Thermodynamic features and enthalpy relaxation in a metal–organic framework glass

Abstract

In this work, we explore the thermodynamic evolution in a melt-quenched metal–organic framework glass, formed from ZIF-62 upon heating to the melting point (T_m), and subsequent enthalpy relaxation. The temperature dependence of the difference in Gibbs free energy between the liquid and crystal states of ZIF-62 in the temperature range from the glass transition temperature (T_g) to T_m is found to be weaker than those of other types of glasses, e.g., metallic glasses. Additionally, we find that the stretched exponent of the enthalpy relaxation function in the glass varies significantly (β = 0.44–0.76) upon changing the extent of sub-T_g annealing, compared to metallic and oxide glasses with similar T_gs, suggesting a high degree of structural heterogeneity. Pair distribution function results suggest no significant structural changes during the sub-T_g relaxation in ZIF-62 glass.