sesquioxides

Sesquioxides are chemical compounds with the formula M2O3, in which the metal is in the +3 oxidation state. The name reflects the 2:3 ratio of metal to oxygen, with “sesqui-” indicating one and a half parts in relation to the oxide ions. They are typically binary oxides of trivalent metals.

Common examples include aluminum oxide Al2O3 (alumina), iron(III) oxide Fe2O3 (hematite), chromium(III) oxide Cr2O3, and manganese(III) oxide Mn2O3. Gallium oxide Ga2O3 and indium oxide In2O3 are also classified as sesquioxides, and many rare-earth oxides RE2O3 (where RE is a lanthanide) are treated as sesquioxides as well.

Structure and polymorphism vary by composition. Al2O3 adopts the corundum structure; Fe2O3 is closely related to this framework. Ga2O3 has several polymorphs, including monoclinic beta-Ga2O3, while In2O3 is commonly seen in a cubic bixbyite structure. Rare-earth sesquioxides can exhibit hexagonal or cubic forms, with bixbyite being common for several members. The wide range of structures contributes to diverse physical properties.

Properties of sesquioxides generally include high melting points and strong chemical stability. Many are insulators or wide-bandgap materials, with performance highly dependent on the specific metal. Alumina is a hard, widely used ceramic; Fe2O3 serves as a pigment and catalyst; Ga2O3 and In2O3 are explored for electronic and optoelectronic applications, including beta-Ga2O3 for wide-bandgap devices and In2O3-based transparent conducting oxides (often doped).

Methods of preparation typically involve high-temperature oxidation of the metal or calcination of corresponding hydroxides, carbonates, or nitrates. Natural occurrences include mineral forms such as corundum (Al2O3) and hematite (Fe2O3), while rare-earth sesquioxides are common constituents in various minerals.