Site connectivities in sodium aluminoborate glasses: multinuclear and multiple quantum NMR results

In a series of sodium aluminoborate glasses, we have applied triple-quantum magic-angle spinning (3QMAS) O-17 NMR to obtain high-resolution information about the connections among various network structural units, to explore the mixing of aluminum and boron species. Oxygen-17 3QMAS spectra reveal changes in connectivites between AlO4 (Al-[4]), AlO5 and AlO5 (Al-[5,Al-6]), BO3 (B-[3]) and BO4 (B-[4]) units, by quantifying populations of bridging oxygens such as Al-O-Al, Al-O-B and B-O-B and of non-bridging oxygens. Several linkages such as Al-[4]-O-Al-[4] and three-coordinated oxygen associated with Al-[5,Al-6] in Al-O-Al, Al-[4]-O-B-[4], Al-[4]-(OB)-B-[13] and ([5,6)]Al-O-B-[3] in Al-O-B as well as B-[4]-O-B-[3] and B-[3]-O- B-[3] in B-O-B can be distinguished for the first time. The fractions of these linkages were calculated from models of random mixing and of mixing with maximum avoidance of tetrahedral-tetrahedral linkages. The results suggest that the structure of all of glasses in this study is well approximated by the latter model. However, the energetic penalty for formation of Al-[4]-O-B-[4] may be somewhat less than for Al-[4]-O-Al-[4] and B-[4]-O-B-[4]. In general, the new results presented here are similar to those obtained on glasses in this system by (Al.-11B)-Al-27} REDOR NMR (J. Phys. Chem. B 104 (2000) 6541), but provide considerably more detail on network connectivity and ordering schemes. (C) 2004 Elsevier Inc. All rights reserved.