Structure of Ag_xNa_1-xPO₃ glasses by neutron diffraction and reverse Monte Carlo modelling

We have performed structural studies of mixed mobile ion phosphate glasses Ag_xNa_1-xPO₃ using diffraction experiments and reverse Monte Carlo simulations. This glass system is particularly interesting as a model system for investigations of the mixed mobile ion effect, due to its anomalously low magnitude in the system. As for previously studied mixed alkali phosphate glasses, with a much more pronounced mixed mobile ion effect, we find no substantial structural alterations of the phosphorous-oxygen network and the local coordination of the mobile cations. Furthermore, the mobile Ag⁺ and Na⁺ ions are randomly mixed with no detectable preference for either similar or dissimilar pairs of cations. However, in contrast to mixed mobile ion systems with a very pronounced mixed mobile ion effect, the two types of mobile ions have, in this case, very similar local environments. For all the studied glass compositions the average Ag-O and Na-O distances in the first coordination shell are determined to be 2.5 ± 0.1 and 2.5 ± 0.1 Å, and the corresponding average coordination numbers are approximately 3.2 and 3.7, respectively. The similar local coordinations of the two types of mobile ions suggests that the energy mismatch for a Na⁺ ion to occupy a site that previously has been occupied by a Ag⁺ ion (and vice versa) is low, and that this low energy mismatch is responsible for the anomalously weak mixed mobile ion effect.