Structure of AgxNa1−xPO3 glasses by neutron diffraction and reverse Monte Carlo modelling

Abstract

We have performed structural studies of mixed mobile ion phosphate glassesAgxNa1−xPO3 using diffraction experiments and reverse Monte Carlo simulations. This glass system isparticularly 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 studiedmixed alkali phosphate glasses, with a much more pronounced mixed mobile ioneffect, we find no substantial structural alterations of the phosphorous–oxygennetwork and the local coordination of the mobile cations. Furthermore, the mobileAg+ and Na+ ions are randomly mixed with no detectable preference for either similar or dissimilar pairsof cations. However, in contrast to mixed mobile ion systems with a very pronouncedmixed mobile ion effect, the two types of mobile ions have, in this case, verysimilar local environments. For all the studied glass compositions the averageAg–O and Na–O distances in the first coordination shell are determined to be2.5 ± 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 fora Na+ ion to occupy a site that previously has been occupied by aAg+ ion (and vice versa) is low, and that this low energy mismatch is responsible for theanomalously weak mixed mobile ion effect.