Abstract
•Viscous relaxation in mixed network forming glasses studied by dynamic light scattering•Addition of GeO2 to NaPO3 increases crosslinking between phosphate chains•Gel-like diffusive mode is seen in the dynamic structure factor•Origin of the diffusive motion is traced to agitation of Ge crosslinks by the α-relaxation
Dynamic light scattering investigations of the α-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 α-relaxation, which we tentatively attribute to gel-like anomalous diffusion of Ge crosslinks formed between phosphate chains.