Experimental insight into the mixed mobile ion effect in glasses

The structures of ion conducting mixed-alkali metaphosphate glasses (A2O)x–(B2O)1−x–P2O5 (A,B=Li,Na,Rb) have been examined by neutron diffraction and Raman spectroscopy. The results show that the short range order of the phosphorous–oxygen network is almost independent of the composition. The length-scale of the intermediate range order correlations increases with increasing size of the cation. For all the investigated compositions the nearest Li–O, Na–O and Rb–O distances have been estimated from the atomic pair correlation functions to be 2.0±0.1, 2.35±0.1 (Na–O I), 2.6±0.1 (Na–O II) and 2.9±0.1 Å, and the corresponding average coordination numbers are approximately 4, 3 (I)+2.5 (II) and 5, respectively. Thus, the cation environments are distinctly different for Li+, Na+ and Rb+ and each of them remain the same for all compositions within experimental errors. The alkali ions are quasi-uniformly distributed and we were not able to observe any significant preference for either similar or dissimilar pairs. This implies that the mixed-alkali effect can be qualitatively understood on a purely structural basis. The A sites are energetically unfavourable to the B+ ions (and vice versa) and jumps to dissimilar sites are then unlikely to occur. Thus, the A sites will have the tendency of blocking the pathways for the B+ ions and vice versa.