Local Structure of Magnesium in Silicate Glasses: A ²⁵Mg 3QMAS NMR Study

Abstract

We have reported the 25Mg triple-quantum magic-angle spinning (3QMAS) NMR spectra of silicate glasses. The two-dimensional spectra suggest that the magnesium ions in MgSiO3, CaMgSi2O6, Ca2MgSi2O7, Mg3Al2Si3O12, and Li2MgSi2O6 glasses are mainly in octahedral environments, although in Na2MgSi2O6, K2MgSi2O6, and K2MgSi5O12 glasses they form tetrahedral species. We discussed the coordination environments of Mg based on the field strength of competing Mg2+, Ca2+, Na+, K+, and Li+ cations, and convincingly demonstrated that there is a correlation between them. These results indicate that the two-dimensional NMR spectroscopy such as MQMAS technique is a very useful method to analyze the local environments of nonframework cations in noncrystalline solids.

Recent Advances in Solid-State 25Mg NMR Spectroscopy

“…A range of glasses prepared from 99%-enriched 25 MgO were investigated, separated into two groups: MgSiO 3 , CaMgSi 2 O 6 , Ca 2 MgSi 2 O 7 and Mg 3 Al 2 Si 3 O 12 (group A) and Na 2 MgSi 2 O 6 and K 2 MgSi 2 O 6 (group B); comparison to diopside and akermanite was also presented. 42,161 The two groups showed distinct spectra as can be seen by a comparison of the studies at two magnetic fields of 16.4 and 21.8T ( Figure 21). 42 The MAS spectra show a broader component that sits on potentially an even unnarrowed component for group A compared to B.…”

“…42,161 The two groups showed distinct spectra as can be seen by a comparison of the studies at two magnetic fields of 16.4 and 21.8T ( Figure 21). 42 The MAS spectra show a broader component that sits on potentially an even unnarrowed component for group A compared to B.…”

“…9,18 The condition n 1 (n q is satisfied for most usual probes, since the RF strength n 1 is typically well below n q value for a low-g nucleus such as 25 Mg. In the cases where more RF power is required, as it is typically the case of MQMAS and related experiments, the use of probes especially designed to attain high RF power levels and appropriate tuning at the low 25 Mg NMR frequency has been reported. 37,[42][43][44] Typical commercial probes used for MAS experiments are equipped with rotors having diameters in the range 2.5-7mm, which allows spinning speeds up to 45kHz. This value is sufficient in many cases for attaining the maximum possible narrowing of the second-order quadrupolar broadened CT in 25 Mg NMR spectra recorded at strong magnetic Δf d 0 Figure 3 Definition of the parameters d 0 (shift of the most intense peak) and Df (total breadth), in the 25 fields.…”

“…Besides the use of large volume samples, there are also other ''brute force'' approaches frequently used since the earlier reports of solid-state 25 Mg NMR, including isotopic enrichment and the use of magnetic fields as strong as possible. 6,37,39,42,43,[45][46][47][48][49][50][51][52] There is no doubt that the increase in the number of reports involving solid-state 25 Mg NMR has been driven by the wider availability of high-field magnets (e.g. !14.1T).…”

Modelling the sulfate capacity of simulated radioactive waste borosilicate glasses

“…Magnesium is known to occur largely as [4] Mg 2+ in the majority of alkali-bearing silicate glasses [29,41,42]. We have assumed [4] Mg 2+ for our model and applied it to all glasses studied here.…”