Non-stoichiometric defects change the ratio of cations to anions in the crystal.

Metal Excess Defect (n-type behaviour)

Type A — Anionic vacancies (F-centres):

• Crystal heated in metal vapour → metal atoms deposit on surface → anion ions migrate to surface → anion vacancies created in interior → electrons from metal ionisation fill these vacancies → F-centres (Farbe = colour in German)

Reaction for NaCl: Na(g) → $Na^{+}$ (surface) + $e^{-}$; $e^{-}$ fills $Cl^{-}$ vacancy → F-centre

• Colour: NaCl → yellow; KCl → violet; LiCl → pink (F-centre absorbs specific wavelengths)
• Electrical behaviour: n-type semiconductor (F-centre electrons are loosely bound, easily excited to conduction band)

Type B — Interstitial cations:

• Extra metal cations occupy interstitial sites; electrons balance charge

Metal Deficiency Defect (p-type behaviour)

• Found in transition metal compounds
• A cation leaves its lattice site (creates vacancy); adjacent cation oxidises to higher state to maintain charge balance
• Results in non-stoichiometric formula

Example: FeO (ideal) → $Fe_{1}$₋ₓO (real) $Fe^{2+}$ vacancy created; 2 $Fe^{2+}$ → 2 $Fe^{3+}$ to compensate (net: 3 $Fe^{2+}$ replaced by 2 $Fe^{3+}$ + 1 vacancy) Electrical behaviour: p-type semiconductor (holes = higher-charge sites that electrons hop into)

Examples: FeO, FeS, NiO, $Cu_{2}O$ (metal deficiency); NaCl heated in Na vapour (metal excess)