frenkel defects, where a vacancy is created by an atom or ion moving into an interstitial position. 

FIGURE 5.1 Schematic illustration of intrinsic point defects in a crystal of composition MX (a) Schottky pair, (b) perfect crystal, and (c) Frenkel pair. 

It is less common to observe an anion Frenkel defect when an anion moves into an interstitial site. This is because anions are commonly larger than the cations in the structure and so it is more difficult for them to enter a crowded low-coordination interstitial site. 

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Smith D. M. and Stocker R. L. (1975). Point defects and non-stoichiometry in forsterite. Phys. Earth. Planet Int, 10 183-192. 

Greenwood, N.N. (1968) Ionic Crystals, Lattice Defects and Non-stoichiometry, Bntterworths. 

Chapter 1 deals with classical non-stoichiometric compounds. By classical, the author means that the basic concept of the phase stability has been well established from a thermodynamical point of view, and does not mean that research in this field has been fully completed. In these compounds the origin of non-stoichiometry is point defects . In the first half of the chapter, the fundamental relation between point defects and non-stoichiometry is described in detail, based on (statistical) thermodynamics, and in the second half various examples, referred to the original papers, are shown. 

N. N. Greenwood, Ionic crystals, lattice defects and non-stoichiometry, Butterworth, London, 1968. 

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