Thursday, October 20, 2011

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:
[2] Beta-alumina:
[3] Anthony R. West: Solid State Chemistry and its Applications. John Wiley & Sons, Chichester, 1984; page 467.

1 comment:

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