Cation distribution in mixed-alkali silicate glasses. NMR studies by Na-23-{Li-7} and Na-23-{Li-6} spin echo double resonance

The relative arrangement of sodium and lithium ions in mixed-alkali silicate glases of composition [(Li2O)(1-y)(Na2O)(y)](x)[SiO2](1-x) (y = 0.25, 0.50, and 0.75; x = 0.40 and 0.50) is studied by Na-23-{Li-7} spin echo double-resonance (SEDOR) NMR spectroscopy, supplemented by additional Na-23-{Li-6} experiments on isotopically labeled materials. The experimental results are compared with various cation distribution scenarios and found to be quantitatively most consistent with a model in which lithium and sodium ions occupy random positions on a cubic lattice mimicking a uniform (homogeneous) spatial cation distribution. The experimental data give no evidence for preferred interactions among unlike cations or cation-pairing models previously invoked to describe the structure of mixed-alkali glasses. To the contrary, if the overall cation distribution were to diverge from homogeneous, the data would be most consistent with preferred interactions among like cations.