The mixed alkali effect in (K, Na)–β-gallate fast ion conductor

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Abstract
K–β‐gallate is a potassium/gallium isomorph of the two‐dimensional fast ion conductor β‐alumina. It exhibits a strong ’’mixed alkali conductivity effect’’ when part of the potassium is replaced by sodium. At ?20 at. % Na the conductivity at room temperature goes through a minimum about 1/5000th of that expected from extrapolation between the conductivities of the single alkali end members. This conductivity minimum is due to a corresponding maximum in the activation energy in the exponent of the conductivity relation, which overrides the opposing effect of a maximum in the pre‐exponent. The effect is attributed to local ordering of the alkali ions—three sodiums are bound in a triplet of interstitial sites around a vacancy, and the potassiums are distributed between normal lattice sites and pairs bound to interstitial oxygens. Raman spectroscopy reveals only one prominent low energy mode that can be related to the alkali ions. This vibrational frequency cannot be related in any obvious way to the mixed alkali effect, however.

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