A misnomer in chemistry and materials science: β-alumina

Alumina is aluminum(III) oxide (or aluminium(III) oxide) with the formula Al₂O₃. Alumina occurs in different crystalline forms, for example, α- and γ-alumina [1]. β-Alumina, however, is not another form or polymorph of alumina, as one would assume from the term with a greek-letter prefix. Its name is a misnomer: β-alumina is a ternary oxide of formula xNa₂O · nAl₂O₃, having typical compositions with x between 1.25 and 1.4 and n about 11 [2].

The term β-alumina has also been used to describe a family of compounds of general formula M₂O · nX₂O₃, where n can have values from 5 to 11, M is a monovalent cation and X is a trivalent cation [3] . With M = Na⁺ and X = Al³⁺,  the most important member of this family is obtained: sodium β-alumina (compare with formula above). Compositions with other alkali, silver, thallium and ammonium ions for M and gallium(III) and iron(III) ions for X make further members of the β-alumina family [3].

Keywords: solid-state ionics, material structure and composition, ternary oxides, non-stoichiometric compounds 

References and more to explore
[1] Alumina: www.infoplease.com/ce6/sci/A0803541.html.
[2] Beta-alumina: authors.library.caltech.edu/5456/1/hrst.mit.edu/hrs/materials/public/Beta-alumina.htm.
[3] Anthony R. West: Solid State Chemistry and its Applications. John Wiley & Sons, Chichester, 1984; page 467.