title: Site connectivities in sodium aluminoborate glasses: multinuclear and multiple quantum NMR results.
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 connectivities between AlO4 ([4]Al), AlO5 and AlO6 ([5,6]Al), BO3 ([3]B) and BO4 ([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.
authors.affiliation: Department of Geological and environmental Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, CA 94305-2115, USA. lsdu@pangea.stanford.edu
authors.forename: Lin-Shu
authors.initials: LS
authors.lastname: Du
authors.forename: Jonathan F
authors.initials: JF
authors.lastname: Stebbins
mesh_terms: D001881:Borates / Q000737:chemistry*; D003738:Dental Cements / Q000737:chemistry; D005898:Glass / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*
chemical_list: D001881:Borates; D003738:Dental Cements; C104335:sodium aluminoborate