Cation Ordering Scenarios in Mixed Alkali Silicate Glasses. Experimental Constraints from ~(23)Na{~6Li} Spin Echo Double Resonance NMR

摘要

The spatial arrangement of sodium and lithium ions in mixed-alkali silicate glasses of composition [(Li_2O)_(1-y)(Na_2O)_y]_(0.4)[SiO_2]_(0.6) (y = 0.25, 0.50 and 0.75) is explored by ~(23)Na{~6Li} spin echo double resonance (SEDOR) NMR spectroscopy on isotopically labelled materials. The results are discussed within the context of two basic models for the spatial cation distribution, both of which are calculated with the constraint of resulting in the same cation density as experimentally measured for the glasses: The first scenario considers a homogeneous distribution of the cations by arranging them at regular distances on a cubic lattice. For this base model, a statistical mutual arrangement of lithium and sodium is in fairly good agreement with the experimental data. The second scenario models a clustered distribution by fractionally occupying the cation sites of the Na_2O lattice in a random fashion. If this base model obtains, the experimental NMR results indicate preferential interactions among like cations. Approximate agreement with the experimental SEDOR decay curves can be obtained by assuming a flat distribution function of M_2~(Na-Li) values, ranging from zero to M_(20), the average second moments calculated for a statistical mutual distribution of Li and Na ions. This model also gives satisfactory fits to previously published ~(23)Na{~7Li} SEDOR data on compositional analogous glasses with the natural isotope distribution.