Ion exchange as a simple and effective tool for screening possible cation conductors

Abstract

Although kinetics of the low-temperature cation exchange in mixed oxide materials (aluminates, gallates, titanates, niobates, tantalates, antimonates, phosphates etc.) cannot provide quantitative information on self-diffusion and ionic conductivity in the starting material due to the mixed cation effect, it is the most direct and simple qualitative indication of the cation mobility in the solid state. It does not need using ceramics and single crystals and thus represents a useful tool for rapid selection of prospective cation conductors for subsequent detailed studies of dense samples with electrical methods. Examples of solid electrolytes discovered owing to their ion-exchange properties are reviewed, and rational principles of the ion-exchange testing are discussed. Laws of ion-exchange equilibria are based on ionic size compliance and the principle of hard and soft acids and bases. The former is most important for alkali/alkali exchange and the latter for exchanging cations of similar size but having different electronic structures: those of the rare-gas type and those having 18- or 18 + 2-electron shells, like Na+ and Ag+ or K+ and Tl+. Ion-exchange testing is especially useful for structures with non-intersecting conduction paths. It is shown that the resistivity of crystals with non-parallel and non-intersecting conduction paths cannot be described by the classical tensor formalism. Significant differences between isotope exchange, chemical ion exchange and ion conduction, quasi-one-dimensional and true one-dimensional conductors and single- and multiple-barrelled non-intersecting channels are disclosed and discussed.