Point defects are imperfections in the crystal lattice at specific atomic sites. Stoichiometric defects maintain the overall chemical formula.

Schottky Defect

• Equal numbers of cation and anion vacancies created
• Ion pairs migrate from interior to surface
• Density decreases (mass decreases, volume unchanged)
• Stoichiometry maintained (equal cation and anion vacancies)
• Found in: compounds with high CN, similar-sized ions
• Examples: NaCl, KCl, CsCl, AgBr

Consequence: Slightly increased ionic conductivity (vacancy mechanism — ions hop into vacancies)

Frenkel Defect

• A cation displaced from lattice site to an interstitial site
• No ions leave the crystal
• Density unchanged (mass and volume both unchanged)
• Stoichiometry maintained
• Found in: compounds with large size difference (small cation, large anion)
• Examples: ZnS, AgCl, AgBr, AgI

Consequence: Increased ionic conductivity (interstitial mechanism — cation moves from interstitial to interstitial)

AgBr Exception

AgBr shows BOTH Schottky and Frenkel defects — a critical NEET fact.

• $Ag^{+}$ is small enough to displace into interstitials (Frenkel)
• $Ag^{+}$ and $Br^{-}$ can also form vacancy pairs (Schottky)
• This dual behaviour makes AgBr uniquely useful in photography

Comparison Table