Identifying and Quantifying Borate Environments in Borosilicate Glasses: ¹¹B NMR-Peak Assignments Assisted by Double-Quantum Experiments

We discuss the prospects for accurate ¹¹B 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^[3] and B^[4] species of the respective BO₃ and BO₄ network groups in borosilicate (BS) glasses. We critically review hitherto proposed ¹¹B^[3] and ¹¹B^[4] NMR-peak assignments relating to their neighboring Si, B^[3] and B^[4] species, as quantified by MAS NMR spectral deconvolution. Guidance to these resonance assignments was offered from double-quantum-single-quantum (2Q-1Q) ¹¹B MAS NMR experiments that inform about the B^[<i>p</i>]-O-B^[<i>q</i>] network linkages. The NMR spectral deconvolutions from two BS glass series with low nonbridging oxygen (NBO) contents and fixed molar ratios <i>n</i>_Si/<i>n</i>_B = {1.0, 2.0} but variable network-modifying cations of alkali metals and Mg²⁺ revealed a dominance of B^[4]-O-Si linkages, yet with a significant dependence on the BO₃ population of the glass, which was rationalized by the different propensities for B^[4]-O-{Si, B^[3], B^[4]} linkage formation. For BS glasses with comparable B and Si contents, we recommend three-peak deconvolutions of the ¹¹B^[4] spectral region, whose ¹¹B^[4](<i>m</i>Si) sites differ in their (average) numbers of <i>m</i> B^[4]-O-Si and 4 - <i>m</i> B^[4]-O-B^[<i>p</i>] bonds, where B^[<i>p</i>] may assume B^[3] <i>or</i> B^[4]. We also discuss the structural origin of the two rather arbitrarily classified "ring" and "non-ring" B^[3] entities, where 2Q-1Q ¹¹B NMR suggests the former to primarily constitute BO₃ groups that coexist with BO₄ moieties in (superstructural) ring units largely devoid of bonds to Si, whereas the "non-ring" B^[3] sites involve linkages to all of B^[3], B^[4], and Si, with B^[3]-O-Si linkages prevailing. The limitations of ¹¹B NMR spectral deconvolutions are discussed, including the remaining challenges in analyzing NBO-rich BS glasses.