Heat capacities of tellurites and restructuring thermodynamics on glass formation

An increased heat capacity of a supercooled liquid, compared to its superliquidus melt, has been measured in situ in some glass-forming tellurite systems by differential scanning calorimetry (DSC), complementing earlier work on fluorozirconates. This increased heat capacity confirms that restructuring occurs in the supercooled liquid regime, in good agreement with structural investigations. The thermodynamics of restructuring decrease the free energy of the super-cooled liquid, and hence diminish the thermodynamic driving force for crystallization, as evaluated using the classical nucleation approach. Glass formation thus results from a combination of restructuring thermodynamics and kinetics. Thermodynamic aspects of glass formation, in fragile, intermediate and strong systems, are systematized by free energy versus temperature diagrams within the supercooled liquid regime. A common basis for glass formation is revealed, that is, a glass-forming liquid has a tendency to retain its high-temperature liquid structure for some temperature range below the liquidus. Major structural change will not take place until much lower temperatures, and culminates in the glass transition. This viewpoint suggests that glass formation is decisively controlled by physical and chemical properties in the high-temperature liquid.