Exploring entropic contributions of modifier cations in supercooled liquids through isotopic substitution

The vibrational and configurational entropic contributions of modifier Li cations to the glass-to-liquid transition and the fragility of supercooled (Li,Na)PO₃ liquids are investigated using calorimetric measurements of their ⁷Li and ⁶Li isotopologues. Intriguingly, a measurable difference in excess entropy ΔS^exc is observed between the isotopologues, which implies that the modifier Li cations must contribute significantly to the vibrational and configurational entropy of these supercooled liquids. These modifier contributions are shown to influence the thermodynamic and kinetic fragility of these liquids, with more pronounced effects observed in mixed-alkali systems. The larger deviation of heat capacity from the Dulong–Petit limit at the glass transition temperature T_g in mixed-alkali glasses compared to their single-alkali counterparts is attributed to the coupled motion of dissimilar alkali ions in the former. Such motions act as a catalytic step allowing the onset of network rearrangement to occur in mixed-alkali systems without the need for full vibrational excitation of the network at T_g. The lighter ⁶Li isotope facilitates this coupled motion more efficiently than the heavier ⁷Li isotope, resulting in a greater departure from the Dulong–Petit limit of ⁶Li-containing mixed-alkali glasses, compared to their ⁷Li isotopologues.