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Point defects are localised irregularities in the crystal lattice. Schottky defect: equal number of cation and anion vacancies (to maintain charge neutrality). Found in ionic compounds with high CN and similar-sized ions (NaCl, KCl, CsCl, AgBr). Effect: density decreases (fewer atoms, same volume). Frenkel defect: smaller ion (usually cation) moves to an interstitial site, creating vacancy-interstitial pair. Found when ions have large size difference and low CN (ZnS, AgCl, AgBr, AgI). Effect: density unchanged (same number of atoms, same volume). AgBr uniquely shows both defects. Key distinction: Schottky = missing atoms, Frenkel = displaced atoms. Number of defects increases exponentially with temperature: n = N exp(-E/2kT). Consequences: ionic conductivity increases (vacancies allow ion migration), diffusion occurs through vacancy mechanism. In Schottky defect, the crystal "swells" slightly. In Frenkel defect, no volume change occurs.