Effect of boron content on atomic structure of boron-bearing multicomponent oxide glasses: A view from the solid-state NMR

Abstract

Understanding the effect of boron content on atomic structures of boron-bearing multicomponent silicate melts is essential to reveal the atomistic origins of diverse geochemical processes involving silica-rich magmas, such as explosive volcanic eruption and the reaction of magma in contact with aqueous solutions. It is also considered crucial in environmental science because a nuclear waste glasses contain B2O3. Despite the importance, the detailed atomic structure of boron-bearing multicomponent silicate glasses remains unsolved. We report experimental results on the effect of boron content on the atomic structures of sodium borate glasses and boron-bearing multicomponent silicate melts [nepheline (NaAlSiO4)- malinkoite (NaBSiO4) and albite (NaAlSi3O8)- reedmergnerite (NaBSi3O8) pseudo-binary glasses] using the high-resolution solid-state NMR (11B, 27Al, and 17O). The 11B NMR spectra of Na2O-B2O3 glasses show that four-coordinated boron ([4]B) increases at the expense of ring (three-coordinated boron, [3]B) not of non-ring ([3]B) with increasing sodium content based on the new interpretation from the results of quantum chemical calculations that 11B NMR chemical shift of non-ring ([3]B) increases with increasing sodium content. The two topological species of [4]B are firstly observed in 11B 3QMAS NMR spectra for Na2O-B2O3 glasses. The 11B NMR spectra of NaAlSiO4-NaBSiO4 and NaAlSi3O8-NaBSi3O8 glasses show that [4]B increases as boron content increases while non-ring ([3]B) decreases. 27Al MAS NMR spectra for NaAlSiO4-NaBSiO4 glasses confirm that four-coordinated aluminum ([4]Al) is dominant. It is also observed that a drastic decrease in the peak widths (full-width at half-maximum, FWHM) of [4]Al with an addition of boron into nepheline glasses, indicating a decrease in structural and topological disorder around [4]Al. The quantitative atomic environments around boron of multicomponent glasses are estimated from the simulation results of 11B MAS NMR spectra, revealing complex-nonlinear variation of boron topology with varying composition. 17O MAS and 3QMAS NMR spectra for NaAlSiO4-NaBSiO4 glasses show that Na-O-Si, B-O-B, and B-O-Si increase with increasing boron content, indicating that a drastic increase in dissolution rate might be due to an increase in B-O-B and B-O-Si which are relatively weaker bridging oxygen (BOs). An increase in non-bridging oxygen (NBO), Na-O-Si, implying a decrease in the degree of polymerization can explain the low viscosity of boron-bearing silicate melts. Na-O-B is firstly observed in 17O 3QMAS NMR spectra for NaAl0.25B0.75SiO4 glasses. The current experimental results with the changes in coordination environment, topological structure, and network connectivity in the nepheline-malinkoite and albite-reedmergnerite glasses can improve understanding of the structure-property relationships including dissolution and viscosity of multicomponent boron-bearing silicate melts.