Identifying and Quantifying Borate Environments in Borosilicate Glasses: B NMR-Peak Assignments Assisted by Double-Quantum Experiments.

We discuss the prospects for accurate B magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) spectral deconvolutions for reaching beyond the readily extracted borate speciations offered by the integrated resonances of the coexisting B and B species of the respective BO and BO network groups in borosilicate (BS) glasses. We critically review hitherto proposed B and B NMR-peak assignments relating to their neighboring Si, B and B species, as quantified by MAS NMR spectral deconvolution. Guidance to these resonance assignments was offered from double-quantum-single-quantum (2Q-1Q) B MAS NMR experiments that inform about the B-O-B network linkages. The NMR spectral deconvolutions from two BS glass series with low nonbridging oxygen (NBO) contents and fixed molar ratios / = {1.0, 2.0} but variable network-modifying cations of alkali metals and Mg revealed a dominance of B-O-Si linkages, yet with a significant dependence on the BO population of the glass, which was rationalized by the different propensities for B-O-{Si, B, B} linkage formation. For BS glasses with comparable B and Si contents, we recommend three-peak deconvolutions of the B spectral region, whose B(Si) sites differ in their (average) numbers of B-O-Si and 4 - B-O-B bonds, where B may assume B B. We also discuss the structural origin of the two rather arbitrarily classified "ring" and "non-ring" B entities, where 2Q-1Q B NMR suggests the former to primarily constitute BO groups that coexist with BO moieties in (superstructural) ring units largely devoid of bonds to Si, whereas the "non-ring" B sites involve linkages to all of B, B, and Si, with B-O-Si linkages prevailing. The limitations of B NMR spectral deconvolutions are discussed, including the remaining challenges in analyzing NBO-rich BS glasses.