Experimental insight into the mixed mobile ion effect in glasses

The structures of ion conducting mixed-alkali metaphosphate glasses (A(2)O) (x)-(B2O)(1-x)-P2O5 (A,B = Li,Na,Rb) have been examined by neutron diffract ion and Raman spectroscopy. The results show that the short range order of the phosphorous-oxygen network is almost independent of the composition. Th e length-scale of the intermediate range order correlations increases with increasing size of the cation. For all the investigated compositions the ne arest Li-O, Na-O and Rb-O distances have been estimated from the atomic pai r 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 Angstrom, and the corresponding average coordination n umbers 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. T he alkali ions are quasi-uniformly distributed and we were not able to obse rve 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 f or the B+ ions and vice versa. (C) 2000 Elsevier Science B.V. All rights re served.