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, xNa(2)O .(100-x)P2O5 (x less than or equal to 55), were investigated using Na-23 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 Na-23 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 Na-23 spin-echo M-2 are described :adequately by both the decimated lattice and the random HS models, it is demonstrated that the slight nonlinear behavior of M-2 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 M-2 is inconsistent with models incorporating Na-Na clustering. The ability to distinguish between Na-Na clusters and nonclustered distributions becomes mon difficult at higher sodium concentrations.