The Collaborative International Dictionary
Sesquioxide \Ses`qui*ox"ide\, n. [Sesqui- + oxide.] (Chem.) An oxide containing three atoms of oxygen with two atoms (or radicals) of some other substance; thus, alumina, Al2O3 is a sesquioxide.
Wiktionary
n. (context chemistry English) Any oxide containing three oxygen atoms for every two atoms of another element.
Wikipedia
A sesquioxide is an oxide containing three atoms of oxygen with two atoms (or radicals) of another element. For example, aluminium oxide is a sesquioxide. Many sesquioxides contain the metal in the +3 oxidation state and the oxide ion, e.g., AlO, LaO. The alkali metal sesquioxides are exceptions and contain both peroxide, and superoxide, ions, e.g., RbO is formulated [(Rb) ]. Sesquioxides of iron and aluminium are found in soil.
Sesquioxidizing, meaning the creation of a sesquioxide, is the highest scoring word that would fit on a Scrabble board, though it does not actually appear in any official Scrabble dictionary. The word first occurred in 1976 in Josepha Heifetz Byrne's Mrs. Byrne's Dictionary of Unusual, Obscure, and Preposterous Words (ISBN 0806504986), though the Oxford English Dictionary already listed sesquioxidized and sesquioxidation. One could theoretically score 2044 points in a single move, when otherwise only words from the official Scrabble word list are used.
Usage examples of "sesquioxide".
If my horse casts a shoe, do you think I will not trust a blacksmith to shoe him until I have made sure that he is sound on the distinction between the sesquioxide and the protosesquioxide of iron?
As an antidote, the best when the poison is in solution is the hydrated sesquioxide of iron, formed by precipitating tinctura ferri perchloridi with excess of ammonia, or carbonate of soda.