STRUCTURE OF FLUOROZIRCONATE GLASSES AND MELTS

We present a thermodynamic model for fluorozirconate melts which takes into consideration the strong negative deviation from ideality manife sted by deep eutectic wells in phase diagrams and exothermic heats of mixing. The model is based on chemical equilibria between complexes wi th 6-8-fold coordination of Zr. Only 6-coordinated complexes are prese nt in the glass-forming regions. The results of the thermodynamic mode l are related to structural data obtained by infrared and Raman spectr oscopy. Frequencies of Zr-F stretching vibrations have been found to d ecrease with increasing n(F)/n(Zr) ratio for both melts and glasses, w hich indicates either an increasing Zr coordination number or decrease d connectivity between ZrFn, polyhedra. At constant n(F)/n(Zr) ratio t he stretching frequencies increase significantly going from the glass to the melt. We propose that the coordination number of Zr in fluorozi rconate glasses is decreasing significantly when heating above the gla ss transition temperature. The fragility of fluorozirconate glasses sh own by the non-Arrhenius behaviour of the viscosity is thereby explain ed by a decreasing coordination number of Zr causing the connectivity between ZrFn polyhedra to decrease. The viscosity of fluorozirconate m elts demonstrates that the fragility is not very sensitive to changes in composition. However, both the glass transition temperature and the viscosity increase with increasing charge of the counter-cations.