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

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