Structure of fluorozirconate glasses and melts

We represent a thermodynamic model for fluorozirconate melts which ta kes into consideration the strong negative deviation from ideality ma nifested deep eutectic wells in phase diagrams and exothermic heats o f mixing. The model is based on chemical equiibria between complexes with 6-8fold coordination of Zr. Only 6-coordinated complexes are pre sent in the glass-forming regions. The results of the thermodynamic m odel are related to structural data obtained be infrared and Raman sp ectroscopy. Frequencies of Zr-F stretching vibrations have been found to decrease with increasing n(F)/n(Zr) ratio for both melts and glas ses, which indicates either an increasing Zr coordination number or d ecreased connectivity between ZrFn polyhedra. At constant n(F)/n(Zr) ratio the stretching frequensies increase significantly going from th e glass to the melt. We propose that the coordination number of Zr in fluorozirconate glasses is decreased connectivity between ZrFn polyh edra. At constant n(F)/n(Zr) ratio the stretching frequencies increas e significantly going from the glass to the melt. We propose that the coordination number of Zr in fluorozirconate glasses is decreasing s ignificantly when heating above the glass transition temperature. The fragility of fluorozirconate glasses shown by the non-Arrhenius beha vior of the viscosity is thereby explained by a decreasing coordinati on number of Zr causing the connectivity between ZrFn polyhedra to de crease. The viscosity of fluorozirconate melts demonstrates that the fragility is not very sensitive to changes in composition. However, b oth the glass transition temperature and the viscosity increase with increasing charge of the counter-cations.