Characterization of the α-relaxation and gel-like dynamics in mixed network forming (GeO2)x(NaPO3)100-x glass melts by photon correlation spectroscopy

Dynamic light scattering investigations of the $\alpha$-relaxation in network-forming (GeO₂)_x(NaPO₃)_100-x glass melts is reported for x = 0 to 40 mol% GeO₂. Addition of GeO₂ results in an increased density of bridging oxygen bonds. This monotonically increases the glass transition temperature but generates complex changes to the glass forming fragility which initially decreases with increasing GeO₂ but increases again at higher GeO₂ contents. A resolution to this odd fragility behavior is obtained by applying a coarse-graining procedure used previously in sodium germanate melts to arrive at a topological network connectivity for which fragility exhibits a universal dependence. Also observed in the light scattering measurements is a secondary relaxation, several orders of magnitude slower than that of the $\alpha$-relaxation, which we tentatively attribute to gel-like anomalous diffusion of Ge crosslinks formed between phosphate chains.