$word_{count}$: 200

Non-stoichiometric defects cause deviation from ideal chemical formula. Metal excess (F-centre type): heating NaCl in Na vapour — Na atoms deposit on surface, Cl- migrates to surface to form NaCl, leaving anion vacancy. Free electron trapped in vacancy = F-centre (Farbe = colour in German). F-centres absorb visible light: NaCl turns yellow, KCl turns violet, LiCl turns pink. Metal excess (interstitial type): extra cation in interstitial site plus free electron. ZnO on heating loses oxygen: $Zn^{2+}$ and electrons remain, giving yellow colour and n-type semiconductivity. Metal deficiency: cation vacancies compensated by increased oxidation state of neighbouring cations. FeO is always Fe(0.93-0.96)O; some $Fe^{2+}$ replaced by $Fe^{3+}$ to maintain neutrality. These defects explain colour, electrical conductivity, and catalytic activity of real crystals.