Germanium selenide glasses of compositions spanning the whole glass-formation range are aged at room temperature for up to twenty years. A prominent enthalpy relaxation process is observed in all glasses and its structural origin is analyzed by Raman spectroscopy. The structural relaxation is manifested in the Raman spectra as a decrease in the ratio of edge to corner-sharing GeSe 4/2 tetrahedral units. This structural evolution can be explained in terms of configurational entropy and density changes. Changes in Raman features and enthalpy follow an identical stretched exponential relaxation function characteristic of ageing in glasses. The compositional dependence of enthalpy relaxation after twenty years is in agreement with kinetic considerations based on the glass transition temperature of each glass. The relaxation behavior and heat-capacity curves are consistent with standard glass relaxation models for all compositions. These results indicate that the non-reversing enthalpy obtained by modulated differential scanning calorimetry (MDSC) which suggests the existence of non-aging glasses is not a reliable measure of the ability of a glass to relax. Instead, it is suggested that an interpretation of MDSC data in terms of complex heat capacity provides a more complete and reliable assessment of the relaxation properties of glasses. I.

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Many phase change materials (PCMs) are found to crystallize without exhibiting a glass transition endotherm upon reheating. In this paper we review experimental evidence revealing that these PCMs and likely other hyperquenched molecular and metallic systems can crystallize from the glassy state when reheated at standard rate. Among these evidences, PCMs annealed below the glass transition temperature Tg exhibit slower crystallization kinetics despite an increase in number of sub-critical nuclei that should promote crystallization speed. Flash calorimetry uncovers the glass transition endotherm hidden by crystallization and reveals a distinct change in kinetics when crystallization switches from the glassy to the supercooled liquid state. The resulting Tg value also rationalizes the presence of the pre- Tg relaxation exotherm ubiquitous of hyperquenched systems. Finally, the shift in crystallization temperature during annealing exhibit a non-exponential decay that is characteristic of structural relaxation in glass. Modeling using a modified Turnbull equation for nucleation rate supports the existence of sub- Tg fast crystallization and emphasizes the benefit of a fragile-to-strong transition for PCM applications due to a reduction in crystallization at low temperature (improved data retention) and increasing its speed at high temperature (faster computing).