Phase change alloy viscosities down to T_g using Adam-Gibbs-equation fittings to excess entropy data: A fragile-to-strong transition

A striking anomaly in the viscosity of Te₈₅Ge₁₅ alloys noted by Greer and coworkers from the work of Neumann et al. is reminiscent of the equally striking comparison of liquid tellurium and water anomalies documented long ago by Kanno et al. In view of the power laws that are used to fit the data on water, we analyze the data on Te₈₅Ge₁₅ using the Speedy-Angell power-law form, and find a good account with a singularity T_s only 25 K below the eutectic temperature. However, the heat capacity data in this case are not diverging, but instead exhibit a sharp maximum like that observed in fast cooling in the Molinero-Moore model of water. Applying the Adam-Gibbs viscosity equation to these calorimetric data, we find that there must be a fragile-to-strong liquid transition at the heat capacity peak temperature, and then predict the "strong" liquid course of the viscosity down to T_g at 406 K (403.6 K at 20 K min^-1 in this study). Since crystallization can be avoided by moderately fast cooling in this case, we can check the validity of the extrapolation by making a direct measurement of fragility at T_g, using differential scanning calorimetric techniques, and then comparing with the value from the extrapolated viscosity at T_g. The agreement is encouraging, and prompts discussion of relations between water and phase change alloy anomalies.