Question

Which type of stoichiometric defect is shown by:
$(i){\text{ Zns (ii) AgBr}}$

Answer 1

Hint – In this question use the concept that in ZnS the size of Zn is different from that of S and when anion is much larger than the cation then it leads to smaller ions leaving its place in the lattice. In AgBr use the concept that $A{g^ + }$ ion is smaller in size and when removed from lattice point they can occupy interstitial sites. Use this concept to comment upon the stoichiometric defects in both the compounds.

Complete answer:
$\left( i \right){\text{ }}ZnS$ shows Frenkel defect.
A Frenkel defect or dislocation defect is a type of point defect in crystalline solids named after its discoverer Yakov Frenkel. The defect forms when an atom or smaller ion leaves its place in the lattice, creating a vacancy, and becomes an interstitial by lodging in a nearby location.
$\left( {ii} \right){\text{ }}AgBr$ shows Frenkel as well as Schottky defect.
A Schottky defect is an excitation of the site occupations in a crystal lattice leading to point defects named after Walter H. Schottky. In elemental crystals it refers to simple lattice vacancies, also called valency defects.
So this is the required answer.

Note – Stoichiometric defects are those defects that do not affect the overall stoichiometry of the compound. That is the ratio of numbers of positive and negative ions remains unaltered. The trick part about stoichiometric defects is that it is generally seen in compounds with equal number of positive and negative ions.