Question

How are nonstoichiometric point defects classified? Explain with diagram the metal deficiency defect.

Answer 1

• Nonstoichiometric point defects are classified into following two types:

1. Metal deficiency defect
2. Metal excess defect

• Metal deficiency defect:

1. This defect is possible only in compounds of metals that show variable oxidation states.
2. In some crystals, positive metal ions are missing from their original lattice sites. The extra negative charge is balanced by the presence of cation of the same metal with higher oxidation state than that of missing cation.
   e.g.: In the compound NiO, one Ni²⁺ ion is missing creating a vacancy at its lattice site. The deficiency of two positive charges is made up by the presence of two Ni³⁺ ions at the other lattice sites of Ni²⁺ ions. The composition of NiO then becomes Ni_0.97O_1.0.