Frenkel defect: smaller ion (usually cation) displaced from normal lattice site to an interstitial position. Creates a vacancy + interstitial pair. Occurs when there is a large difference in ion sizes and low CN. Examples: ZnS, AgCl, AgBr, AgI. Effect: density UNCHANGED (no atoms leave the crystal, just rearranged). AgBr is special — it shows BOTH Schottky and Frenkel defects. CsCl and NaCl do not show Frenkel defect (ions are too similar in size).