Mixed alkali glass spectra and structure

The far infrared and Raman spectra of several series of mixed alkali metaphosphate glasses have been investigated as a function of the mole fraction x of the network-modifying ionic oxides in xM₂O(1?x)M₂′O · P₂O₅. The frequencies of the cation-motion bands in the far infrared spectra, which correspond to cationsite vibrations, do not shift with x, indicating that the vibrationally significant local geometry and forces associated with a particular cation are unaffected by the introduction of the second cation into the glass structure. Each Raman-active band due to vibrations of the metaphosphate network occurs at a different frequency for each pure glass (x = 0 or 1), but for mixed alkali glasses only one band occurs for each type of mode and it varies linearly with x. This indicates that the cations in these mixed alkali glasses are homogeneously distributed, there is no significant molecular-level domain formation and the phosphate chains are associated with an averaged cation environment whose effect on the chain modes varies with x. A simple vibrational model is presented which shows that the cation-dependent shifts are due to small changes in network bond angles and variation of the cationsite forces.