Structural disorder and the effects of aging in a phosphate glass: Results from two-dimensional 31P PASS NMR spectroscopy

The nature of the short-range structural disorder and its dependence on the fictive temperature are investigated in a phosphate glass of composition 60ZnO·40P2O5 using 31P two-dimensional phase adjusted spinning sidebands nuclear magnetic resonance (2D PASS NMR) spectroscopy. The 2D PASS experiment is shown to be a robust method for determining the correlations between the 31P isotropic shift and chemical shift tensor parameters and their distributions in this glass that allows for a remarkably detailed quantification of structural disorder. The spectra in the isotropic dimension indicate a Q-species disproportionation reaction in this glass of the type 2Q1 = Q2 + Q0 that is driven to the right hand side with increasing fictive temperature. Such a reaction can be a source of configurational entropy as well as a mechanism of Q-species exchange associated with viscous flow in the supercooled liquid. In the anisotropic dimension, the 31P chemical shift anisotropy (CSA) parameters and their distributions are quantified for the constituent Q-species and are found to be sensitive to the thermal history of the glass. Specifically, the span and the asymmetry parameter of the 31P chemical shift tensor decrease with lowering of the fictive temperature implying a concomitant increase in the P site symmetry resulting from the structural relaxation of the network.