Question

Which of the following statements for crystals having Frenkel defect is 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. All of the above

Answer 1

The correct option is D All of the above

Frenkel 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.

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 (b) is correct.

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