Abstract

In a series of sodium aluminoborate glasses, we have applied triple-quantum magic-angle spinning (3QMAS) 17O 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 AlO 4 ( [4]Al), AlO 5 and AlO 6 ( [5,6]Al), BO 3 ( [3]B) and BO 4 ( [4]B) 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 [4]Al–O– [4]Al and three-coordinated oxygen associated with [5,6]Al in Al–O–Al, [4]Al–O– [4]B, [4]Al–O– [3]B and [5,6]Al–O– [3]B in Al–O–B as well as [4]B–O– [3]B and [3]B–O– [3]B 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 [4]Al–O– [4]B may be somewhat less than for [4]Al–O– [4]Al and [4]B–O– [4]B. In general, the new results presented here are similar to those obtained on glasses in this system by 27Al{ 11B} REDOR NMR (J. Phys. Chem. B 104 (2000) 6541), but provide considerably more detail on network connectivity and ordering schemes.

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