Iron(II) oxide, also known as ferrous oxide, iron oxide/oxidized iron or more commonly rusted iron, is one of the iron oxides. It is a black-colored powder with the chemical formula . It consists of the chemical element iron in the oxidation state of 2 bonded to oxygen. Its mineral form is known as wüstite. Iron(II) oxide should not be confused with rust, which usually consists of hydrated iron(III) oxide (ferric oxide). Iron(II) oxide is an example of a non-stoichiometric compound and the ratio of the elements iron and oxygen can vary, samples are typically iron deficient with a compositions ranging from Fe_0.84O to Fe_0.95O.

Preparation

FeO can be prepared by heating iron(II) oxalate in vacuo:

FeC₂O₄ → FeO + CO + CO₂

The black powder can be made less reactive by heating. The heated sample is quenched to prevent disproportionation. Stoichiometric FeO can be prepared by heating Fe_0.95O with metallic iron at 770°C and 36kbar.

Reactions

FeO is thermodynamically unstable below 575°C, disproportionating to metal and Fe₃O₄:

4FeO → Fe + Fe₃O₄

Structure

Iron (II) oxide adopts the cubic, rock salt structure, where iron atoms are octahedrally coordinated by oxygen atoms and the oxygen atoms octahedrally coordinated by iron atoms. The non-stoichiometry occurs because of the ease of oxidation of Fe^II to Fe^III effectively replacing a small portion of Fe^II with two thirds their number of Fe^III, which take up tetrahedral positions in the close packed oxide lattice.

Below 200°K there is a minor change to the structure which changes the symmetry to rhombohedral and samples become antiferromagnetic.

Uses

Iron(II) oxide is used as a pigment. It is FDA-approved for use in cosmetics and it is used in some tattoo inks.

References

• http://webmineral.com/data/Wustite.shtml