Dynamic light scattering in mixed alkali metaphosphate glass forming liquids.

We report the first ever photon correlation spectroscopy performed on single alkali and mixed alkali metaphosphate glasses at refractory temperatures above the glass transition. We find not only a significant decrease in the glass transition temperature but also a decrease in fragility for the mixed alkali composition as compared with the single akali glasses. We argue that structural relaxation in these polymeric oxide glasses is largely controlled by the cross linking cations and that the changes in fragility that we observed are a reflection of changes in the cooperativity of structural relaxation wrought by the substantial decrease in the ion mobility that accompanies the mixing of alkali ions.