Rapid communication Quantification of five- and six-coordinated aluminum ions in aluminosilicate and fluoride-containing glasses by high-field,

Aluminum cation sites with five (5Al) or six (6Al) anion neighbors are minor species in aluminosilicate glasses that are interesting in models of structure, diusion, and viscous flow. Using 27Al NMR, we present the first direct evidence for 5Al in a calcium-aluminosilicate glass without excess aluminum over charge-balancing cations, and quantify small concentrations of both 5Al and 6Al in several fluoride-containing aluminosilicate glasses. NMR techniques that en-hance resolution by decreasing the eects 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.