a Frenkel defect
/ FRENK-el /
A Frenkel defect is the other honest way an ionic crystal makes point defects without upsetting its charge balance: instead of removing an ion, it simply MOVES one. An ion leaves its normal site and squeezes into an interstitial gap nearby, leaving behind an empty site. Now there is a vacancy (the site it left) and an interstitial (the crowded gap it moved into) — but the ion is still inside the crystal, so no charge has been added or taken away. The whole thing is a self-balancing shuffle.
Because the ion just relocates rather than leaving, a Frenkel defect does not change the crystal's density or formula at all — it is a vacancy and an interstitial of the SAME ion, created as a pair. Which ion moves depends on which one fits the gaps: usually the smaller ion, most often the cation. Silver chloride (AgCl) is the classic case — the small Ag+ ion readily hops off its site into an interstitial position, giving a silver vacancy plus a silver interstitial. Anion Frenkel defects also occur where the anion is the mobile one, as in fluorite-structured CaF2, where the fluoride ions slip into interstitial sites.
The choice between Schottky and Frenkel is basically a size question: if the ions are similar in size and the interstitial gaps are tight, it is cheaper to make vacancy pairs (Schottky); if one ion is small enough to slip into a gap, it is cheaper to displace it (Frenkel). Frenkel defects are what make silver halides such good ionic conductors and underpin the light-sensitivity of photographic film, where mobile interstitial silver ions gather to form the latent image. As with all intrinsic defects, their equilibrium number climbs with temperature by a Boltzmann law.
In silver bromide, the basis of black-and-white photographic film, light frees electrons that combine with mobile interstitial Ag+ ions (Frenkel interstitials) to deposit tiny specks of metallic silver — the latent image later amplified by development. The whole process rests on the Frenkel defect's gift of mobile silver ions.
A Frenkel defect displaces an ion into an interstitial site, creating a vacancy plus interstitial of the same ion.
A Frenkel defect leaves the crystal's density and formula unchanged (nothing left), whereas a Schottky defect slightly lowers density (atoms removed). Which one dominates a given crystal is set by ion sizes and the tightness of its interstitial gaps, not by preference.