Fe1xO

Fe1−xO, commonly known as wüstite, is a non-stoichiometric iron oxide with the formula Fe1−xO. The value of x reflects iron vacancies in the cation sublattice and varies with temperature and the ambient oxygen partial pressure, typically ranging from 0 to about 0.15.

Structure and composition: Fe1−xO adopts a rock-salt (NaCl-type) crystal structure. Non-stoichiometry is achieved through iron vacancies and, to balance charge, small amounts of Fe3+ in the lattice. This defect structure gives Fe1−xO a variable composition and influences its physical properties.

Phase relations and occurrence: Fe1−xO is part of the iron–oxygen system and is stable under reducing conditions, forming a non-stoichiometric solid solution near FeO. Natural wüstite is usually found in oxide assemblages formed under low oxygen fugacity, near the magnetite–wüstite boundary, and can occur in meteorites or high-temperature, low-oxygen environments.

Properties: Wüstite is typically gray to brown. Its electrical behavior is influenced by defects, and it behaves as a narrow-gap semiconductor with relatively higher conductivity for an oxide. Magnetic properties are temperature dependent; near-stoichiometric FeO is antiferromagnetic below about 198 K, while Fe1−xO with vacancies tends to show altered magnetic behavior at higher temperatures, often paramagnetic at room temperature.

Synthesis and applications: Fe1−xO can be prepared by reducing FeO or related oxides under controlled atmospheres, or by synthesis under low oxygen fugacity. In materials science and geochemistry, Fe1−xO serves as a model defect oxide for studying diffusion, defect chemistry, and phase relations in iron oxides. See also: wüstite, iron oxide, magnetite, hematite.