Iron oxide (FeO) has rock salt structure. In this case, O₂ ions adopt on sites and Fe^{2+ ions should occupy non- stoichiometric.}

This is the ideal arrangement. This oxide is always non-stoichiometric i.e., the composition of Fe²⁺ and O₂ ions is not 1 : 1. It is 0.95 : 1 i.e. FeO₉₅O(Wustite)

This composition can be obtained if a small number of Fe²⁺ ions are replaced by two-thirds of Fe³⁺

Eventually there would be less amount of metal as compared to stoichiometric composition.