A model for phosphate glass topology considering the modifying ion sub-network (Journal of Chemical Physics (2014) 140 (154501))

In a recent paper, Hermansen, Mauro, and Yue [J. Chem. Phys. 140, 154501 (2014)] applied the temperature-dependent constraint theory to model both the glass transition temperature, T_g, and fragility, m, of a series of binary alkali phosphate glasses of the form R 2 O x P 2 O 5 1 - x, where R represents an alkali species. Key to their success seems to be the retention of linear constraints between the alkali ion (R⁺) and the non-bridging oxygens near T_g, which allows the model to mimic a supposed minimum for both T_g(x) and m(x) located near x = 0.2. However, the authors have overlooked several recent studies that clearly show there is no minimum in m(x). We argue that the retention of the alkali ion constraints at these temperatures is unjustified and question whether the model calculations can be revised to meet the actual experimental data. We also discuss alternative interpretations for the fragility based on two-state thermodynamics that can accurately account for its compositional dependence.