A sesquioxide is an oxide of an element (or radical), where the ratio between the number of atoms of that element and the number of atoms of oxygen is 2:3. For example, aluminium oxide Al2O3 and phosphorus(III) oxide P4O6 are sesquioxides.Many sesquioxides contain a metal in the +3 oxidation state and the oxide ion O, e.g., aluminium oxide Al2O3, lanthanum(III) oxide La2O3 and iron(III) oxide Fe2O3. Sesquioxides of iron and aluminium are found in soil. The alkali metal sesquioxides are exceptions because they contain both peroxide O2−2 and superoxide O−2 ions, e.g., rubidium sesquioxide Rb4O6 is formulated (Rb)4(O2−2)(O−2)2. Sesquioxides of metalloids and nonmetals are better formulated as covalent, e.g. boron trioxide B2O3, dinitrogen trioxide N2O3 and phosphorus(III) oxide P4O6; chlorine trioxide Cl2O3 and bromine trioxide Br2O3 do not have oxidation state +3 on the halogen.
Many transition metal oxides crystallize in the corundum structure type, with space group R3c. Sesquioxides of rare earth elements crystalize into one or more of three crystal structures: hexagonal (type A, space group P3m1), monoclinic (type B, space group C2/m), or body-centered cubic (type C, space group Ia3).
