Unified approach for uncovering the structural role of Mg in amorphous materials: Diopside glass

The ²⁵Mg nuclear magnetic resonance (NMR) spectroscopy of diopside (CaMgSi₂O₆) glass is revisited by comparing the data acquired at moderate (14.1 T) magnetic fields with literature data at ultrahigh (35.2 T) magnetic fields to probe the informational content of measured field dependent spectra. The glass structure was modeled using ab initio molecular dynamics (AIMD) and classical molecular dynamics (CMD) simulations using Pedone potentials. It shows a distribution of 4-, 5- and 6-coordinated magnesium atoms which yields an average Mg-O coordination number of 4.7 ± 0.1 (AIMD) or 5.0 ± 0.1 (CMD), close to that found from diffraction experiments. Density functional theory was then used to provide the nuclear magnetic resonance (NMR) parameters for the individual magnesium sites and the ²⁵Mg magic angle spinning (MAS) NMR spectra were calculated accordingly. The results establish a ²⁵Mg chemical shift scale as a function of the mean Mg-O distance but do not show a convincing correlation between the ²⁵Mg isotropic chemical shift and the coordination number. They show that while ultrahigh field NMR significantly enhances the chances of detecting bimodal distributions, owing to the diminishing effect of line broadening by quadrupolar interactions, there will be ambiguity in the assignment and quantification of the features to different Mg coordination states. The findings are a prerequisite for establishing quantitative structure-property relations for Mg-bearing silicate glasses.