Schottky Defect:
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This defect in ionic solids is equivalent to the vacancy defect in nonionic solids.
In order to maintain electrical neutrality, the number of missing cations and anions are equal
o Schottky defect also decreases the density of the substance.
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This defect is shown by ionic substances in which the cation and anion are of almost similar sizes.
In NaCl there are approximately 10 6 Schottky pairs per cm 3 at room temperature. In 1 cm 3 there are about 10 22 Thus, there is one Schottky defect per 10 16 ions.
o For example, NaCl, KCl, CsCl and AgBr. o Out of these AgBr shows both, Frenkel as well as Schottky defects.
Impurity defect
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Crystallization of molten NaCl with little amount of SrCl2 results in Sr 2 + occupying some of the sites of Na +
o Each Sr 2 + replaces two Na + ions. o It occupies the site of one ion and the other site remains vacant.
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The cationic vacancies thereby produced are equal in number to that of Sr 2 +
o Solid solution of CdCl2 and AgCl.