Sesquioxide

A sesquioxide is a chemical compound containing oxygen that is present in a 2:3 ratio to a metal. The chemical formula for a typical metal sesquioxide is M2O3, where M represents a metal cation. These compounds are also referred to as trioxides, because they contain three oxide ions for every two metal ions. Sesquioxides are usually formed when binary metal oxides are partially oxidised or when metal oxides react with water or carbon dioxide at elevated temperatures.

Aluminium oxide (Al2O3) is the most common example of a sesquioxide. Other frequently encountered sesquioxides include iron(III) oxide (Fe2O3), chromium(III) oxide (Cr2O3), and gallium oxide (Ga2O3). Rare‑earth sesquioxides such as lanthanum sesquioxide (La2O3) and neodymium sesquioxide (Nd2O3) possess high thermal stability and are used in the manufacture of ceramic materials. Some sesquioxides, including iron oxides and titanium sesquioxides, exist in several polymorphic forms which differ in crystal structure and can exhibit different colours, catalytic properties and electrical conductivities.

Through their high hardness, resistance to corrosion, and relatively low reactivity they find many practical applications. Ferric oxide, for instance, serves as a pigment and a catalyst in the production of sulfuric acid, while chromium sesquioxide is widely used as a protective coating and in the manufacture of refractory materials. Cadmium sesquioxide is applied in certain gas‑sensing devices. In addition, many sesquioxides are used as precursors in the synthesis of other oxides, alloys or ceramic products, or as catalysts in organic transformations. The study of sesquioxides is important in metallurgy, materials science, and mineralogy, and their properties are continually leveraged in technological development.