Question

Describe the various types of non-stoichiometric defects found in ionic crystals.

Answer 1

1. Metal excess defects due to anion vacancies: This type of defect arises when an anion is missing from its lattice site, creating a hole, and the hole is occupied by an electron to maintain the electrical neutrality of the crystal.
   
   These kinds of flaws are frequently found in crystals that are likely to have Schottky faults, such as crystals of alkali halides. Anion vacancies are created when alkali metal halides are heated in an alkali metal vapour environment.
2. Metal excess defects due to interstitial cations: This type of defect arises when an extra positive ion is present in an interstitial site. The electrical neutrality is maintained by the presence of an electron in some other interstitial site.
   
3. Metal deficiency defects due to cation vacancies: This defect arises when a positive ion (metal ion) is missing from its normal lattice site and the charge is balanced by a nearby metal ion having a higher positive charge (i.e., in the higher oxidation state).