Investigation of Sodium Distribution in Phosphate Glasses Using Spin-echo ²³Na NMR

The spatial arrangements of sodium cations for a series of sodium phosphate glasses, xNa2O•(100-x)P2O5 (x ≤ 55), were investigated using 23Na spin-echo NMR spectroscopy. The spin-echo decay rate is a function of the Na-Na homonuclear dipolar coupling, and is related to the spatial proximity of neighboring Na nuclei. The spin-echo decay rate in these sodium phosphate glasses increases nonlinearly with higher sodium number density, and thus provides a measure of the Na-Na extended range order. The results of these 23Na NMR experiments are discussed within the context of several structural models, including a decimated crystal lattice model, cubic dilation lattice model, a hard sphere (HS) random distribution model, and a pairwise cluster hard sphere model. While the experimental 23Na spin-echo M2 are described adequately by both the decimated lattice and the random HS models, it is demonstrated that the slight nonlinear behavior of M2 as a function of sodium number density is more correctly described by the random distribution in the HS model. At low sodium number densities the experimental M2 is inconsistent with models incorporating Na-Na clustering. The ability to distinguish between Na-Na clusters and nonclustered distributions becomes more difficult at higher sodium concentrations