Suboxides

Suboxides are oxide-like compounds in which the metal is in a lower oxidation state than in the most common oxide of that element, or in which oxygen is present in less than the ideal stoichiometry. They are often non-stoichiometric and can contain mixed-valence metal centers, with defects such as oxygen vacancies. Suboxides are typically formed by partial reduction of a higher oxide, by synthesis under limiting oxygen, or by disproportionation of oxides. Their electronic properties can range from insulating to metallic or semiconducting, and they frequently exhibit distinctive colors and catalytic or pigmentary behavior.

A classic example is lead suboxide, Pb3O4 (minium), which contains both Pb2+ and Pb4+ and is widely used as a pigment and in historical corrosion-protection contexts. Copper suboxide, Cu2O, represents copper in the +1 state and is often discussed as a lower-valence oxide relative to CuO. Iron(II) oxide, FeO, is nonstoichiometric and can harbor vacancies and small amounts of Fe3+, and is sometimes treated as a suboxide in older literature. Tin(II) oxide, SnO, is another lower-oxidation-state oxide sometimes described as a suboxide of tin.

Because the term suboxide is applied variably across sources, many chemists prefer the terms nonstoichiometric oxide or mixed-valence oxide for greater precision. Suboxides are of interest for their distinctive electrical, catalytic, and pigmentary properties, and they provide insight into how oxygen content and oxidation state influence solid-state structure.