Question

why AgBr shows both Frenkel and Shirley defect

Answer 1

The radius ratio for AgBr is intermediate. Thus, it shows both frenkel and schottky defects.

The major defect in AgBr is the Frenkel defect. It has a rock salt structure i.e. CCP lattice of Br with atoms of Ag occupying all octahedral holes. Ag moves from octahedral to tetrahedral sites causing only cations to precipitate.

Schottky defect arise due to missing of ions from their lattice point and frenkel arise when the missing ions occupy interstitial sites.

in AgBr, Ag+ ion is small in size  and when removed from lattice point they can occupy interstitial site  and therefore show both frenkel and schottky defect. SCHOTTKY Defect in AgBr is exhibited due to precipitation of both Cations and Anions.