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

Abstract In alkali silicate glasses with ≤ 50 mol% M 2 O ( 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 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 ⇔ Q 1 + Q 3 ). In doing so, polymerization reactions (e.g. 2 Q 2 ⇔ 2 Q 3 + 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 ≥ 7 mol%, while MD simulations of Na 4 SiO 4 yield ∼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.