Detection of “free” oxide ions in low-silica Ca/Mg silicate glasses: Results from 17O → 29Si HETCOR NMR

• 17O to 29Si HETCOR NMR can select oxygen sites with and w/o Si neighbors in silicates.
• We tested this selectivity on crystalline Ca3SiO5, which contains ‘free’ oxide ions.
• In Ca/Mg silicate glasses with 29–33% SiO2, HETCOR NMR reveals such ‘free’ oxide ions.
• These ‘free’ oxide ions are largely coordinated by Mg2 +, not a Ca/Mg mixture.

Silicon and oxygen are the two most abundant elements in most technological and geological glasses and glass-forming liquids, and extensive studies of the structures of these materials have been made using 29Si and 17O high-resolution solid-state NMR. Double resonance NMR experiments hold great potential for revealing new details about the network structures of such important materials but have not yet been done for this particular pair of spins because of several technical challenges. Using a crystalline model compound, Ca3SiO5 (calcium trisilicate or the ‘alite’ phase of Portland cement), which contains oxide ion sites bonded either to one Si4 + or only to Ca2 +, we demonstrate that the 17O → 29Si Cross-Polarization Heteronuclear Correlation (CP-HETCOR) experiment can clearly distinguish between these two types of oxygen sites. The latter species, so-called ‘free’ oxide ions that are bonded to no network cations such as Si4 +, play an important role in models of glass and melt structure but have been difficult to directly detect in many systems. In two Ca/Mg silicate glasses near to the orthosilicate composition (O/Si ≥ 4), the HETCOR experiment clearly shows the presence of such ‘free’ oxide ions, whose 1D 17O NMR signals are actually completely hidden under those for the predominant non-bridging oxygens. Their observed chemical shifts suggest coordination by predominantly Mg2 + cations, an ordering that may be expected from the smaller size and stronger bonding interactions of this cation relative to those of Ca2 +. The estimated abundances of ‘free’ oxide ions in these glasses are consistent with previous, indirect estimates based on analyses of 1D 29Si NMR spectra.