Correlative activation free energy (CAFE) model: Application to viscous processes in inorganic oxide glass-formers

We explore the concept of correlative activation free energy (CAFE) for the analysis and interpretation of viscosity data of a collection of 847 inorganic oxide glass formers that exhibit various degrees of non-Arrhenius behavior. The CAFE model formalism is strictly based on transition state theory and accounts for the variation in the activation barrier height for the viscous dissipation due to the structural evolution the system undergoes upon traversing the glass transition regime. Thus, fitting parameters are meaningful in a statistical thermodynamic context. Compared to the VFT and MYEGA equations, fits using the CAFE model are more robust when extrapolating to infinite temperature because the latter encodes non-enthalpic contributions to the rate coefficient more realistically. The CAFE model-based analysis reveals a strong connection between melt fragility and the degree of change in the potential energy landscape with temperature. Accordingly, while the average ground-state potential energy of the glass forming liquid gradually increases with temperature, the energy of the activated state remains relatively invariant. Our analysis also allows one to estimate the number of atoms involved in the viscous relaxation process, which ranges from approximately 10 to 50 atoms for the oxide glass formers studied. We observe that thermally activated dynamic phenomena exhibited by glassy networks, such as the mixed alkali effect, persist into the supercooled liquid state. Graphical abstract for “Correlative activation free energy (CAFE) model: Application to Viscous Processes in Inorganic Oxide Glass-Formers,” by C. Beg, J. Byeong, N. Berman, and J. Kieffer. This graphic depicts how the temperature dependence of viscosity coefficients, measured by means of a steady-state shear deformation at a macroscopic scale, is interpreted using the proposed correlative activation free energy (CAFE) model to yield information about a glass forming system's energy landscape and the viscous dissipation mechanism. Typical glass forming liquids exhibit viscoelastic responses that ubiquitously deviate from Arrhenius behavior, which gave rise to the strong-fragile classification. The CAFE model quantifies such behavior in terms of a free energy landscape that changes with temperature, i.e., both the ground state potential energy and configurational entropy. Accordingly, the more fragile the glass former, the more substantial the change in the activation free energy as a functio