This issue of the MRS BULLETIN contains three articles relating to the general field that has come to be known as Solid State Ionics. The central feature of this area of science and emerging technology is the rapid transport of atomic or ionic species within solids, and the various phenomena, of both scientific and technological interest, that are related to it.

Attention to this area has grown greatly in recent years because of the rapidly increasing recognition of the possibility of a wide range of interesting technological applications. One example already widespread is the use of an oxygen-conducting solid electrolyte as the critical element in the oxygen sensors installed in the exhaust systems of almost all current automobiles to reduce deleterious emissions and improve the efficiency of the combustion process.

Work is under way in a number of other directions, including static and dynamic chemical sensors, solid state electrochemical reactors, low impedance selective atomic filters, new concepts for the direct conversion of heat to electricity by the use of sodium- or hydrogen-transporting cycles, a novel method for the low cost electrolysis of water at intermediate temperatures, batteries that can store greatly increased amounts of energy, ion exchange materials, solid state laser hosts, high efficiency fuel cells, electrochromic materials and configurations for both optical displays and “smart windows,” advanced catalysts, atomic reservoirs and pumps, high temperature superconductors, and possibly solid state fusion hosts.

Despite this recent attention, however, it is worth noting that interest in solids in which ionic species can move with unusual rapidity is actually not new at all. As early as 1839, Michael Faraday reported measurements on several materials that showed an unusual increase in electrical conductivity at elevated temperatures, contrary to that found in normal metals.

Beta and β″-alumina are remarkable solid electrolytes whose study has been a principal theme in the field of solid state ionics ever since Yao and Kummer first reported the exceptionally high conductivity of sodium β-alumina in 1967. The unusual properties of sodium β-alumina stimulated widespread interest in both the science and technology of high conductivity solid electrolytes. Since Yao and Kummer's seminal work, many solid electrolytes have been explored, but interest in the β and β″-aluminas has endured, principally because of their virtually unique ability to undergo ion exchange in which the sodium ions are replaced by a wide variety of monovalent, divalent, and trivalent cations as well as various protonic species. The β and β″-aluminas thus are not single compounds, but a family of solid electrolytes with diverse properties and potential technological applications.

Kummer first pointed out the ion exchange properties of sodium β-alumina by showing that the sodium ions in the structure can be replaced by different monovalent cations, such as Li+, K+, as well as H(H2O)x+ and NH4+. More recent studies have shown that the ion exchange chemistry of sodium β″-alumina, is far richer than that of sodium β″-alumina. In fact, the sodium ion content of β″-alumina can be exchanged for virtually any +1, +2, or +3 cation in the periodic chart.

Only in the last two decades has the full realization been made that many materials can incorporate atoms or ions into their structures around room temperature. This incorporation frequently occurs with minimal structural changes so that the reaction can be reversed by appropriate chemical or electrical means. These materials include metals, inorganics, and organics. The driving force for reaction is a gain in free energy and is frequently associated with a transfer of electron density between the guest and host species. Thus by definition the host material must contain an electronic structure that can be readily oxidized or reduced, and hence for inorganic materials normally contains transition metals with their variable valences, or for organics a delocalized electron system.

The systems described here frequently exhibit both electronic and ionic conductivity, i.e., they are mixed conductors over at least part of their composition range. They tend to have variable composition; this contrasts with solid electrolytes such as β-alumina which may also be nonstoichiometric but are of fixed composition at normal temperatures. Materials in this last category include the β-aluminas and aluminosilicates such as vermiculite and montmorillonite, both of which can be used as electrolytes due to their high ionic conductivity and low electronic conductivity.

The preparation, utilization, and understanding of high polymers represents one of the great triumphs of chemistry and materials science in the 20th century. Synthetic polymers have traditionally been used as structural materials and electrical insulators. Biopolymers often exhibit interesting electrical response phenomena. A recent article in the MRS BULLETIN, for example, discussed piezoelectric properties of both synthetic and biopolymer systems. The newer, synthetic electroactive polymeric materials, however, represent one of the most exciting current areas of polymer materials science.

Many synthetic ionic polymer materials are known; perhaps the first were the polyelectrolytes and crosslinked ion exchange materials. These are materials whose backbone contains charges of one sign, balanced by small counter ions of the opposite sign. Such polyelectrolytes have found very important applications in analytical chemistry, water purification, and chemical processing.

Complexes, in which salts are dissolved in neutral polymer hosts, have until recently received less attention. The area of polymer/salt complexes became extremely active following the work of P.V. Wright, who first clearly showed that polyethylene oxide (PEO) is an excellent polymer host for a number of salts, and that the resulting solid polymer/salt complexes are electrical conductors. M. Armand broadened the investigation of electrical properties of polymer/salt complexes and pointed out that these materials might be useful in electrochemical devices, especially batteries.

This article will discuss the formation, properties, behavior, and applications of polymer electrolytes and mixed conductors—that is, polymeric materials in which charge is transported either by ions or by ionic and electronic charge motion. Our concentration will be on solvent-free materials—materials in which no small molecule solvents are present. There is substantial interest, and substantial progress, in the area of solvent-swollen polymer electrolytes.