Dynamic light scattering in network-forming sodium ultraphosphate liquids near the glass transition

The viscoelastic relaxation of glass-forming (Na(2)O)(x)(P(2)O(5))(1-x) liquids was measured by photon correlation spectroscopy at temperatures near the glass transition for compositions extending from pure phosphorus pentoxide to the metaphosphate (x=0.5). Over this compositional range, alkali addition produces a continuous depolymerization of the covalently bonded structure from one of a three-dimensional network to that of polymer chains. Substantial increases in the fragility accompany the depolymerization and are shown to be identical to those seen in certain ion-free chalcogenide glass formers suggesting the time scale for viscoelastic relaxation in network-forming liquids is controlled only by the topology of the covalent structure. The relaxation is nonexponential and the stretching exponent shows a complex variation with regards to both composition and temperature that is believed to arise from a decoupling of ionic motions from those of the network occurring as the glass transition is approached.