Q.What are point defects ? Explain the non-stoichiometric point defects in ionic crystals.

Point defects are localised lattice irregularities; non-stoichiometric defects specifically arise when the ratio of cations to anions departs from the ideal formula, via metal-excess or metal-deficiency mechanisms.

Point defects: irregularities or deviations from the ideal, perfectly ordered arrangement of constituent particles (atoms, ions or molecules) around a point or an atom in a crystalline solid. They can arise due to atoms/ions missing from their normal lattice sites, occupying interstitial sites, or being incorrectly placed. Point defects are broadly classified as stoichiometric defects (Frenkel and Schottky defects, which do not change the overall chemical composition), impurity defects, and non-stoichiometric defects.

Non-stoichiometric point defects: In these defects, the ratio of cations to anions becomes DIFFERENT from that indicated by the ideal chemical formula of the compound, though the crystal as a whole remains electrically neutral. They are of two main kinds:

1. Metal excess defect: there is an excess of the metal (cation) relative to the anion.
   1. Metal excess due to anionic vacancies: an anion is absent from its regular lattice site, and the vacant site is occupied by an electron to maintain overall electrical neutrality. Such an electron-occupied hole is called an F-centre and is responsible for imparting colour to the crystal - e.g., NaCl heated in sodium vapour develops F-centres and turns yellow.
   2. Metal excess due to extra cations at interstitial sites: e.g., when ZnO is heated it loses oxygen and turns yellow; the excess Zn²⁺ ions formed move to interstitial sites, with an equal number of electrons trapped nearby in neighbouring interstitial sites to preserve neutrality - this gives the crystal n-type semiconducting behaviour. …