Explanation:
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Frenkel defect, first discovered by and named by Yakov Frenkel is a type of point defect found in crystalline solids.
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This defect forms when a smaller atom or ion, generally cation, leaves its usual lattice position and occupies an interstitial position instead.
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By leaving its position, it creates a vacancy at its lattice point.

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Since Frenkel defect only involves the migration of ions within the lattice, the density and volume remains unchanged (mostly).
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This defect is usually found in those ionic crystals in which there is a large size difference between the anion and the cation.
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Some examples of solids showing Frenkel defect are zinc sulfide, silver chloride, etc.
Final Answer:
Frenkel defect is a point defect found in crystalline solids due to migration of smaller ion from its position to interstitial position.