FeOä

FeOä is not a standard chemical name, and it does not designate a distinct, widely recognized compound in mainstream chemistry or mineralogy. In some sources, the symbol FeOä is used informally to refer to non-stoichiometric iron(II) oxide, commonly described in the literature as Fe1−xO (wüstite). The suffix “ä” has no defined chemical meaning in formal nomenclature, and FeOä should be treated as an informal label rather than a formal mineral or compound name.

Chemistry and structure

Fe1−xO represents a solid solution where iron deficiency (vacancies) and mixed oxidation states occur within the rock-salt (NaCl-type) lattice. As the non-stoichiometry x varies with temperature and the oxygen partial pressure, the material exhibits defect structures including cation vacancies and some Fe3+ ions, which influence its physical properties. The lattice parameter can change with the level of vacancy disorder and defect chemistry, and the material is often described as defective wüstite.

Occurrence and synthesis

Non-stoichiometric iron oxide forms under reducing or controlled oxygen conditions, typically at elevated temperatures. It can be produced by reducing Fe2O3 or Fe3O4 with gases such as hydrogen or carbon monoxide, or as a transient phase in steelmaking slags and high-temperature oxide systems. At room temperature and in air, more iron oxide products such as Fe3O4 or Fe2O3 may form from Fe1−xO upon oxidation.

Properties and applications

Fe1−xO behaves as a defective magnetic oxide with properties that depend on defect concentration. It is generally black to dark gray and exhibits antiferromagnetic ordering at low temperatures. Its electrical and catalytic behaviors are influenced by non-stoichiometry, making it a subject of research in defect chemistry and oxide materials.

See also

FeO, wüstite, iron oxide.