Problem 3 - Entrance Test

A Frenkel defect is a type of stoichiometric point defect in ionic crystals. It occurs when an ion (usually the smaller cation) leaves its normal lattice site and occupies an interstitial position within the crystal lattice. This process creates a vacancy at the original lattice site and an interstitial defect. Since no ions are truly missing from the crystal, the overall density of the crystal remains unchanged. It is typically found in compounds where there is a large difference in size between the cation and anion (cations being much smaller) and also where ions have low coordination numbers. Therefore: - Option A is incorrect because a Frenkel defect does not change the overall density of the crystal as no ions leave the crystal. - Option B is incorrect because while it does create a vacancy, it also creates an interstitial, and the 'missing' ion is just displaced, not truly gone from the crystal. This description better fits a Schottky defect if the ion is missing from the crystal. - Option C is correct as it accurately describes the mechanism of a Frenkel defect: an ion moves from its lattice site to an interstitial position. - Option D is incorrect. Frenkel defects are more common in ionic compounds where there is a *large* difference in ionic sizes, allowing the smaller ion to fit into an interstitial void. Schottky defects are more common when the sizes are similar.