Question

AgBr crystallizes in the NaCl lattice. how many Ag+ and Br-ions are present in the unit cell? justify your answer.

Answer 1

silver bromide ie, AgBr shows both frenkel and schottky effect. so if we talk about NaCl it is an example of rock salt structure and frenkel defect. CCP lattice of Br with atoms of Ag occupying all octahedral holes. Ag moves from octahedral to tetrahedral sites causing only cations to participate in frenkel type defect.

because in frenkel defect, ions occupy the intersatitial sites which are small enough to move.

Ag + = 12/4 + 6/2 = 6

Contribution of Corner Spheres to Unit Cell = 12 x 1/6 = 2
Contribution of Face Corner Spheres to Unit Cell = 2 x 1/2 = 1
Contribution of Spheres Inside the Unit Cell = 3 x 1 = 3
Contribution of All Types of Spheres to Unit cell = 6