A Frenkel defect involves which pair?

• A. A cation-vacancy and a cation-interstitial pair
• B. Two cation vacancies
• C. Anion-vacancy and anion-interstitial
• D. A neutral defect cluster

Answer: (A)

A Frenkel defect is a point defect formed when an ion leaves its regular lattice site and moves into a neighboring interstitial site, creating a vacancy and an interstitial of the same species. This arrangement keeps the overall composition and charge balance of the crystal intact.

In the common case for ionic crystals, the moving ion is a cation, so the defect pair is a cation vacancy together with a cation interstitial. This combination exactly matches the described pair and is the classic Frenkel defect.

Two cation vacancies would leave no interstitial counterpart and would alter stoichiometry unless compensated elsewhere, so that doesn’t describe a Frenkel defect. An anion vacancy paired with an anion interstitial is a Frenkel defect for the anion species, which is a related but distinct case and not the standard example often tested. A neutral defect cluster is too vague to capture the specific interstitial–vacancy pairing that defines a Frenkel defect.