Study of the Al coordination in mullites with varying Al:Si ratio by²⁷Al NMR spectroscopy and X-ray diffraction

Abstract

Mullite is an aluminosilicate of the composition Al 2 (Al 2ϩ2x Si 2Ϫ2x)O 10Ϫx with x generally ranging between 0.2 and 0.5. XRD and 29 Si and 27 Al nuclear magnetic resonance (NMR) have been used to investigate the structure of various mullite compositions (x ϭ 0.26, 0.36, 0.42, and 0.69) as well as sillimanite (x ϭ 0) and ␥-Al 2 O 3 (x ϭ 1). 27 Al 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 AlO 4 and AlO 6 units. The isotropic chemical shifts of the various units are 5.9 ppm for AlO 6 , 69.1 ppm for AlO 4 (T), 55.0 ppm for AlO 4 (TЈ), and 45.7 ppm for AlO 4 (T*) where AlO 4 (T) denotes the aluminum tetrahedra in the double chains, and AlO 4 (TЈ) and AlO 4 (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 ␥-Al 2 O 3 in the mullites with x ϭ 0.42 and 0.69.

Towards understanding the microstructural and structural changes in natural hierarchical materials for energy recovery: In-operando multi-scale X-ray scattering characterization of Na- and Ca-montmorillonite on heating to 1150 °C

“…The representative chemical formula for mullite is: Al (4+2x) Si (2-2x) O (10-x) where × varies from 0.2 and 0.9, depending on temperature. 59,60 …”

High‐Pressure Behavior of Mullite: An X‐Ray Diffraction Investigation

“…The chemical composition of the mullite samples was verified by using the well‐known linear relationship between the a ‐lattice parameter and the molar Al 2 O 3 content of the Al 4+2 x Si 2−2 x O 10− x phases . The mullite sample with a = 7.58154 (6) Å, was determined to have a chemical composition of 65.3 mol% Al 2 O 3 , which corresponds to that of 2:1‐mullite with x = 0.4 (designated as 2:1‐mullite).…”

Carbonate and silicate phase reactions during ceramic firing

“…Goldsmith (1953) indicates that mullite could be another example of his "simplexity principle" for the "ease" of crystallization, if compared with other aluminum silicates. In fact, the mullite structure has a larger amount of Al in 4-coordination and less Al in 6-coordination than sillimanite (Rehak et al, 1998) for instance, which may account for the abundance of the former and the non-existence of the latter in the studied bricks.…”