Colloid formation in irradiated insulators

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Abstract
Irradiation of ionic crystals causes the displacement of lattice ions and the formation of primary defects in the form of vacancies and interstitials. At high temperatures when these defects are mobile secondary defect reactions will produce various types of defect clusters. In some compounds clustering can lead to the formation of small particles of the metal constituent, referred to as colloids. A well-known example of this effect occurs in the alkali halides, where the colloids form as the result of large-scale aggregation of the primary F centres, so that the metallic region in this case derives from primary defects on the anion sublattice. The latent image of the photographic process in silver halides is also an example of the formation of a small metal colloid, and other crystals such as hydrides and azides can also be partially decomposed into metallic particles by irradiation with ionizing radiation. Recently metal colloids have been found as a result of displacement damage in the oxides Li2 and Al2O3. This article reviews some of the background properties of colloids in ionic crystals and describes some examples of colloid formation by irradiation. Colloid growth in NaCl is described in more detail, since recent experimental and theoretical work provides a more complete picture than in other compounds. The Jain-Lidiard theory explains many features of the behaviour observed during high dose irradiation at high temperatures, and some comments are made about ways in which the theory could be developed further.