Free oxide ion abundances in Na, Ba, and K silicate glasses from O 1s XPS, ²⁹Si NMR, Raman, and MD simulations

In alkali silicate glasses with ≤ 50 mol% M₂O (M = 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 ²⁹Si nuclear magnetic resonance (NMR) studies since 1980 have shown that two or more Qⁿ (n = 0–4) species are present in silicate glasses over a wide range of compositions. For example, M₂SiO₃ crystals contain only Q² species; however, glasses of the same composition exhibit Q¹ and Q³ in addition to Q². Previous Raman and NMR studies on alkali silicate glasses have related the abundances of these three species solely through disproportionation reactions (e.g. 2Q² ⇔ Q¹ + Q³). In doing so, polymerization reactions (e.g. 2Q² ⇔ 2Q³ + FO) were completely neglected. By combining published O 1s x-ray photoelectron spectroscopy (XPS) spectra, ²⁹Si NMR and Raman results for 40 mol% and 50 mol% Na₂O, K₂O, and BaO glasses, together with new molecular dynamics (MD) simulations of Na₄SiO₄ glass, we provide consistent and compelling evidence for the existence of > 1 mol% FO in these glasses and melts. In particular, for 50 mol% K₂O silicate glass, all three experimental techniques estimate FO to be ≥ 7 mol%, while MD simulations of Na₄SiO₄ yield ∼5 mol% FO. Our analysis requires revised assignments (challenging decades of “conventional wisdom”) for ²⁹Si 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% Na₂O, K₂O, 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 H₂O and CO₂), bioactivity, and physical properties (e.g. melting) of silicate glasses is discussed.