Heat capacity of glass-forming fluorozirconates

Heat capacities of fluorozirconates between 25 and 800°C have been determined by differential scanning calorimetry (DSC) and transposed-temperature-drop calorimetry. Heat capacities of glasses are relatively similar, while the heat capacities of melts are more distinct among different compositions in the superliquidus range. The average heat capacity between the glass transition temperature (T_g) and the liquidus temperature (T₁) has a value close to the heat capacity of the supercooled liquid just above T_g, but is higher than the heat capacity of high-temperature melts. Therefore, the fluorozirconate exhibits an increased heat capacity in the T_g-T₁ interval. The region of high heat capacity is related to the restructuring in the supercooled liquid regime. This behavior supports our previous study which suggests a substantial structural change from melt to glass. The restructuring gives excess contributions to heat capacity, enthalpy, and entropy and allows the supercooled liquid to significantly lower its free energy (relative to one which does not change structural state), resulting in a diminished thermodynamic driving force for crystallization. Glass formation in fluorozirconates is not only controlled by kinetics but also assisted by restructuring thermodynamics.