Study of the Al coordination in mullites with varying Al:Si ratio by 27Al NMR spectroscopy and X-ray diffraction

Mullite is an aluminosilicate of the composition Al2(Al212xSi222x)O102x with x generally ranging between 0.2 and 0.5. XRD and 29Si and 27Al nuclear magnetic resonance (NMR) have been used to investigate the structure of various mullite compositions (x 5 0.26, 0.36, 0.42, and 0.69) as well as sillimanite (x 5 0) and g-Al2O3 (x 5 1). 27Al magic angle spinning (MAS) NMR of the central and satellite transitions have been used to determine the chemical shifts and quadrupole interaction parameters for the various AlO4 and AlO6 units. The isotropic chemical shifts of the various units are 5.9 ppm for AlO6, 69.1 ppm for AlO4(T), 55.0 ppm for AlO4(T9), and 45.7 ppm for AlO4(T*) where AlO4(T) denotes the aluminum tetrahedra in the double chains, and AlO4(T9) and AlO4(T*) those next to the oxygen Oc*. Quantitative numbers of the aluminum occupancy of these sites have been determined for the various powder samples. These results are in good agreement with the average structure model of mullites. NMR and XRD proved the presence of impurities of kyanite in natural sillimanite and of g-Al2O3 in the mullites with x 5 0.42 and 0.69.