POINT DEFECT STABILIZATION IN IONIC CRYSTALS AT HIGH DEFECT CONCENTRATIONS

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Abstract
Stabilization of point defects in ionic crystals differs from that in metals or other monatomic solids in that there is the need to maintain both stoichiometry (or accommodate non-stoichiometry) and order. Primary point defects in these solids, whether produced thermally, chemically, or by irradiation, seldom are present or aggregate in exactly stoichiometric proportions, and in consequence at high defect concentrations extended secondary defect structures arise which can be quite distinct from those formed in monatomic solids. Non-stoichiometry can be accommodated in several ways. In oxygen-deficient transition metal oxides, oxygen vacancies can be effectively eliminated by alteration of the mode of linking metal-oxygen octahedra together, giving rise to so-called crystallographic shear planes. This process effectively delocalizes a change in cation valency. In cation deficient/anion excess systems (e. g. Fe1-xO, UO2+x, CaF2 : YF3), accommodation occurs by a localized change in cation valency (and often position) and formation of point defect cluster complexes (cation vacancies, anion interstitials) which when stacked together can form embryonic nuclei for a new phase of altered stoichiometry. Where valency changes are not possible, for example in alkali halides or undoped alkaline earth halides and oxides, stoichiometric extended defects (such as dislocation loops) are generated, but these must be accompanied by other more stable forms of point defect species. Aggregation of point defects at high density can additionally result in precipitation of separate elemental phases, and thus decomposition of the solid

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