Structure of Ag x Na 1−x PO 3 glasses by neutron diffraction and reverse Monte Carlo modelling

Artikel i vetenskaplig tidskrift, 2007

We have performed structural studies of mixed mobile ion phosphate glasses Ag x Na 1−x PO 3 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. (Some figures in this article are in colour only in the electronic version).