Recent developments in fast ion transport in glassy and amorphous materials

We review new developments in the expanding phenomenology of amorphous solid electrolytes. We consider separately (a) the vitreous materials in which the charge carriers are highly decoupled from the supporting matrix, and (b) the polymer-electrolyte solutions, in which the charge carriers are coupled to, and move cooperatively with, a locally fluid (though globally elastic) matrix. To these we add briefly some observations on orientationally amorphous solids (plastic crystals in their interesting temperature range) which show some potential for combining the best features of both (a) and (b) above. In the vitreous cases, new methods of preparation, including vapor deposition and sol-gel processes, are allowing exploration of compositions in previously inaccessible regions - also of new vitreous "states" within known composition regions. Use of new heavy element oxide chalcogenide matrixes has revealed some new and pleasing fast-conducting compositions. Among conventionally prepared glasses, the highest conductivities are still obtained with Ag^± and Cu⁺ containing glasses. New Ag⁺ glasses containing only halide anions yield σ_{25°C = 4·7 × 10^-2 Ω^-1 cm^-1}, even higher than for the AgI + oxyanion-containing record-holders. Fast anion-conducting (F^-, Cl^-), and possibly fast divalent cation-conducting, (Pb²⁺) galsses have been developed. We emphasize the importance of studying the fast ion motions by mechanical response, in addition to electrical response, measurements. Data on several systems are now available from low frequency tensile, ultrasonic, and hypersonic (Brillouin light scattering) techniques covering in all ten decades of frequency. These show that the mechanical response has the same average time constant as the electrical response, and the same activation energy but quite different spectral widths (relaxation time distributions). Furthermore, the greatest widths are found for a case where only one "type" of Ag⁺ is present suggesting that the spectral widths do not simply reflect the mobile ion site distributions. Of importance is the observation that the change in mechanical modulus due to relaxation of the fast ions decreases with increasing temperature. This seems incompatible with the weak electrolyte models, which require the displaceable ion concentration to increase with increasing temperature. In the polymer electrolyte solution field, new polymeric solvents with more flexible backbones, hence lower "local" viscosities at ambient temperatures, have been developed. The "solid" electrolytes based on these show behavior which is predictable from models developed for ordinary concentrated electrolyte solutions. Comparisons are made with the properties of low temperature "plicfics".