Why is ferrous oxide hard to be prepared with ideal stoichiometric composition?

Explanation

The chemical formula suggests a stoichiometric composition for the metal oxide )FeO). However, it is found to have compposition of `Fe_(0.95)O`. This means that Fe and O are not in stoichiometric proportions. In fact in the cystal lattice of ferrous oxide (FeO), some `Fe^(2+)` ions are missing and the loss of positive charge is made up by some `Fe^(3+)` ions. Since lesser number of `Fe^(3+)` ions are needed to maintain the electrical neutrality of the lattice, this shows that FeO is not formed in stoichiometric composition.