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

When a cation of higher valence is added to an ionic solid as an impurity to it, the cation of higher valence replaces more than one cation of lower valence so as to keep the crystal electrically neutral. As a result, some sites become vacant. For example, when Sr²⁺ is added to NaCl, each Sr²⁺ ion replaces two Na⁺ions. However, one Sr²⁺ ion occupies the site of one Na⁺ ion and the other site remains vacant. Hence, vacancies are introduced

Answer 2

Let us take an example NaCl doped with SrCl, impurity when SrCl2 is added to NaCl solid as an impurity, two Na+ ions will be replaced and one of their sites will be occupied by Sr21- while the other will remain vacant. Thus, we can say that when a cation of higher valence is added as an impurity to an ionic solid, two or more cations of lower valency are replaced by a cation of higher valency to maintain electrical neutrality. Hence, some cationic vacancies are created.