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The beta-alumina family of materials has played a central role in the field of solid-state ionics. β- and β-alumina have been a principal theme in the field ever since Yao and Kummer reported the extraordinarily high conductivity for sodium β-alumina. The material “came of age” at a time when there was great interest in the science and technology of highconductivity solid electrolytes. A previous MRS Bulletin review introduced this broad family of materials, examining the range of compositions and the two major structures. The beta-alumina family emerged as almost an ideal system in which to explore structure-property relations, as its unusual structure is responsible for its remarkable ion-transport properties. Although the initial interest in this material, and the one which endures to this day, is its rapid sodium-ion transport, the rich ion-exchange chemistry added a dimension to this material that few inorganic systems can match, let alone exceed. β-alumina, in particular, is an almost universal solid electrolyte for cations. It constitutes a broad family of solid electrolytes whose properties are dependent upon the nature of the ion inserted into the conduction plane. As a result, the β-aluminas have served as model systems for a wide range of studies: proton transport and mixed-ion diffusion, order-disorder reactions, and superlattice phenomena. Moreover, these β-alumina compositions demonstrated that fast-ion transport was not limited to a few monovalentions, but could be extended to divalent and even trivalent cations. With the latter materials, the interest was not necessarily ion transport, but optical properties instead. The presence of trivalent cations in this unusual structure, coupled with the ability to control the chemistry of the local environment, enabled the β-aluminas to exhibit a number of novel optical properties.

Results are reported of three‐electrode measurements in sodium polysulfide melts. The composition range covered was Na2S3‐Na2S5 and the temperature range 300°–400°C. The electrode material used was vitreous carbon. Voltammetric, chronopotentiometric, and current interruption techniques were used to investigate cathodic and anodic processes. Evidence is produced that sulfide films are formed at the cathode and that these result in limiting currents. The lowest value of limiting current density was 28 mA · cm−2 obtained for Na2S3 at 350°C. At the anode the current appears to be limited by accumulation of liquid sulfur. The value of the limiting current density in Na2S5 was 100 mA · cm−2 at 350°C. A reaction scheme which best fits our results is suggested.