Thermodynamics and glass formation in fluorozirconates and related systems

High-temperature calorimetric techniques were used to measure enthalpies of mixing of melts and enthalpies of formation of glasses and crystals in ZrF{dollar}sb4{dollar}-based glass forming systems. Due to the strong acid-base chemistry in fluorozirconate melts, high-temperature calorimetry provides an objective basis for making a distinction between complex-formers and fluoride ion donors, and gives insights into molten structures. The data suggest that glass formation from the melt involves a substantial structural change. The melt is dominated by ZrF{dollar}sbsp{lcub}5{rcub}{lcub}-{rcub}{dollar} species while the glass is composed of ZrF{dollar}sb7{dollar} and ZrF{dollar}sb8{dollar} polyhedra. This restructuring was evident as a relatively high average heat capacity of the supercooled liquid compared to those of its glass and high-temperature melt. Such an increased heat capacity of the supercooled liquid was also confirmed in situ in tellurite glass-forming systems by differential scanning calorimetry.; The thermodynamics of restructuring lowers the free energy of the supercooled liquid, and hence diminishes the thermodynamic driving force for crystallization, as evaluated using the classical nucleation approach. Glass formation is further discussed in terms of restructuring thermodynamics and kinetics within the supercooled liquid regime using free energy versus temperature diagrams in systems varying from fragile fluorozirconates to strong silicates. The entire supercooled liquid state can be considered as a gradual phase transition between the superliquidus molten state and the glassy state. A common basis for glass formation is shown, that is, a glass-forming liquid has a tendency to retain its high-temperature molten structure for a significant range below the liquidus. Major structural change will not take place until much lower temperatures, culminating in the glass transition.; In addition, emphasis is placed upon developing a relation among thermodynamics of high-temperature melts, deep eutectics in phase diagrams, and glass formation. Special attention is given to the thermodynamic basis of immiscibility. A glass formation principle is concluded, suggesting that retention of high-temperature molten structure in the supercooled liquid regime is assisted by (a) high viscosity, (b) formation of stable complex species, and/or (c) entropy stabilization from the mixing of distinct but energetically similar species in the melt.