PHYSICAL THERMODYNAMIC MODEL OF ALKALI AND ALKALINE-EARTH FLUOROZIRCONATE GLASS-FORMING MELTS

A physical model for the thermodynamics of glass-forming alkali and al kaline-earth fluorozirconate melts is presented that takes into consid eration the strong negative deviation from ideal behavior that is mani fested by deep eutectic wells in the phase diagrams and large exotherm ic heats of mixing. The model is based on chemical equilibria between anionic complexes with six- to eight-fold coordination of zirconium an d a simple Temkin model for the free energy of mixing. Calculated phas e diagrams and liquid-liquid heats of mixing agree well with the exper imental data, The model predicts six-fold coordination of zirconium in glass-forming melts, which implies that the fragile nature of fluoroz irconate glasses can be explained by a significant increase in the coo rdination of zirconium when the melt is quenched to the glassy state.