Question

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

Answer 1

Cation of higher valency in an ionic solid replaces more than one cation of lower valency. To keep the crystal electrically
neutral some sites become vacant

Example
When $S{r}^{2+}$ is added to $NaCl,\text{each}S{r}^{2+}$ ion replaces two $N{a}^{+}$ ions. However, one $S{r}^{2+}$ ion occupies the site of one $N{a}^{+}$ ion and the other site remains vacant.

Hence, vacancies are introduced.