Local dimensionality and intermediate range ordering of ion conduction pathways in borate glasses

Conduction pathways in metal halide doped silver, lithium and sodium diborate glasses have been examined by bond valence analysis of Reverse Monte Carlo (RMC) produced structural models of the glasses. Although all glass compositions have basically the same short-range structure of the boron–oxygen network it is evident that the intermediate-range structure is strongly dependent on the type of mobile ion. The differences are particularly large for the highly doped glasses, where the AgI salt is homogenously introduced to the glass causing an expansion of the B–O network with the formation of pronounced conduction pathways between neighboring borate segments. In contrast, the dopant salts LiCl and NaCl are considerably more inhomogeneously introduced forming microscopic clusters of salt ions, but also regions containing almost no salt ions. The network of the conduction pathways is directly related to the real (atomic) structure, and therefore reflects its features. The characterization of the conduction pathways is discussed in relation to the experimentally observed ionic conductivity.