Asesquioxide is anoxide of anelement (orradical), where the ratio between the number ofatoms of that element and the number of atoms ofoxygen is 2:3. For example,aluminium oxideAl₂O₃ andphosphorus(III) oxideP₄O₆ are sesquioxides.Many sesquioxides contain ametal in the +3oxidation state and the oxide ionO²⁻, e.g.,aluminium oxideAl₂O₃,lanthanum(III) oxideLa₂O₃ andiron(III) oxideFe₂O₃. Sesquioxides ofiron andaluminium are found insoil. Thealkali metal sesquioxides are exceptions because they contain bothperoxideO2−2 andsuperoxideO−2 ions, e.g.,rubidium sesquioxideRb₄O₆ is formulated(Rb⁺)₄(O2−2)(O−2)₂.^[1] Sesquioxides ofmetalloids andnonmetals are better formulated as covalent, e.g.boron trioxideB₂O₃,dinitrogen trioxideN₂O₃ and phosphorus(III) oxideP₄O₆;chlorine trioxideCl₂O₃ andbromine trioxideBr₂O₃ do not have oxidation state +3 on the halogen.

Manytransition metal oxides crystallize in thecorundum structure type, withspace group R3c. Sesquioxides ofrare 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).^[2][3]

Sesquioxidizing, meaning the creation of a sesquioxide, is the highest scoring word that would fit on aScrabble board,^[4] though it does not actually appear in any official Scrabble dictionary.^[5] Though theOxford English Dictionary already listed the noun and the past participle adjective — sesquioxidation and sesquioxidized, respectively — the verb, sesquioxidize, and its conjugated forms, have been absent from the dictionaries used as sources for the official Scrabble word lists. An early appearance of the noted present participle had occurred in the 1860 publication of the State of New York's Legislative Assembly's Transactions of the State Medical Society,^[6] yet the word's first appearance in a dictionary was in the 1976 edition of Josepha Heifetz Byrne'sMrs. Byrne's Dictionary of Unusual, Obscure, and Preposterous Words (ISBN 0806504986),^[7] One could theoretically score 2044 points in a single move, when otherwise only words from theofficial Scrabble word list are used.^[8]

• Sesquioxides

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