Explain metal deficiency defect.

(N/A) Many solids are prepared in which the metal ions are less in number than as required by the stoichiometric proportion.
For example,$FeO$ is mostly found with a composition of $Fe_{0.95}O$.
It may actually range from $Fe_{0.93}O$ to $Fe_{0.96}O$.
In crystals of $FeO$,some $Fe^{2+}$ cations are missing and the loss of positive charge is made up by the presence of the required number of $Fe^{3+}$ ions.