Fe- and Cr-substituted mullites: Mossbauer spectroscopy and Rietveld structure refinement

Fe-substituted mullite samples Al4.5-yFeySi1.5O9.75 (y = 0.25 and 0.50) hav e been prepared using a sol-gel route involving Si and Al alkoxides and iro n nitrate as precursors. Mossbauer spectroscopy reveals the presence of thr ee different Fe(III) sites but does not allow to conclude concerning the io n environments, tetrahedral or octahedral. Rietveld refinements reveal that Fe ions are distributed between octahedral and tetrahedral positions, the proportion of octahedral iron increasing from y = 0.25 to y = 0.50. On the other hand, for Cr-substituted mullite, Cr ions are mainly located on octah edral positions. This difference between Fe and Cr has been attributed to t he fact that Cr shows a clear trend to occupy octahedral sites compared to Fe and that it is incorporated inside the mullite structure only at high te mperature.