Which kind of defect is observed in AgBr?

Both Frenkel and Schottky defects

The correct answer is Option (3) → Both Frenkel and Schottky defect

Silver bromide shows both Frenkel and Schottky defects.

Reasoning: In silver bromide, silver ions are relatively small and mobile. Some silver ions leave their normal lattice sites and move to interstitial positions producing a Frenkel defect. At the same time, pairs of silver and bromide ions may be missing from the lattice producing a Schottky defect.

Option (1) → Vacancy defect -A vacancy defect occurs when an atom or ion is missing from its regular lattice position. This is a general type of defect where empty lattice sites are created. In ionic solids it usually appears as a pair of missing cation and anion and is known as a Schottky defect. AgBr does not show only a simple vacancy defect; therefore this option is not correct.

Option (2) → Interstitial defect - An interstitial defect occurs when an atom or ion occupies a space between the regular lattice positions.  In ionic crystals this corresponds to the Frenkel defect where a smaller ion moves from its lattice site to an interstitial position. AgBr can show this defect, but it is not the only defect present, so this option is incomplete.

Option (4) → Only Schottky defect- A Schottky defect occurs when equal numbers of cations and anions are missing from the crystal lattice. This defect is common in ionic solids such as NaCl where ions of similar size leave the lattice in pairs. However, in AgBr the silver ions are small enough to move to interstitial sites, so Frenkel defect also occurs. Hence AgBr does not show only Schottky defect.