The non-stoichiometry occurs because of the instability in the FeO molecule as undergoing the oxidation of Fe2+ to Fe3+ effectively replacing a small portion of Fe2+ with two thirds their number of Fe3+.
Thus for every three "missing" Fe2+ ions, the crystal contains two Fe3+ ions to balance the charge. This change in composition violets the law of constant composition as the composition of the compound usually varies in a continuous manner over a narrow range. Thus FeO becomes a non-stoichiometric compound, and the formula is written as Fe(1-x)O, where x is a small number representing the deviation from the "ideal" formula.
Generally, it is written as Fe0.95O or found in the form of FeO.Fe2O3 represented as Fe3O4.
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