Structure of mixed alkali phosphate glasses by neutron diffraction and Raman spectroscopy

The structures of ion conducting mixed alkali metaphosphate glasses (Li₂O)ₓ-(Rb₂O)_1-x-2P₂O₅ 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 only observable change is that the second peak in $G(r),$ arising from the first O-O and P-P distances, increases slightly for increasing rubidium concentration. The intermediate range order seems to increase continuously with increasing rubidium content. The cation environments are distinctly different for Li⁺ and Rb⁺ and each of them remains about the same for all compositions. The nearest Li-O and Rb-O distances are determined to be 2.0±0.1 and 2.9±0.1, and the corresponding average coordination numbers are approximately 4 and 5, respectively. The Li⁺ and Rb⁺ ions are quasiuniformly distributed and we are not able to observe any significant preference for either similar or dissimilar pairs of cations. The clearly different cation environments suggest that the Li⁺ sites are energetically unfavorable to the Rb⁺ ions and vice versa.