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- Antiferromagnetic coupling (1)
- Boehmite (1)
- Corundum (1)
- EPR Spectroscopy (1)
- ESR fine structure (1)
- ESR spectroscopy (1)
- Exchange-coupled units (1)
- Inorganic glasses (1)
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The 57Fe-Moessbauer spectrum of (dmpipzH2)2[FeIII(H2O)2Cl4][FeIII Cl4]Cl2 (1) consists of two doublets caused by a strongly distorted octahedral and a non-distorted tetrahedral component. Both (dmpipzH2)[FeIIIBr4]2 (2) and (trienH2)[FeIIIBr4]Br (3) show only one doublet indicating a more strongly distorted iron(III) species for 2 and a less-distorted one in the case of 3. For all compounds investigated, the ESR spectra reveal the existence of ground states with predominant antiferromagnetic coupling. The Néel temperatures amount to <4.2 K for 1, 4 K for 3 and 48 K for 2. In the case of 2, both the Moessbauer and the ESR spectra reveal the formation of exchange-coupled {FeBr4} units which has been attributed to a significant cation influence. The fine structure of residual paramagnetic [FeBr4]- subunits remaining in the crystalline matrix could be directly determined. Temperature dependent AC and DC susceptibility measurements of 2 and 3 confirmed the results.
The role of Fe3+ ions in the transformations from boehmites and pseudoboehmite xerogels via transition aluminas to corundum was studied here. Especially, the active iron species responsible for the decrease of the temperature of transformation to corundum were looked for. To enable the formation of various Fe3+
and Fe2+ species, samples were subjected to thermal treatments in different atmospheres as well as mechanically activated. Thermal analysis and ESR spectroscopy served to follow the processes and to characterise the resulting products. It was found that (i) isolated Fe3+ ions can indicate local structural changes but have (almost) no influence on the temperature of corundum formation, (ii) the temperature of corundum formation decreases in the result of action of small
α-Fe2O3 particles and (iii) during thermal treatments Fe3+ ions are distributed between different phases or precursors thereof: transition aluminas, corundum, Fe2O3, and a Fe3+ pool.