Schottky and Frenkel defects
/ SHOT-key, FRENK-el /
The neat diagrams of crystal structures show perfect, endless order — but no real crystal above absolute zero is perfect. Thermal energy means that some ions are always missing from where they should be, or sitting where they should not. These small imperfections are point defects, and the two most important intrinsic kinds in ionic solids are named after their discoverers: Schottky and Frenkel.
A Schottky defect is a matched pair of vacancies: one cation and one anion are both missing from their proper sites, leaving two holes in the lattice. They come in pairs because the crystal must stay electrically neutral overall — you cannot remove a positive ion without also removing a negative one. A Frenkel defect is different: an ion (usually the smaller one, often the cation) leaves its normal site and squeezes into an interstitial hole nearby. No ion is lost, just displaced, so neutrality is automatically preserved. Schottky defects lower the crystal's density slightly (atoms are genuinely missing); Frenkel defects do not (the atom is still there, just relocated). Which kind dominates depends on the structure: Frenkel defects favour solids with a big size difference between ions and roomy interstitial holes (silver halides), Schottky defects favour solids with similar-sized ions and tight packing (alkali halides like NaCl).
Far from being mere flaws, point defects make ionic solids do useful things. They are the reason ionic solids can conduct electricity at all (an ion can hop into a neighbouring vacancy, and then another ion into the vacancy it left, so charge creeps through the lattice) — the basis of solid electrolytes in batteries and fuel cells. They underlie the light-sensitivity of silver halides in photographic film (a Frenkel defect of silver), and they are the gateway to nonstoichiometry and to colour centres. The deeper lesson is thermodynamic: a perfect crystal is not even the most stable state above absolute zero, because creating some disorder raises the entropy enough to lower the free energy.
Silver bromide, AgBr, in photographic film shows Frenkel defects: small Ag+ ions slip out of their lattice sites into interstitial holes. The mobile silver ions and the vacancies they leave behind let light trigger the formation of tiny silver specks — the latent image that develops into a photograph.
Frenkel defects of Ag+ in silver halides make photographic film light-sensitive.
Both defects keep the crystal electrically neutral and stoichiometric — Schottky by removing a cation-anion pair, Frenkel by merely moving an ion. They are intrinsic and increase with temperature; do not confuse them with the composition-changing nonstoichiometry.