High field 25Mg NMR spectroscopy of silicate crystals and glasses up to 35.2 T: Establishment of chemical shift scale and Mg-O coordination environments

Abstract

In this study, we have established an empirical 25Mg chemical shift scale for silicates based on experimentally determined isotropic shift δ iso values for Mg sites in crystalline compounds spanning coordination numbers from four to eight. These data reveal a robust linear correlation between δ iso and the average Mg–O bond distance in the coordination polyhedra, defining approximate δ iso ranges of 50 to 30, 27 to 17, 17 to −5, and ≤ −15 ppm for MgIV, MgV, MgVI, and MgVIII environments, respectively. This scale is applied to a comprehensive slice-by-slice analysis of previously reported ultra-high field (35.2 T) 25Mg 3QMAS NMR spectra of three glasses of composition CaMgSi2O6, Na2MgSi3O8, and K2MgSi5O12, revealing a positive correlation between the quadrupolar coupling constant C Q and δ iso within two structurally distinct Mg environments assignable to MgVI and MgIV. Quantitative analysis indicates that ∼70–75% of Mg is present as MgIV in all three glasses, yielding an average Mg–O coordination number of ∼4.6 for CaMgSi2O6 glass, in close agreement with the value of ∼4.4 reported in a recent isotope-substituted neutron diffraction study. The finding of the lack of any significant dependence of the Mg-O coordination number on the field strength of the second network modifying cation in these glasses is in sharp contrast with previous reports of a preference of Mg for fourfold coordination in silicate glasses containing low field strength alkali cations.