Types of Point Defects

Point defects are of three types: (i) stoichiometric, (ii) impurity and (iii) non-stoichiometric defects.

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(a) Stoichiometric defects

These do not disturb the stoichiometry of the solid, and are also called intrinsic or thermodynamic defects. In non-ionic solids they take two forms:

• Vacancy defect — some lattice sites are empty (Fig. 1.27). This decreases the density. It can also develop on heating.
• Interstitial defect — some particles occupy interstitial sites (spaces between normal sites) (Fig. 1.28). This increases the density.

Ionic solids must stay electrically neutral, so instead of simple vacancy/interstitial defects they show:

• Frenkel defect — a smaller ion (usually the cation) leaves its normal site for an interstitial site (Fig. 1.29), creating a vacancy at the original site and an interstitial defect at the new one. Also called a dislocation defect, it does not change the density. It occurs when there is a large size difference between the ions — for example ZnS, AgCl, AgBr and AgI (small Zn$^{2+}$ and Ag$^+$).
• Schottky defect — essentially a vacancy defect in which equal numbers of cations and anions are missing, preserving neutrality (Fig. 1.30). It decreases the density. It is common when cation and anion are of similar size — for example NaCl, KCl, CsCl and AgBr. In NaCl there are about $10^{6}$ Schottky pairs per cm$^3$ at room temperature, against about $10^{22}$ ions per cm$^3$ — roughly one Schottky defect per $10^{16}$ ions. (Note: AgBr shows both Frenkel and Schottky defects.)

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(b) Impurity defects

If molten NaCl containing a little SrCl$_2$ is crystallised, some Na$^+$ sites are taken by Sr$^{2+}$ ions (Fig. 1.31). Each Sr$^{2+}$ replaces two Na$^+$ — it occupies one site while the other Na$^+$ site is left vacant. So the number of cationic vacancies equals the number of Sr$^{2+}$ ions. The solid solution of CdCl$_2$ and AgCl behaves similarly.

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(c) Non-stoichiometric defects

Many inorganic solids contain their elements in a non-stoichiometric ratio because of crystal defects. These are of two types.

(i) Metal excess defect

• Due to anionic vacancies: alkali halides such as NaCl and KCl show this. Heating NaCl in sodium vapour deposits Na atoms on the surface; Cl$^-$ ions diffuse to the surface and combine with them (each Na atom losing an electron to form Na$^+$), and the released electrons occupy the vacant anionic sites (Fig. 1.32). The crystal now has excess sodium. These anion sites occupied by unpaired electrons are called F-centres (from the German Farbenzentrum, colour centre) and give NaCl a yellow colour — the trapped electrons absorb visible light and get excited. Similarly, excess lithium turns LiCl pink and excess potassium turns KCl violet (lilac).
• Due to extra cations at interstitial sites: zinc oxide is white at room temperature but turns yellow on heating as it loses oxygen: …