Local coordination and spatial distribution of cations in mixed-alkali borate glasses

Abstract

A comprehensive NMR study is presented to elucidate the local coordination and spatial distribution of the alkali-metal cations in mixed alkali borate glasses [(M₂O)_x(Na₂O)_1−x]_0.3(B₂O₃)_0.7 (M = Li, K and x = 0.2, 0.4, 0.6, 0.8, 1.0). ¹¹B MAS-NMR results indicate that the network structure in these glasses is constant and remains unaffected by the cation substitution process. ¹¹B{²³Na} rotational echo double resonance (REDOR) results reveal that the anionic BO_4/2 ⁻ groups and the neutral BO_3/2 units interact equally strongly with sodium indicating the absence of cation clustering. The corresponding dipolar second moments extracted from the ¹¹B{²³Na} REDOR curves scale linearly with Na content, consistent with random mixing of the alkali ions. More quantitative information is available on the basis of ²³Na–²³Na and ²³Na–^6,7Li dipole–dipole couplings measured by spin-echo double resonance (SEDOR) spectroscopy. The experimental results are compared with various cation distribution scenarios and found to be most consistent with a model in which the different types of ions are statistically mixed within a homogeneous distribution of the entire cation population. These results are consistent with the view that the mixed alkali effect is due to the site mismatch between unlike cations A and B. Close inspection of the ²³Na chemical shift trends reveals that the cation sites in mixed alkali glasses are modified in a universal manner: the sites of the larger cation are compressed while those of the smaller ion are expanded. This effect creates a secondary type of site mismatch impeding ion transport between regular A sites and A′ sites in the vicinity of a substituent cation.