Structure of diopside, enstatite, and magnesium aluminosilicate glasses: A joint approach using neutron and x-ray diffraction and solid-state NMR

Abstract

Neutron diffraction with magnesium isotope substitution, high energy x-ray diffraction, and 29Si, 27Al, and 25Mg solid-state nuclear magnetic resonance (NMR) spectroscopy were used to measure the structure of glassy diopside (CaMgSi2O6), enstatite (MgSiO3), and four (MgO) x(Al2O3) y(SiO2)1−x−y glasses, with x = 0.375 or 0.25 along the 50 mol. % silica tie-line (1 − x − y = 0.5) or with x = 0.3 or 0.2 along the 60 mol. % silica tie-line (1 − x − y = 0.6). The bound coherent neutron scattering length of the isotope 25Mg was remeasured, and the value of 3.720(12) fm was obtained from a Rietveld refinement of the powder diffraction patterns measured for crystalline 25MgO. The diffraction results for the glasses show a broad asymmetric distribution of Mg–O nearest-neighbors with a coordination number of 4.40(4) and 4.46(4) for the diopside and enstatite glasses, respectively. As magnesia is replaced by alumina along a tie-line with 50 or 60 mol. % silica, the Mg–O coordination number increases with the weighted bond distance as less Mg2+ ions adopt a network-modifying role and more of these ions adopt a predominantly charge-compensating role. 25Mg magic angle spinning (MAS) NMR results could not resolve the different coordination environments of Mg2+ under the employed field strength (14.1 T) and spinning rate (20 kHz). The results emphasize the power of neutron diffraction with isotope substitution to provide unambiguous site-specific information on the coordination environment of magnesium in disordered materials.

Molecular dynamics simulations of magnesium aluminosilicate glass structure: High‐coordinated alumina and oxygen tricluster formation

“…The Huang potentials utilized in this work report an average of 5% less Al [5] in comparison. However, these percentages agree with NMR work conducted by Gammond F I G U R E 1 3 Fraction of Al [5] with 50-55 mol% SiO 2 from this work compared with nuclear magnetic resonance (NMR) results from Lamberson, 44 Mohammadi et al, 43 Neuville et al, 45 Guignard and Cormier, 39 and with ReaxFF from Yeon et al 40 et al in compositions with 60 mol% SiO 2 at the same [Al 2 O 3 ]/[MgO] ratios (Table 2). At the equivalence point (R = 1), reported literature results show 10-21% Al [5] for 50-60 mol% SiO 2 which agrees with the Pedone and Wang potentials.…”

“…Instead, Mg coordination changes depending on the molar percentage of SiO 2 in the composition. This is a possible explanation for the observations made by Mohammadi et al 43 in which it was concluded that there was little evidence of four-coordinated Mg acting as a network former. From their work using 25 Mg NMR, the absence of spectroscopic differences between Ca-containing aluminosilicates and Mg compositions suggested that Mg behaves similarly to Ca (calcium ions are understood to behave primarily as network modifiers).…”

“…Figure 13 compares the fraction of Al [5] collected from this work to both NMR and ReaxFF reports in the literature for 50-55 mol% SiO 2 . 24,39,40,[43][44][45] In the metaluminous range, NMR results range from 5% to 20% Al [5] . Work by Neuville et al, 45 Guignard and Cormier, 39 and Mohammadi et al 43 using NMR, show approx.…”

“…24,39,40,[43][44][45] In the metaluminous range, NMR results range from 5% to 20% Al [5] . Work by Neuville et al, 45 Guignard and Cormier, 39 and Mohammadi et al 43 using NMR, show approx. 10-18% Al [5] when R < 1 which agrees with results using the Pedone and Wang potentials.…”

“…Work by Mohammadi et al concluded that there was no evidence of four-coordinated Mg behaving as a network former using NMR and neutron diffraction experiments. 43 Other works conclude that Mg competes with Al to form its own network. 39 To investigate the Mg environment within this work, we first plotted the average Mg coordination as a function of the cutoff value from the pair distribution function.…”