why does AgBr show both frenkel n schottky defect? and why does ONLY AgBr show both d defects?
Schottky defect arise due to missing of ions from their lattice point and Frenkel defect arise when the missing ions occupy interstitial sites.
The radius of AgBr is intermediate hence it shows both Frenkel and Schottky defects.
The major defect in AgBr is Frenkel defect. It has ccp lattice, where atoms of Br with atoms of Ag occupies all octahedral holes. Ag moves from octahedral to tetrahedral sites causing only cations to precipitate.
Schottky defect in AgBr is exhibited due to precipitation of both cations and anions.
In AgBr Ag+ ion is small in size and when removed from lattice point they can occupy interstitial sites and hence show both Frenkel and Schottky defects.
Lets understand What is Schotty defect and Frenkel defect:-
- Schottky defect arises due to departure of ions (both cation and anion in equal ratio) from crystal lattice leaving holes that favour conductivity. The equal no of ions depart because to maintain the electrical neutrality of crystal.
- Frenkel defect ions(cations) are missing from their normal lattice site and occupying an interstitial site between the lattice points. They don’t depart from crystal lattice.
In AgBr, the Ag + ions (and corresponding Br - ions) are absent from the crystal lattice causing Schottky defects. However, Ag +ions are exceptionally mobile and they have a tendency to move about inside the lattice. So they complement the Schottky defects with Frenkel defects with the Ag+ ions trapped in interstitial spaces. This is because of the similar size of Ag + ions and Br - ions and the small size of Ag + ions and the fact that Ag + ions have a property to be mobile in the structure.