On the thermodynamic activity of Na2O in Na–beta-alumina

Recently a paper [Mater. Res. Bull. 35 (2000) 1153] has been published in which well known aspects of thermodynamic investigations on Na–beta-alumina were disregarded. These aspects are discussed below in the light of the current knowledge about the subject. As a consequence, the conclusions drawn in the cited paper that Na–β″-Al2O3 is not a stable phase must be considered as not justified.