What is it about?
The structure of vitreous enstatite (MgSiO3) and diopside (CaMgSi2O6) was investigated by static wideline and high-resolution magic angle spinning 25Mg NMR using samples prepared from isotopically pure 25MgO. Also, the potential of spin echo decay spectroscopy to measure 25Mg-25Mg magnetic dipole-dipole couplings was investigated.
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Why is it important?
Both the static and rotor-synchronized Hahn-echo MAS NMR spectra indicate a broad distribution of the electric field gradient components at the 25Mg position caused by disorder, which could be fitted consistently to a common set of interaction parameters and distribution widths using the Czjzek approach. The best fits to the measured spectra gave a chemical shift which suggests that the Mg^(2+) ions are sixfold coordinated. However, this conclusion is based on the simplifying assumption that the 25Mg spectrum comprises a Czjzek distribution centered at a single isotropic chemical shift value and is in contradiction to the results found from experiments using neutron diffraction with magnesium isotope substitution. It is concluded that multiple Mg sites with different coordination numbers are present in the glass structure and that, for the investigated materials, 25Mg NMR at typically used spinning rates and magnetic field strengths (20 kHz, 14.1 T) is not capable of resolving them due to excessive broadening by quadrupolar interactions.
Perspectives
Our work shows the benefit of combining solid-state NMR with neutron diffraction to assess the information content of the measured spectra for quadrupolar nuclei such as 25Mg.
Professor Philip S Salmon
University of Bath
Read the Original
This page is a summary of: Analysis and information content of quadrupolar NMR in glasses: 25Mg NMR in vitreous MgSiO3 and CaMgSi2O6, Journal of Magnetic Resonance Open, December 2022, Elsevier,
DOI: 10.1016/j.jmro.2022.100067.
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