Question

Explain why doping produces cation vacancies and not the anion vacancies.

Answer 1

1. Doping produces cation vacancies and not anion vacancies because the dopant ions usually replace cations and have a higher positive charge than the original cations.
2. For example, when a divalent cation like Mg²⁺ is added to an NaCl crystal, it replaces a monovalent Na⁺ ion. To maintain electrical neutrality, one extra Na⁺ ion is removed from the lattice, creating a cation vacancy. This is because removing a cation balances the excess positive charge introduced by the dopant.
3. On the other hand, creating anion vacancies would disturb the charge balance even more and is not favoured in such cases. Hence, cation vacancies are formed during doping to preserve the electrical neutrality of the crystal.