So you might then think of Fe(III) ferrate Fe2(FeO4)3 = Fe5O12. But Fe(III) is strongly acidic, and [FeO4]2- is stable only in alkaline conditions. FeO2 containing Fe(IV) rather than being Fe(II) ferrate (VI) may be possible, but for Fe(IV) so far only mixed oxides such as BaFeO3 are known, and various complexes with organic molecules.