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

When a cation of higher valence is added as an impurity, it substitutes for the normal cations while preserving neutrality. For example, adding $SrCl_2$ to $NaCl$: each $Sr^{2+}$ (charge $+2$) replaces two $Na^+$ ions (charge $+1$ each) but occupies only one site; the second $Na^+$ site is left vacant. Thus each higher-valent cation introduces …

Step 1 – Charge imbalance introduced.

Suppose an ionic solid $NaCl$ (host cation $Na^+$, charge $+1$) is doped with a higher-valence cation such as $Sr^{2+}$ (charge $+2$).

Step 2 – Substitution with neutrality.

The impurity ion enters cation sites of the lattice. To keep the crystal electrically neutral, the positive charge introduced must equal the positive charge removed. A single $Sr^{2+}$ carries the charge of two $Na^+$ ions.

Step 3 – Vacancy creation.

So each $Sr^{2+}$ takes one cation site, and the second $Na^+$ that its charge accounts for is removed, leaving that site empty:

Step 4 – General rule. …

Apply charge-neutrality with an aliovalent dopant: the higher-charge cation balances two (or more) host cations but sits on one site, forcing the surplus host- …

• Saying anion vacancies form; a higher-valence cation creates cation vacancies.
• Claiming the impurity makes no vacancy because it just substitutes 1:1 — it substitutes 1 site for the charge of 2, hence a vacanc …