Question

Why AgI is exception for rock salt(Nacl )
Type structure. Where all halides of ag are suitable for nacl type structure.

Answer 1

Dear Student,
The rock salt structure has ccp/fcc anions with octahedral sites fully occupied by cations and tetrahedral sites empty. Each cation is surrounded by six anions and
similarly, each anion is octahedrally coordinated by cations. The (NaCl6) or (ClNa6) octahedra share common edges.

All halide of Ag are stable in NaCl type structure but AgI forms ZnS type structure that is Wurtzite structure because of the size of Iodide ion The iodide ion is quite large is size hence it deviate from NaCl type structure and tend to show wurtzite type structure.
The relative size of Ag⁺ and I^- is different from relative size of Na⁺ and Cl^- hence, there is a deviation in structure.​​​​
In AgI, if there are nI^- ions, there will be nAg⁺ ions. As I^- ions form the lattice, number of tetrahedral voids = 2n. As there are nAg⁺ ions to occupy these voids, therefore fraction of tetrahedral voids occupied by Ag⁺ ions = n/2n = ½ = 50%.
Regards