Explain how vacancies are introduced in an ionic solid when a cation of higher valence is added as an impurity in it.
If we mix a small amount of molten strontium chloride (SrCl2) with molten sodium chloride (NaCl) and the resulting solution is cooled, in the crystals of NaCl some Na+ ions will get replaced by Sr2+ ions. In order to maintain electron neutrality too Na+ ions are to leave their respective lattice sites. However, one out of them will be occupied by the Sr2+ion while the other will remain unoccupied or it will be vacant. Thus vacancies are introduced in an ionic solid.