Impact of nitridation of metaphosphate glasses on liquid fragility

• We study glass transition, dynamics, and structure of oxynitride metaphosphate glasses.
• Nitridation causes increase in glass transition width (ΔTg) and decrease in liquid fragility (m).
• Jump in heat capacity at glass transition (ΔCp) does not vary systematically with N/P.
• Our results imply that the liquids become “stronger” upon nitridation.

Oxynitride glasses are mixed-anion glasses, in which the oxygen atoms have been partially substituted by either two-fold (Nd) or three-fold (Nt) coordinated nitrogen atoms, introducing additional bonds and thereby constraining the glass network and enhancing the atomic packing density. Phosphate glasses are more prone to undergo nitridation via ammonolysis at relatively low temperature compared to other types of oxide glasses, but here the question is how the nitridation influences the glass transition, structure, and dynamics in phosphate glasses. Here, we answer this question by taking sodium and lithium metaphosphate oxynitride glasses as model systems. Using differential scanning calorimetry (DSC), we measure the effect of the atomic nitrogen/phosphorus (N/P) ratio on the glass transition temperature (Tg), jump in isobaric heat capacity at the glass transition (ΔCp), glass transition width (ΔTg), and liquid fragility index (m) of these model glasses. The results show that the nitridation causes an increase in the atomic packing density and Tg. In addition, ΔTg increases and m decreases with increasing N/P, both implying that the liquids become stronger upon nitridation, whereas ΔCp does not vary systematically with N/P within the error range. We discuss the origin of these changes in terms of the underlying changes in the short- and intermediate-range order structures, as detected by Raman spectroscopy.