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Free oxide ion abundances in Na, Ba, and K silicate glasses from O 1s XPS, 29Si NMR, Raman, and MD simulations
G. Michael Bancroft, H. Wayne Nesbitt, John S. Tse, Grant S. Henderson, and Ben J. A. Moulton
2026 (5):
56102-056102.
doi: 10.1088/1674-1056/ae3606
摘要
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In alkali silicate glasses with $\le 50$ mol% $M_{2}$O ($M={\rm Na}$, K, Rb, Cs), the existence of $>1$ mol% reactive ``free'' oxide (FO, where O is not bonded to Si) has been a highly controversial topic over the past 15 years. Unlike their crystalline analogues, Raman and $^{29}$Si nuclear magnetic resonance (NMR) studies since 1980 have shown that two or more $Q^{n}$ ($n=$0-4) species are present in silicate glasses over a wide range of compositions. For example, $M_{2}$SiO$_{3}$ crystals contain only $Q^{2}$ species; however, glasses of the same composition exhibit $Q^{1}$ and $Q^{3}$ in addition to $Q^{2}$. Previous Raman and NMR studies on alkali silicate glasses have related the abundances of these three species solely through disproportionation reactions (e.g. $2{ Q}^{2}\Leftrightarrow { Q}^{1}+{ Q}^{3}$). In doing so, polymerization reactions (e.g. $2{ Q}^{2}\Leftrightarrow 2{ Q}^{3}+{\rm FO}$) were completely neglected. By combining published O 1s x-ray photoelectron spectroscopy (XPS) spectra, $^{29}$Si NMR and Raman results for 40 mol% and 50 mol% Na$_{2}$O, K$_{2}$O, and BaO glasses, together with new molecular dynamics (MD) simulations of Na$_{4}$SiO$_{4}$ glass, we provide consistent and compelling evidence for the existence of $>1$ mol% FO in these glasses and melts. In particular, for 50 mol% K$_{2}$O silicate glass, all three experimental techniques estimate FO to be $\ge 7$ mol%, while MD simulations of Na$_{4}$SiO$_{4}$ yield $\sim 5$ mol% FO. Our analysis requires revised assignments (challenging decades of ``conventional wisdom'') for $^{29}$Si NMR and Raman spectra, based on O mass balance, recognition of M-BO bonding effects first identified in O 1s XPS spectra, and quantitative analysis of Raman spectra for 40-50 mol% Na$_{2}$O, K$_{2}$O, and BaO glasses. These FO values are comparable to those now accepted for alkaline-earth silicate glasses. The importance of this reactive FO for chemical reactivity (e.g. with H$_{2}$O and CO$_{2}$), bioactivity, and physical properties (e.g. melting) of silicate glasses is discussed.
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