Iron incorporation in mullite

Abstract

The mode of iron incorporation in synthetic sinter mullites and in commercially fused mullites has been studied by chemical, X-ray diffraction, Mössbauer, and electron paramagnetic resonance techniques. Sinter mullite incorporates up to about 11 wt% Fe 2O 3. The sinter mullites exhibit three Fe 3+ centers, which are attributed to Fe 3+ at octahedral and tetrahedral lattice sites, and to Fe 3+ clusters, respectively, though most of the iron is incorporated octahedrally in each case. The distribution of Fe 3+ over the octahedral and tetrahedral lattice sites depends on the annealing temperature: increasing annealing temperatures cause a slight increase of Fe 3+ incorporation at tetrahedral positions, while increasing amounts of Fe 3+ incorporation induces a preferable occupation of the octahedron. In the presence of additional TiO 2 a preferable Fe 3+ entry at tetrahedral sites seems to occur. Only very low iron concentrations are observed in fused mullites: Fe 3+ almost exclusively occupies tetrahedral lattice sites.