Explain how vacancies are introduced in an ionic solid when a cation of higher valence is added as an impurity in it.

(N/A) When a cation of higher valence is added to an ionic solid as an impurity,it replaces more than one cation of lower valence to maintain electrical neutrality of the crystal.
As a result,some lattice sites remain vacant.
For example,when $Sr^{2+}$ is added to $NaCl$,each $Sr^{2+}$ ion replaces two $Na^{+}$ ions.
One $Sr^{2+}$ ion occupies the site of one $Na^{+}$ ion,while the other site remains vacant.
Thus,cationic vacancies are introduced in the crystal lattice.