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What are stiochiometric defect and explin ?

The stoichiometric defects (Schottky and Frenkel defects) in ionic crystals are formed due to irregularities in ionic arrangement all the while maintaining the stoichiometry and electric neutrality. It is a fact that AgBr shows both Frenkel and Schottky defects while most other compounds show only one of these defects. AgCl shows only Frenkel defects.

The ratio of the sizes of anion and the cation determines which type of defect is shown by the crystal. If the ratio is closer to one, it shows Schottky defects and if it deviates significantly from one, it shows Frenkel defects. Intuitively, this is correct since to show Frenkel defects, the cation (or the smaller ion) has to move in the interstitial space which is indeed easy if the ion is very small as compared to the other ion and hence is easily mobile. The activation energy for cationic movement is less.And if the ionic size are comparable, filling the interstices is not feasible, and hence Schottky defects can only be exhibited by the crystal.