Spatial distributions and chemical environments of cations in single- and mixed alkali borate glasses: Evidence from solid state NMR

The structural environments and the spatial distributions of the alkali ions in single- and mixed-alkali borate glasses are studied by complementary solid state NMR techniques. Specifically, spin echo decay spectroscopy is used to extract homodipolar second moments for Na-23 and Cs-133 in binary sodium and cesium borate glasses. These values are found to be quantitatively most consistent with spatially homogeneous cation distributions, except in sodium borate glasses with cation contents less than or equal to 16 mole %. Complementary isotropic chemical shifts extracted from field-dependent magic-angle spinning (MAS)-NMR depend linearly on alkali ion content, revealing a continuous concomitant change in the oxygen environment of the alkali ions. This effect can be related to structural changes in the network, where trigonal BO3/2 units are progressively converted to tetrahedral BO4/2- sites as the alkali oxide content is increased. Taken together these data argue strongly against cation clustering models previously proposed for other types of glass systems. Isotropic Li-7 and Na-23 chemical shift data measured for mixed-alkali Li,Na and K,Na-borate glasses containing 30 mole % alkali oxide indicate universal compositional trends that can be understood in terms of the site-mismatch concept of Bunde's dynamic structure model: Consistent with current semi-empirical predictions, mismatching the cation of interest, e.g. Na+ to a smaller Li site produces a low-frequency shift, while mismatching to a larger K site produces high-frequency Na-23 isotropic chemical shifts.