Question

Explain why cdcl2 will induce Schottky defect if added to Agcl crystal

Answer 1

A really useful tool for understanding this sort of reaction is Kröger-Vink notation, in which an atom is described not just by its charge and state but by which sort of lattice site it sits on.
For example, dissolved Ag++ and Cl−− ions precipitating onto the surface of an AgCl crystal do so according to the reaction
Ag++(aq) + Cl−−(aq) →→ AgAgAg + ClClCl
That is, the Ag sits on the Ag sublattice and the Cl sublattice. Since the charge on each ion matches the charge expected at that lattice site, we don’t write any charge.
Now let’s see what happens when CaCl22 is added instead:
Ca2+2+(aq) + 2Cl−−(aq) →→ Ca⋅AgAg⋅ + v′Ag