Evidence for Mixed Mg Coordination Environments in Silicate Glasses: Results from ²⁵Mg NMR Spectroscopy at 35.2 T

The Mg-O coordination environment of silicate glasses of composition CaMgSi₂O₆, Na₂MgSi₃O₈, and K₂MgSi₅O₁₂ is probed using ultrahigh-field (35.2 T) ²⁵Mg magic angle spinning nuclear magnetic resonance (MAS NMR) and triple-quantum MAS NMR spectroscopy. These spectra clearly reveal the coexistence of 4-fold- (Mg^IV) and 6-fold- (Mg^VI) coordinated Mg in all glasses. The Mg^IV/Mg^VI ratio implies an average Mg-O coordination number of ∼5 for CaMgSi₂O₆ glass, bringing NMR results for the first time in good agreement with those reported in previous studies based on diffraction and X-ray absorption spectroscopy, thus resolving a decade-long controversy regarding Mg coordination in alkaline-earth silicate glasses. The Mg-O coordination number decreases to ∼4.5 in the alkali-Mg silicate glasses, indicating that Mg competes effectively with the low field strength alkali cations for the nonbridging oxygen in the structure to attain tetrahedral coordination. This work illustrates the promise of ultrahigh-field NMR spectroscopy in structural studies involving nuclides with low gyromagnetic ratio.