The glass formation of sodium metaphosphate: Perspective from the correlation of thermodynamic and kinetic parameters

Abstract In this work, the glass-forming ability (GFA) of sodium metaphosphate (NaPO3) is focused, and the cooperative nature of its thermodynamics and kinetics is explored. The critical cooling rate (Rc) of this substance is determined, giving a value as low as ~ 0.5 K/s, and the fragility m-index of 83±3 is calculated based on the calorimetric measurements. Among oxide systems with a mixture of covalent-ionic bonds, the compatibility of relatively high fragility of NaPO3 and its good GFA seems an exception. The good GFA is ascribed to its impressive liquidus viscosity (ηL) and the low entropy of fusion normalized by the structural parameter 'beads' (ΔSm/beads). Strikingly, an extended survey of various glass formers delivers a correlation between ηL and ΔSm/beads, verifying the significance of low entropies of fusion for the glass formation. A critical liquidus viscosity (ηL ~ 0.01 Pa-s) is featured, below which this thermo-kinetic correlation cannot be detected.