Towards a universal behaviour of ion dynamics in Na- and Rb-oxide glasses

Activation enthalpies of ionic conductivities in Na-Rb alumino-germanate and borate glasses are reviewed. Correlations between the activation enthalpy and the ratio of average distances between like alkali ions, 〈dion〉, to the average distances between network-former atoms, 〈dnetwork〉, are elucidated for single- and mixed-alkali glasses. The Haven ratio is shown to decrease with decreasing 〈dion〉/〈dnetwork〉. Interstitial-like and substitutional-like subnetworks of ion sites are suggested. The experimentally observed dependence of the Haven ratio on 〈dion〉/〈dnetwork〉 is consistently reproduced by a Monte Carlo simulation of ion dynamics on this random network including single and collective ion jumps.