Structure and Fragility of Zn-phosphate glasses: Results from multinuclear NMR spectroscopy and calorimetry

Abstract

The structure of (ZnO)x(P2O5)100-x glasses with 35≤x ≤ 67 is investigated using 31P and ultra-high field 67Zn NMR spectroscopy. The 31P NMR results confirm a nearly binary Q-species distribution. The progressive decrease in the connectivity of the phosphate network upon addition of ZnO is manifested in a monotonic drop in Tg in the composition range 35≤x ≤ 55. The 67Zn NMR spectra indicate that the average Zn-O coordination number varies between 4 and 6 and increases progressively with increasing Zn content. The resulting increase in the connectivity of the Zn-O-P network compensates for the depolymerization of the P-O-P network, which is reflected in a reversal in the compositional trend in Tg for x > 55. In contrast to Tg, the fragility index m shows negligible composition dependence until x∼50, beyond which it increases rapidly with increasing Zn content in response to the progressive shortening of the Q2 chains.