Question

How does a crystal having metal excess defect maintain its electrical neutrality?

Answer 1

1. A crystal with a metal excess defect maintains its electrical neutrality through the formation of F-centres or by the presence of extra cations in interstitial sites.
2. In the F-centre mechanism, an anion (negative ion) is missing from its lattice site, creating a vacancy. To maintain electrical neutrality, the missing negative charge is compensated by a free electron that gets trapped in this vacancy, forming an F-centre.
3. This free electron acts as a negative charge and balances the excess positive charge of the metal cations. This type of defect is commonly observed in crystals like NaCl, KCl, and LiCl, where it also imparts a characteristic colour to the crystal.
4. In some cases, extra metal cations occupy interstitial sites within the lattice, and to balance this, an equal number of electrons are also present in the interstitial sites, maintaining overall electrical neutrality.
5. This type of defect is observed in ZnO, which turns yellow upon heating due to the loss of oxygen and the formation of interstitial zinc and free electrons.