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

Explanation

Each cation of higher valence will replace two or more cations of lower valence in the crystal lattice in order to maintain electrical neutrality. Thus some cation vacancies are created. For example, if `SrCl_2` is added to NaCl during crystalisation, some of the sities of `Na^+` are replaced by `Sr^(2+)`. Each `Sr^(2+)` ions replaces two `Na^+` to maintain electrical neutrality, `Sr^(2+)` occupies one of the sites of `Na^+` and the other site remains vacant.