Defects: Schottky, Frenkel, F-centres
Solid State · Chemistry
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NaCl heated in sodium vapour turns yellow. Explain via F-centres. Step 1: Na atoms deposit on crystal; Cl⁻ leaves as... actually Na⁺ + e⁻ enter. Step 2: Some Cl⁻ sites become vacant; electrons get trapped there. Step 3: Trapped electron absorbs visible light (yellow) — an F-centre (Farbe = colour). Step 4: The crystal is now non-stoichiometric: excess Na. Defects paint crystals: F-centres make KCl violet, ZnO yellow when hot.