Solid State Ionics
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ionic materials. Electronically insulating materials in which ionic transport is rapid hâve electrical properties comparable to those of molten salts, a n d t h u s can be e m p l o y e d as solid electrolytes. PbF 2 is now recognized to be an outstanding example of an anionconducting solid electrolyte, in which electrical charge is carried primarily by the transport of fluoride ions. Ag 2 S, on the other hand, is a prototypical mixed conductor, in which b o t h ionic a n d electronic species can transport charge. As s h o w n by W a g n e r 2 in t h e early 1930s, the nature of adjacent phases can influence whether charge is carried primarily by ions or by électrons in mixed conductors. As early as 1914, Tubandt and Lorenz 3 showed that the ionic conductivity of solid electrolytes can be extremely high. For the high température alpha phase of Agi, it even exceeds the values observed above the melting point. Interprétation of the unusual behavior of such materials in terms of an u n u s u a l l y large degree of structural disorder in one of the sublattices was presented by Strock4 in 1934 on the basis of x-ray diffraction observations. This important characteristic is also manifested by thermodynamic évidence of "sublattice melting" in a number of such materials at températures well below their macroscopic melting points. The great accélération of interest in fast ionic conductors, and especially their use as solid electrolytes, stems from observations that this is not just a high t e m p é r a t u r e p h e n o m e n o n a n d that a number of solids can hâve high ionic conductivities even at ambient températures. 5 " 9 Particularly important was the work of Yao and Kummer, who showed that a number of différent ionic species can exhibit rapid transport in the /3-alumina structure. This led to the concept 10 of a radical approach to high performance rechargeable batteries involving liquid électrodes and a solid electrolyte, based on the reaction of s o d i u m with sulfur by transport of s o d i u m ions t h r o u g h solid ceramic materials in the /3-alumina family.
Since that rime there has been a great deal of progress in understanding the phenomena in the /3-alumina type of materials, and in developing methods for their fabrication into practical configurations and devices. As discussed in the article by B. Dunn, G.C. Farrington, and J.O. Thomas in this issue, it is now recognized that this is not just a single compound, but a broad family of materials, with a wide range of compositions and two major structural variants. They point out that the ability to insert a wide variety of monovalent, divalent, and trivalent guest species into one of thèse variants, the j8" structure, leads to interesting new possibilities. Thèse include possibilities based on their rapid ionic transport, and also in solid state optical devices in which the local ionic environment plays a critical rôle. The accompanying article by M.S. W h i t t i n g h a m discusses some of the interesting properties and phenomena in three classes of m
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