Crystal Defects: Structure and Consequences

Crystal defects profoundly affect the physical properties of solids — density, colour, electrical conductivity, and magnetic behaviour — which is why NEET tests them heavily.

Schottky defects arise when thermal energy is sufficient to remove cation-anion pairs from the interior lattice, which then migrate to the crystal surface. Because the interior loses mass but the volume changes only minimally (surface ions reconstruct), density decreases. The vacancies created also allow nearby ions to hop, increasing ionic conductivity. NaCl, KCl, CsCl, and AgBr are classic examples. The condition for Schottky defect is that the ions should be similar in size (so that both cation and anion vacancy formation is energetically similar) and the coordination number should be high.

Frenkel defects require a significantly smaller cation than anion. The small cation ($Ag^{+}$, $Zn^{2+}$) can "jump" from its normal lattice site into an adjacent interstitial site, leaving the original site vacant. Because the ion remains within the crystal, total mass and volume are unchanged and density is constant. However, like Schottky, Frenkel defects create mobile species (an interstitial cation and a vacancy) that enable ionic conduction. ZnS, AgCl, AgBr, AgI all show Frenkel defects.

Non-stoichiometric defects are more subtle. Metal excess defects create F-centres — electrons trapped in anion vacancies. These loosely bound electrons absorb visible light (giving colour) and can be excited into the conduction band (n-type semiconductor behaviour). Metal deficiency defects (FeO, FeS) create cation vacancies where adjacent cations increase their oxidation state; this p-type behaviour arises because electrons can hop between different-oxidation cations, effectively moving a positive "hole."

For semiconductors, the band theory framework is essential: Group 15 dopants in Si provide extra electrons (n-type, majority carrier = electrons); Group 13 dopants create electron holes (p-type, majority carrier = holes). Unlike metals (where conductivity falls with rising temperature due to increased phonon scattering), semiconductor conductivity rises with temperature as more thermally excited charge carriers are generated.