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Assessing the network connectivity of modifier ions in metaphosphate glass melts: A dynamic light scattering study of Na-Zn mixtures.

Photon correlation spectroscopy conducted on polymeric metaphosphate melts [NaPO3]1-y[Zn(PO3)2]y shows a systematic decrease in glass fragility as the more strongly bonding Zn cation replaces the more weakly bonding Na cation as a crosslinking agent between PO3 chains. This decrease is similar to that observed previously in Na-Al melts and the decrease in fragility for both systems is shown to be fully consistent with a recently reported universal pattern of fragility in network forming glasses as a function of network connectivity. Unique to the Na-Zn system is the appearance of an ultraslow relaxation in the dynamic structure factor (slower than the viscoelastic decay) that is not present in either Na-Al or Na-Li metaphosphate mixtures. This relaxation appears to originate from the diffusion of the Zn cation within the melt which is partially coupled to the oxide network. Taken together, these results underscore the need to distinguish between network-forming cations of high ionic bond strength that contribute to the connectivity of the oxide network and those of lower bond strength that do not contribute.