Question

Doping of $AgCl$ crystal with $CdC{l}_2$ results in:

Answer 1

Silver is monovalent while cadmium is divalent. Their ionic sizes do not differ much.
When $AgCl$ is doped with $CdC{l}_2$, to balance the charge difference each doubly charged $C{d}^{2+}$ ion has to displace two silver monovalent ions from two cationic sites. When cadmium ions substitute silver ions, only one of the two vacant silver ion sites will be substituted by cadmium ion and the other will remain vacant.
So, the doping of $AgCl$ crystals with $CdC{l}_2$ results in the creation of impurity defect.