Ionic motion in various forms of solid-state alkali-metal hydroxides: Individual compounds, eutectic mixtures, and crystalline hydrates

Ionic motion in a solid, both chaotic (self-diffusion, Dsd) and electric-field-sustained (ionic conductivity, σ), is mediated by the chemical nature of the compound, the structure of its crystal lattice, and texture of the material. One of the ways to controlling these characteristics be it for a basic study of the mechanism underlying ion transport or a search for high conductivity ionic conductors, lies in complicating the composition of a chemical compound and development of composites. This report demonstrates convincingly that individual LiOH(DsdrefA, σref), NaOH(Dsd, σref), KOH(Dsd, σref) and H2O, which can hardly be considered as attractive candidates for this purpose because of their low conductivity below 500 K, can be used to advantage as a basis for production of a new family of inorganic, high-conductivity (>10−4 S cm−1) protonic conductors at temperatures in the 500–250 K range. The first members of this family studied thus far were chosen drawing from phase diagrams of the corresponding binary systems, more specifically, crystalline hydrates KOH·H2O(Dsd, σrefA), KOH·2H2O(Dsd, σ), and NaOH·H2O(Dsd, σ) and eutectics KOH + NaOH(Dsd, σrefA), LiOH + 1.5 NaOH(Dsd, σ), and KOH + KOH H2O(Dsd, σrefA). (Symbols in the parentheses, σ, σref and σrefA, refer to, accordingly, the conductivity measured in the present paper, published by other authors, and by the present author; the same applies to Dsd and DsdrefA as the hydrogen self-diffusion coefficients.) We are reporting on the first measurements of the self-diffusion coefficients of all the three types of ions in KOH, of oxygen and hydrogen in NaOH, and of hydrogen in crystalline hydrates and eutectics. The cubic phases of the individual hydroxides have Dsd of hydrogen, ~7 · 10−7 cm2 s−1, which only weakly depend on temperature. The Dsd of hydrogen in crystalline hydrates and eutectics measured in the 470–300-K interval is 10−7–10−9 cm2 s−1, and the activation energy, 0.4–0.5 eV. The dc conductivities of the eutectics exceed by two to three orders of magnitude those following from the Nernst–Einstein relation, and by five to six orders of magnitude, those of the individual starting components. It is suggested that this effect is actually a result of the self-organized micro-heterogeneity of the eutectics.