Question

why a schottky defect is formed when calcium chloride is added to silver chloride crystal?

Answer 1

When Agcl and cacl2 are mixed,each added ion of ca2+ replaces ag+.
since the charge of ca and ag is +2,+1 respectively,as they have similar radii.
The formation of positive it leads to the vacancies(Schottky defect) accordingly reaction:
cacl2+2Agcl-> caO Ag+2 clx cl+v+ Ag
Which mean one mole of cacl2 provides one mole of ag vacancies in Crystal structure.