Fe3+ Ions Acting as Probes and Agents in Aggregation Processes and Solid‐State Reactions in AlO(OH)/Al2O3 Matrices

Abstract

The fate of doped iron species during various mechanically and thermally initiated redox reactions and aggregation processes in crystalline and amorphous AlO(OH) or Al2O3 matrices was investigated. The amorphization of the matrix was performed by chemical (i.e., via sol–gel processes) or mechanochemical treatment (high‐energy ball milling). Thermal analysis, coupled with mass spectrometry (TA‐MS), Mössbauer spectroscopy, and electron spin resonance (ESR), was used. The TA under various gas atmospheres allowed preparing samples under a controlled temperature regime, together with a controlled gas influence. Both the effect of mechanical activation and the influence of the iron doping could be followed macroscopically via the down‐shift of the peak temperature of the corundum formation. The ESR data characterize the Fe3+ ions and their interaction with the magnetic surrounding based on the fine structure parameters. The Mössbauer data allowed the characterization of the Fe(0), Fe2+, and Fe3+ species, together with providing information about their coordinative surrounding. Both methods provided general complementary spectroscopical information. Unexpectedly, (FeOx)n and (FexAl1−x)2O3 aggregates could also be detected in the range of low Fe concentrations. It was demonstrated that even in the low‐level doped systems [(FexAl1−x)2O3 with x≥0.01], all the essential spectroscopic phenomena occur. At higher Fe concentrations, they were discovered to be caused by magnetic and spin exchange interactions as well as by solid‐state reactions during and after the mechanical activation.