Behaviour of viscosity in metaphosphate glasses

• Fragility of phosphate glasses depends upon the model used for fitting viscosity.
• Fragility increases with the modifier cationic potential showing a minimum for Zn.
• Ea for viscous flow above Tg is greatly influenced by bond strength and molar volume.
• Activation energy of the melt depends slightly upon the modifier cationic potential.
• There seems to be a close relationship between Doremus ratio and the NMR anisotropy.

The experimental viscosity data of a series of alkali, alkaline earth and zinc metaphosphate glasses have been analysed using Vogel–Fulcher–Tamman, Avramov–Milchev and Mauro–Yue–Ellison–Gupta–Allan models. The kinetic fragility and activation energy for viscous flow have been obtained and studied as a function of the composition, paying special attention to the short and intermediate range order structure as studied by Nuclear Magnetic Resonance and Raman spectroscopy. The fragility shows an increasing tendency within the alkali and alkaline earth series and presents the lowest value for the Zn(PO3)2 composition. Meanwhile the modifiers influence on the local structure of the PO4 tetrahedra follows a direct relationship with their cationic potential; both static Nuclear Magnetic Resonance and Raman experiments showed evidence of a change in the rings versus chains configurations at the medium range order that could have an important contribution on the variation of activation energy. Furthermore, the molar volume and average bond strength of the glasses have shown to play a most important role, having a similar variation as the activation energy in the high viscosity range. However, the low viscosity range data seem to increase with the modifier's cationic potential, thus suggesting a different flow mechanism when compared with the high viscosity range.