Question

Which of the following statement(s) for crystals having Frenkel defect is/are correct?

• A. Frenkel defects are observed where the difference in sizes of cations and anions is larger.
• B. Usually alkali halides do not have Frenkel defect.
• C. $AgBr$ crystal have both Frenkel defect and Schottky defect.
• D. The density of crystals having Frenkel defect is lesser than that of a pure perfect crystal.

Answer 1

Answer: (A), (B), (C)

$AgBr$ crystal have both Frenkel defect and Schottky defect.

Frankel defect is due to the movement of a smaller ions (usually cations) from the lattice sites and ocupying the interstial sites of the crystal. This defect consists of a vacant lattice site (a “hole”) and cations occupying interstitial position.
This defect is observed in ionic compound where there is a large difference in size between the positive and negative ions.
Hence, statement (a) is correct.

Since, the ions does not leaves the crystal, hence the density of the crystal remains unchanged.
Hence, statement (b) is incorrect.

Frenkel defect are observed in ionic compound whose coordination number is usually low (4 or 6). Since, alkali halides have higher coordination numbers they do not have Frenkel defect.
Hence, statement (c) is correct.

$AgBr$ shows both, Frenkel as well as Schottky defects.
Hence, statement (d) is correct.