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Abstract: 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. [Copyright &y& Elsevier]

Abstract: Structural changes in mechanically treated CaF2 powders have been studied by X-ray diffraction, transmission electron microscopy, thermal analysis, 19F and 1H MAS Nuclear Magnetic Resonance and Electron Paramagnetic Resonance methods. Applying the same methods, the results could be compared with those of mechanochemically synthesized CaF2 samples, prepared for the first time in this study by high-energy ball milling. The applied methods indicate that the mechanically treated samples become, under the applied conditions, nanocrystalline. Unexpectedly, the mechanochemically synthesized samples show the same effects, i.e., nanocrystalline samples were formed. In contrast to many oxide compounds, a weak amorphization takes place only after a strong mechanical impact, and essentially in grain boundaries of spherical particles. Observed effects after the application of mechanical impact like broadening of XRD reflections, broadening of 19F resonances, or the decreasing ability for H trapping at ambient temperature, are mainly due to decreasing particle sizes as well as very small structural changes in the bulk of the particles. Surprisingly, the spin relaxation of both nuclear spins (19F) and electron spins (H ) appears to be the most sensitive tool for the investigation of mechanically and chemically induced changes indicating the increasing surface to bulk ratio with increasing mechanical impact. [Copyright &y& Elsevier]