Physical Thermodynamic Model of Alkali and Alkaline-Earth Fluorozirconate Glass-Forming Melts

A physical model for the thermodynamics of glass-forming alkali and alkaline-earth fluorozirconate melts is presented that takes into consideration the strong negative deviation from ideal behavior that is manifested by deep eutectic wells in the phase diagrams and large exothermic heats of mixing. The model is based on chemical equilibria between anionic complexes with six- to eight-fold coordination of zirconium and a simple Temkin model for the free energy of mixing. Calculated phase diagrams and liquidliquid heats of mixing agree well with the experimental data. The model predicts six-fold coordination of zirconium in glass-forming melts, which implies that the fragile nature of fluorozirconate glasses can be explained by a significant increase in the coordination of zirconium when the melt is quenched to the glassy state.