The title compounds have received a considerable amount of attention because of their high protonic conductivities at 100–300 °C. However, because of their complex characters there remain many areas of uncertainty. The current status is reviewed here.

Synthesis

It is well known that the β- and β″-aluminas are sodium ion conductors with promising applications as an electrolyte for the sodium–sulphur secondary battery. They have been widely studied because of their technological and scientific interest.

The sodium ions in the conduction planes of β- and β″-alumina type structures are exchangeable with many other monovalent and divalent cations. The main host crystals are β- and β″-alumina, and the Ga, Fe analogues: β- and β″-gallate, β- and β″-ferrite. The guest ions which contain protons include H+, H3O+ (in this review the use of H3O+ does not necessarily mean a hydronium ion), and NH4+. Many combinations of host crystals and guest ions have been prepared; details are given in Table 13.1.

The crystals, even of β-alumina, are attacked corrosively by hot sulphuric acid. Furthermore, crystals are inclined to cleave during ion exchange because the ionic radii of NH4+ and H3O+ are much larger than that of Na+14. These factors have serious implications for the fabrication of high density, polycrystalline, flaw-free ceramics (see Chapter 33).