Sodium Environments in Dry and Hydrous Albite Glasses: Improved 23Na Solid State NMR Data and Their Implications for Water Dissolution Mechanisms

Abstract

Abstract-The sodium environments in albite glasses with water concentrations ranging from 0 to 60 mol% were studied using 23 Na off-resonance quadrupole nutation and magic angle spinning (MAS) NMR spectroscopy. Crystalline albite was used as a model compound to demonstrate that off-resonance nutation is a suitable method for determination of the quadrupole coupling constant (C q ) for 23 Na. Off-resonance nutation experiments gave a mean C q ϭ 1.75 Ϯ 0.2 MHz for all the albite glasses studied here. MAS NMR experiments were performed at three magnetic fields, 7.05 T, 9.4 T, and 14.1 T in order to deduce the mean isotropic chemical shift, ␦ iso , and to provide an independent measurement of the values of C q . The mean isotropic chemical shift is a strong function of dissolved water concentration, but the mean C q is essentially constant at 2.1-2.2 Ϯ 0.2 MHz over the water concentration range studied. The distributions of both chemical shift and quadrupolar interactions decreases markedly with increasing water concentration, consistent with earlier suggestions that the hydrous glasses have a much more ordered structure. These new data using off-resonance nutation and faster MAS combined with higher applied magnetic fields supersede the 23 Na NMR data of Kohn et al. (1989a) and should be used in preference in devising or testing models for water dissolution mechanisms in albite melts and glasses. Our revised data provide no evidence for a change in water dissolution mechanism at 30 mol% H 2 O, but the other conclusions of Kohn et al. (1989a) and the principal features of the dissolution mechanism developed by Kohn et al. (1989aKohn et al. ( , 1992Kohn et al. ( , 1994 are essentially unchanged.

Doping of a high calcium oxide metaphosphate glass with titanium dioxide

“…The lineshapes are broad and asymmetric with a tail on the low frequency side. This shape is characteristic of sodium observed in a glass where there is a spread of quadrupolar interactions [18,19]. Despite the lack of sharp features, a fit of these spectra is possible if multiple field data is considered, as in Fig.…”

Recent Advances in Solid-State 25Mg NMR Spectroscopy

“…22 As the MAS rate increased, there was a shift in the peak maximum, and the linewidth increased as sites with larger quadrupole coupling constants became successively narrowed. 159,160 A larger range of structures are probably present in the diopside glass compared to the leucite composition as there was a greater change in the spectrum observed in increasing the spinning speed from 10 to 20kHz. A key observation from this initial work was that the most deshielded (positively shifted) edge of the spectra of glassy and corresponding crystalline materials did not change very much, indicating insignificant differences in d iso values and thus suggesting that the magnesium coordination in the glass was similar to that in the original crystal.…”

Recent technique developments and applications of solid state NMR in characterising inorganic materials

“…The other point about the wider availability of particularly high field magnets and appropriate MAS probes is the variable field approach to determining quadrupolar interaction parameters [3,8] becoming more widely used. The differing field dependencies of the second-order quadrupolar interaction ( B o 1 , in Hz) and chemical shift interaction ( B o , in Hz) means that measurements at two fields, with three or more preferred, much better constrain the NMR interactions determined.…”

“…A further study using MAS at three magnetic fields on a different range of model inorganic compounds found Q in the range 1.6-23.4 MHz. In looking at the isotropic chemical shift range it appears that the chemical shift in these compounds was dominated by the local coordination number, with ScO 6 and ScO 8 showing a shift difference of ~200 ppm [56]. Applying 45 Sc MAS NMR to the solid solution Sc 2 (WO 4 ) 3 -Sc 2 (MoO 4 ) 3 suggested a small average Q of 3.6 MHz, with spectra changing in a way consistent with random mixing of the tungsten and molybdenum over tetrahedral sites [57].…”