Atomic structure of BeO-B2O3-Al2O3 glasses: 11B and 27Al MAS NMR spectroscopy at 21.8 Tesla

The structures of BeO-Al2O3-B2O3 glasses with a range of BeO/Al2O3 ratios at fixed BeO/B2O3 ratios of 1:1.5 and 1:1 have beers investigated using B-11 and Al-27 MAS NMR spectroscopy at ultrahigh magnetic field (21.8 T). These B-11 and Al-27 MAS NMR spectra are characterised by remarkably superior resolution and thus provide more accurate results for B and Al speciation compared to the lour field (11.7 T) NMR results reported in previous studies. All glasses are characterised by a BO3:BO4 ratio of similar to 86:14, and approximately 70%, of all Al atoms are found to be five- and six-fold coordinated. Such large concentrations of BO3, AlO5 and AlO6 species are consistent with the highest field strength of Be among all alkali and alkaline earth metals. The compositional variation of cation speciation, when combined with bond valence calculations, indicates that most of the oxygens in these glasses would have to be 3-coordinated and shared between B-III-Al-VI-Be-IV, B-III-Be-IV-Be-IV, B-IV-Al-IV-Be-IV, Al-IV-Be-IV-Be-IV, or Al-IV-Al-V-Be-IV coordination polyhedra. This bonding scenario world be expected to give rise to a dense structure with increased rigidity, consistent with the remarkably high T-g values reported in the literature for these glasses.