Cation field strength effects on high pressure aluminosilicate glass structure: Multinuclear NMR and La XAFS results

We examined aluminosilicate glasses containing a variety of network modifying to intermediate cations (Li, La, Sc, and Fe), quenched from melts at 1 atm to 8 GPa, to further investigate the role of cation field strength in Al coordination changes and densification. ^(27)Al Nuclear Magnetic Resonance Spectroscopy (NMR) reveals that the mean Al coordination increases with increasing pressure in the Li-containing glasses, which can be explained by a linear dependence of fractional change in Al coordination number on cation field strengths in similar K-, Na-, and Ca-containing aluminosilicate glasses (K < Na < Li < Ca). Measured recovered densities follow a similar linear trend. In contrast, the La-containing glasses have significantly lower mean Al coordination numbers at given pressures than the cation field strength of La and glass density would predict. La L_3 X-ray absorption fine structure (XAFS) spectroscopy results indicate a significant increase with pressure in average La–O bond distances, suggesting that La and Al may be “competing” for higher coordinated sites and hence that both play a significant role in the densification of these glasses, especially in the lower pressure range. However, in Na aluminosilicate glasses with small amounts of Sc, ^(45)Sc NMR reveals only modest Sc coordination changes, which do not seem to significantly affect the mean Al coordination values. For a Li aluminosilicate glass, ^(17)O MAS and multiple quantum magic angle spinning (3QMAS) NMR data are consistent with generation of more highly coordinated Al at the expense of non-bridging oxygen (NBO), whereas La aluminosilicate glasses have roughly constant O environments, even up to 8 GPa. Finally, we demonstrate that useful ^(23)Na and ^(27)Al MAS NMR spectra can be collected for Ca–Na aluminosilicate glasses containing up to 5 wt.% Fe oxide. We discuss the types of structural changes that may accompany density increases with pressure and how these structural changes are affected by the presence of different cations