Which of the following shows both Frenkel as well as Schottky defects?

The correct answer is Option (1) → AgBr

The type of point defect in an ionic solid depends on the relative sizes of the ions:

• Schottky defect (equal numbers of cations and anions missing) is shown when cation and anion are of similar size.

• Frenkel defect (smaller ion moved to an interstitial site) is shown when there is a large difference in size, as with the small \( \ce{Ag^{+}} \) ion.

AgBr meets both conditions to an extent, so it shows both Frenkel and Schottky defects.

Why the other options are incorrect

• Option (2) NaCl — Incorrect. NaCl is the textbook example of the Schottky defect only.

• Option (3) KCl — Incorrect. \( \ce{K^{+}} \) and \( \ce{Cl^{-}} \) are of similar size; KCl shows only the Schottky defect.

• Option (4) AgCl — Incorrect. The small \( \ce{Ag^{+}} \) ion moves into interstitial sites; AgCl shows the Frenkel defect.

Note 1: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Frenkel Defect – “This defect is shown by ionic solids. The smaller ion (usually cation) is dislocated from its normal site to an interstitial site (Fig. 1.25). It creates a vacancy defect at its original site and an interstitial defect at its new location. Frenkel defect is also called dislocation defect. It does not change the density of the solid. Frenkel defect is shown by ionic substance in which there is a large difference in the size of ions, for example, ZnS, AgCl, AgBr and AgI due to small size of Zn²⁺ and Ag⁺ ions.”

Schottky Defect – “… Schottky defect is shown by ionic substances in which the cation and anion are of almost similar sizes. For example, NaCl, KCl, CsCl and AgBr. It may be noted that AgBr shows both, Frenkel as well as Schottky defects.”

Note 2: The same/similar question has been asked in different years by NTA as per the details given below: