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 O2−, 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 O2 ions, e.g., rubidium sesquioxide Rb4O6 is formulated (Rb+)4(O2−2)(O2)2.[1] Sesquioxides of metalloids and nonmetals are exceptions too, e.g. boron trioxide B2O3, dinitrogen trioxide N2O3 and phosphorus(III) oxide P4O6.

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).[2][3]

Sesquioxidizing, meaning the creation of a sesquioxide, is the highest scoring word that would fit on a Scrabble board,[4] though it does not actually appear in any official Scrabble dictionary.[5] Though the Oxford 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's Mrs. 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 the official Scrabble word list are used.[8]

List of sesquioxidesEdit

H He
Li Be B2O3 C N2O3 O F Ne
Na Mg Al2O3 Si P4O6 S Cl2O3 Ar
K Ca Sc2O3 Ti2O3 V2O3 Cr2O3 Mn2O3 Fe2O3 Co2O3 Ni2O3 Cu2O3 Zn Ga2O3 Ge As2O3 Se Br2O3 Kr
Rb4O6 Sr Y2O3 Zr Nb Mo2O3 Tc Ru Rh2O3 Pd Ag Cd In2O3 Sn Sb2O3 Te I Xe
Cs4O6 Ba Lu2O3 Hf Ta W2O3 Re Os Ir2O3 Pt Au2O3 Hg Tl2O3 Pb2O3 Bi2O3 Po At Rn
Fr Ra Lr Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og
La2O3 Ce2O3 Pr2O3 Nd2O3 Pm2O3 Sm2O3 Eu2O3 Gd2O3 Tb2O3 Dy2O3 Ho2O3 Er2O3 Tm2O3 Yb2O3
Ac2O3 Th Pa U Np2O3 Pu2O3 Am2O3 Cm2O3 Bk2O3 Cf2O3 Es2O3 Fm Md No


  1. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  2. ^ Eyring, LeRoy; Holmberg, Bo (1963-01-01). "Ordered Phases and Nonstoichiometry in the Rare Earth Oxide System". Advances in Chemistry. Washington, D. C.: American Chemical Society. pp. 46–57. doi:10.1021/ba-1964-0039.ch004. ISBN 978-0-8412-0040-1. ISSN 0065-2393.
  3. ^ Marezio, M. (1966-11-01). "Refinement of the crystal structure of In2O3 at two wavelengths". Acta Crystallographica. International Union of Crystallography (IUCr). 20 (6): 723–728. doi:10.1107/s0365110x66001749. ISSN 0365-110X.
  4. ^ The Scrabble Omnibus, Gyles Brandreth, ISBN 0-00-218081-2
  5. ^ [1], David K. Israel, "Scrabble Word Records", March 22, 2010, accessed March 31, 2018
  6. ^ New York State, Legislature, Assembly (1860). Documents of the Assembly of the State of New York, Eighty-third Session. — 1860. Volume IV ; No. 111: Transactions of the State Medical Society, p. 19
  7. ^ Keith W. Smith Total scrabble, page 67
  8. ^ Record for the Highest Scoring Scrabble Move at scrabulizer.com, accessed 2008-05-30