AlO4/SiO4 Distribution in Tetrahedral Double Chains of Mullite

The order–disorder of the tetrahedrally coordinated aluminum and silicon atoms in mullite has been investigated by means of 29Si nuclear magnetic resonance (NMR) spectroscopy. Sinter (3/2) and fused (2/1) mullites in the as‐received state and reheated at 1750°C, and a reference sillimanite were used for this study. All mullites display similar 29Si NMR spectra: The strongest peak occurs at about −88 ppm, with two subpeaks close to −92 and −96 ppm. The −88 ppm signal is assigned to a sillimanite‐type environment with three aluminum oxygen tetrahedra as next nearest neighbors of the silicon oxygen tetrahedra. The two 29Si NMR signals near −92 and −96 ppm are assigned to silicon oxygen tetrahedra surrounded by two aluminum oxygen and one silicon oxygen tetrahedra, and one aluminum oxygen and two silicon oxygen tetrahedra, respectively. 29Si NMR spectra with different short‐range‐order parameters were simulated by an array of 2 × 10 000 tetrahedral positions by means of an adapted random generator. The comparison between measured and simulated mullite and sillimanite 29Si NMR spectra yields a moderate degree of tetrahedral aluminum–silicon order, with no tendency toward cation demixing.