Diffusive dynamics in an amorphous superionic conductor

The fast diffusion of alkali ions and resulting high ionic conductivity is a defining feature of alkali borate glasses. Here we report impedance spectroscopy and atomic-scale x-ray photon correlation spectroscopy measurements on rubidium borate systems (Rb2O)(x)(B2O3)(100-x) with particular focus on x = 30 mol %. We find that the coherently scattered intensity does not show temporal fluctuations on timescales corresponding to ionic diffusion. We conclude that the spatial configuration of alkali sites as stepping stones for ionic diffusion evolves on the same timescales as the borate backbone, and that the defect density is low, i.e., at a given instant, nearly all alkali sites are singly occupied.