SPATIAL DISTRIBUTIONS AND CHEMICAL ENVIRONMENTS OF CATIONS IN SINGLE-ALKALI AND MIXED-ALKALI BORATE GLASSES - EVIDENCE FROM SOLID-STATE NMR

The structural environments and the spatial distributions of the alkal i ions in single- and mixed-alkali berate glasses are studied by compl ementary solid state NMR techniques. Specifically, spin echo decay spe ctroscopy is used to extract homodipolar second moments for Na-23 and Cs-133 in binary sodium and cesium berate glasses. These values are fo und to be quantitatively most consistent with spatially homogeneous ca tion distributions, except in sodium borate glasses with cation conten ts less than or equal to 16 mole %. Complementary isotropic chemical s hifts extracted from field-dependent magic-angle spinning (MAS)-NMR de pend linearly on alkali ion content, revealing a continuous concomitan t 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 str ongly against cation clustering models previously proposed for other t ypes of glass systems. Isotropic Li-7 and Na-23 chemical shift data me asured for mixed-alkali Li,Na and K,Na-borate glasses containing 30 mo le % alkali oxide indicate universal compositional trends that can be understood in terms of the site-mismatch concept of Bunde's dynamic st ructure model: Consistent with current semi-empirical predictions, mis matching the cation of interest, e.g. Na+ to a smaller Li site produce s a low-frequency shift, while mismatching to a larger K site produces high-frequency Na-23 isotropic chemical shifts.