Cation Distribution in Mixed-Alkali Silicate Glasses. NMR Studies by 23Na−{7Li} and 23Na−{6Li} Spin Echo Double Resonance

The relative arrangement of sodium and lithium ions in mixed-alkali silicate glases of composition [(Li 2 O) 1- y (Na 2 O) y ] x [SiO 2 ] 1- x ( y = 0.25, 0.50, and 0.75; x = 0.40 and 0.50) is studied by 23 Na−{ 7 Li} spin echo double-resonance (SEDOR) NMR spectroscopy, supplemented by additional 23 Na−{ 6 Li} 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.