Materials that conduct both electrons and ions are important technologically for use in electrodes, permeation membranes, sensors and catalysts. They often consist of two or more components in a composite, with each mode of conduction catered for separately. This paper shows that a composite of the ‘super-ionic’ conductor rubidium silver iodide (RbAg4I5) and the electronic conductor graphite exhibits extremely fast diffusion of silver ions at the interface between the two materials, generating both a silver excess and a silver deficiency, analogous to single-phase mixed conductors, even though such behaviour is not possible in the individual phases.
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