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Co-milling of AlO(OH) xerogels with various iron(III) compounds such as α-Fe2O3 or K3[Fe(CN)6] has been used for mechanochemical activation leading to Fe/Al oxide phases in coexistence with the activated main component. The results obtained allow a deeper insight into the chemical processes occurring during activation, doping, and thermal treatment in atmospheres of different chemical activity (H2O; N2; O2/N2; H2/N2). Evidence will be given for Fe3 + species being able to form suitable crystallization germs and to lower the crystallization temperature. One can distinguish between the incorporation of Fe3 + ions into the alumina matrix at the crystallite growth (e.g. on Fe2O3-like seeds) and the incorporation of Fe3 + ions during the last steps of crystallization. These last-mentioned Fe3 + ions are provided by coexisting Fe/Al oxide phases. In contrast to alumina matrices equilibrated at high temperatures (e.g. at 1,200–1,500°C), the matrices formed in situ are able to incorporate Fe3 + ions by diffusion at temperatures of ∼900–1,100°C in a short time regime. [ABSTRACT FROM AUTHOR]

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]

Focussed on the understanding of the complex Fe3+ electron spin resonance (ESR) spectra in crystalline and amorphous systems, the simulation program ESR-MAKRO-1 was developed, which allows the calculation of powder spectra of paramagnetic centers with 1/2 ≤ S ≤7/2. For the reproduction of the ESR spectra of amorphous systems Gaussian distributions of the zero-field-splitting parameters of second and fourth order ( D, E, a, F) were implemented into the program. Applications of ESR-MAKRO-l are given both for crystalline powders and for glassy systems. A stepwise analysis of the ESR spectra, allowing an approach to Fe3+ ESR spectra of amorphous matrices and showing the influence of D, E, a, and F as well as their distributions, is given. [ABSTRACT FROM AUTHOR]

Searching for a microscopic description of structural phase transitions in AlF3, these transitions were followed by electron paramagnetic resonance (EPR) in situ measurements in X-band in the temperature range 290 K ≤ T ≤ 800 K with Cr3+ — and Fe3+-doped powder samples of α-AlF3. For each temperature the zero-field splitting (zfs) parameters of the paramagnetic transition metal ions were determined by simulation. Starting from the axial zfs parameters b ( T) and bearing in mind the trigonal symmetry, the application of the superposition model allowed the calculation of the angle θ( T) (θ: <( c-axis, Al-F)) with b ( T) = 3 b 2( R)(3cos2θ( T) − 1) ( c-axis ∥ 3-fold axis). Direct relations between θ and the rotational angle ω of the matrix in dependence on temperature could be obtained by geometrical considerations. Local displacements of the anions were taken into account in terms of the parameter x characterizing their lattice position in the hexagonal cell of aluminium fluoride. [ABSTRACT FROM AUTHOR]

Two-frequency continuous-wave and pulse EPR (electron paramagnetic resonance) spectroscopical techniques are applied to determine static and dynamic EPR parameters of Cu2+ ions in oxide and fluoride glasses. The investigations are focussed on the analysis of strain effects in the glassy matrices, the identification of the magnetic nuclei in the vicinity of Cu2+ ions as well as the determination of the dependence of the phase memory time T M on temperature and resonance field. The results obtained by X-band continuous-wave EPR, X- and S-band echo-detected EPR, and X- and S-band electron spin echo envelope modulation studies of Cu2+-doped inorganic glasses yield information on the local symmetry of the Cu2+ coordination polyhedra, the chemical nature of the atoms in the second and higher coordination spheres, the distribution of the parameters of the static spin Hamiltonian and the low-temperature motions of the dopant-containing structural units. Special techniques like 2-D Mims ENDOR (electron nuclear double resonance) and hyperfine-correlated ENDOR are applied for the first time to doped inorganic glasses. From the spin relaxation measurements a stronger tendency of the Cu2+ ions to aggregate is found for fluoride glasses in comparison to aluminosilicate and phosphate glasses. [ABSTRACT FROM AUTHOR]

Stepwise amorphization of a low-level doped α-Al2O3 single crystal yielded powders without changes of the concentration and chemical nature of the dopants. Kind and degree of disturbances could be unambiguously monitored by the allowed and forbidden transitions of the Fe3+-cw-EPR in X-band using a Bruker bimodal cavity. Based on simulations diagonalizing the spin Hamiltonian for S = 5/2 it was possible to discriminate between regions as well as samples of structural order and disorder. The spectral pattern of the allowed and forbidden transitions of Fe3+ ions together with the field-position-dependent saturation behaviour (4.2 K) allowed us to differentiate between crystalline and amorphous components or regions of a sample. [ABSTRACT FROM AUTHOR]