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

Abstract

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[p]-O-B[q] network linkages. The NMR spectral deconvolutions from two BS glass series with low nonbridging oxygen (NBO) contents and fixed molar ratios nSi/nB = {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](mSi) sites differ in their (average) numbers of m B[4]-O-Si and 4 - m B[4]-O-B[p] bonds, where B[p] may assume B[3] or 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.