What is metal excess defect? Give examples.

Explanation

It is a non-stoichiometric defect in which metal ions are more than the anions with maintaining the electrical neutrality of the solid.
Example: (i) The defect arises when sodium chloride crystal is heated in presence of sodium vapour.
Example: (ii) This defect is produced when ZnO is heated (white coloured ZnO turns yellow).
`ZnO overset("heat") to Zn^(2+) + 1/2 O_(2) + 2e^(-)`.