Quantification of five- and six-coordinated aluminum ions in aluminosilicate and fluoride-containing glasses by high-field, high-resolution 27Al NMR

Aluminum cation sites with five (Al-[5]) or six (Al-[6]) anion neighbors are minor species in aluminosilicate glasses that are interesting in models of structure, diffusion, and viscous flow. Using Al-27 NMR, we present the first direct evidence for Al-[5] in a calcium-aluminosilicate glass without excess aluminum over charge-balancing cations, and quantify small concentrations of both Al-[5] and Al-[6] in several fluoride-containing aluminosilicate glasses. NMR techniques that enhance resolution by decreasing the effects of second-order quadrupolar broadening, including triple-quantum magic-angle spinning (3QMAS), analysis of spinning sidebands, and data collection at very high external magnetic fields (14.1 and 18.8 T), are particularly effective in accurately observing such species. (C) 2000 Elsevier Science B.V. All rights reserved.