Oxygen 1s XPS spectra of a Pb-silicate glass containing 76.6 mol% PbO provide the first accurate, direct measurement of free oxide ion (O2−) in these glasses. O2− constitutes 35 (±3) mol% of total oxygen, with NBO and BO constituting, respectively, 52 (±3) and 13 (±3) mol%. All 29Si NMR and O 1s XPS results for Pb-silicate glasses indicate mol% levels of O2− containing more than ~30 mol% PbO. The O2− abundances are consistent with equilibrium thermodynamic considerations where K ~ 12 for the mass action equation involving NBO, BO, and O2−.
Raman and 17O NMR spectra of two CaMg-silicate glasses indicate ~10 (±4) mol% O2− in CaMgSiO4 glass and ~18 (±4) mol% O2− in a Ca0.36Mg0.36Si0.28O1.28 glass. Oxygen species abundances are calculated using experimental results from 13 separate 29Si NMR, 17O NMR, and Raman measurements of Mg-, Ca-, and CaMg-silicate glasses. All reveal mol% levels of O2− with ~1 to 2.6 mol% in metasilicate glass and ~5 to 10 mol% in orthosilicate glass. Recent Raman experimental results also indicate O2− in CaMg-silicate glasses at levels ranging from about 1 to 10 mol%. In all there are 23 separate 29Si NMR, 17O NMR, and Raman measurements indicating mol% levels of O2− in alkaline earth silicate glasses. Eight recent MD simulations of Mg, Ca, and CaMg-silicate glasses include 21 separate simulations over a wide compositional range. All indicate mol% levels of O2− in the glasses demonstrating that the MD simulations and experimental results on these systems are in accord.
There are two fundamentally important implications of these studies. First, free oxygen (O2−) is an essential constituent of Pb, Mg, Ca, and CaMg binary silicate melts and glasses. It is not an “accidental” product associated with glass or melt defects. It is instead, a thermodynamically important constituent of these binary melts (and glasses). Second, where melts are equilibrated, the mass action equation relating BO, NBO, and O2− must hold across the entire Ca, Mg, CaMg, and Pb binary systems, thereby requiring the activities and mole fractions of all three species to be defined and finite in the melts. Free oxygen, however, may be too low to be detected in highly siliceous glasses using conventional spectroscopic techniques.