Cs. Fetzer

Phase transition in nanomagnetite

Recently, the application of nanosized magnetite particles became an area of growing interest for their potential practical applications. Nanosized magnetite samples of 36 and 9nm sizes were synthesized. Special care was taken on the right stoichiometry of the magnetite particles. Mossbauer spectroscopy measurements were made in 4.2–300K temperature range. The temperature dependence of the intensities of the spectral components indicated size dependent transition taking place in a broad temperature range. For nanosized samples, the hyperfine interaction values and their relative intensities changed above the Verwey transition temperature value of bulk megnetite. The continuous transition indicated the formation of dendritelike granular assemblies formed during the preparation of the samples.

Surface characterization and catalytic CO + H2 reaction on Fe82.2B17.8 amorphous alloy

Abstract Fe82.2B17.8 amorphous ribbon has been used as a catalyst for the Fischer-Tropsch-type reaction of CO+H2. Specific activity has been found to be at least an order of magnitude higher than that of either the crystallized ribbon of identical composition or the supported iron catalyst. Before and after the catalytic tests the ribbons were characterized by XRD, XPS, UPS and Mossbauer spectroscopy in transmission and in conversion electron modes. Conversion electron Mossbauer spectroscopy and UPS proved that the surface of the amorphous ribbons is being partially crystallized during 8000 min reaction time at a maximum reaction temperature of 560 K. The superior catalytic activity has been explained by stabilization of the small iron particles and Fe2O3 by boron atoms at the surface and by suppressed carbide formation.

Metastable phases of cobalt-ironsilicide formed by sequential implantation of Co and Fe in Si (111)

By implanting Co and Fe in sequence into Si (111), metastable ternary Co1−xFexSi2 phases were formed. Mossbauer effect measurements showed three resonance line components in the spectrum. Comparison of the central shift (CS) values of the components with those appearing in the stable ternary phases indicated that iron atoms are positioned in the substitutional Co site, in the empty cube of the fluorite-type lattice and in CsCl-like B2 structures. It was found that the CS values of two components are in the velocity range of the values obtained for the metastable γ-FeSi2 synthesized using various methods. This result suggests the existence of a similar structure.

Ternary CoxFe(1−x)Si2 and NixFe(1−x)Si2 formed by ion implantation in silicon

Co1−xFexSi2 and Ni1−xFexSi2 metastable ternary phases were formed by sequentially implanting Co, Ni, and Fe into Si (111) at 623 K. In order to compare the phases formed by ion implantation, the Ni1−xFexSi2 stable bulk ternary phase with a wide variety of x values was synthesized. The samples were studied by Mossbauer effect, transmission electron microscopy (TEM), x-ray diffraction, and Rutherford backscattering and channeling. X-ray diffraction and TEM results on the as-implanted samples with x=0.5 indicate a cubic (fluorite) structure. 57Fe Mossbauer spectra show three resonanceline components. Comparison of the isomer shift values of the components with those measured in the stable and metastable transition-metal silicide phases indicated three different sites for iron atoms: Fe substituting Co or Ni; Fe in the empty cubes of the fluorite-type lattices; and Fe populating sites in the CsCl-type B2 lattice. In samples of Ni1−xFexSi2 annealed at 1273 K, α-FeSi2 and a fraction of Fe dissolved in NiSi2 app...

Local interactions of 57Fe after electron capture of 57Co implanted in -Al2O3 and in -Fe2O3

-Al2 O3 and -Fe2 O3 were implanted with 57 Co at room temperature. -Al2 O3 showed complex Mossbauer spectra indicating paramagnetic hyperfine splitting for high-spin Fe3+ and electric quadrupole-split spectra for Fe2+ ions. In canted antiferromagnet -Fe2 O3 , the implanted Co atoms were positioned substitutionally in Fe2 O3 and at higher doses in Fe3 O4 and in Fe1-x O. In the different phases, the ions were in Fe2+ and/or in Fe3+ states after the electron capture of 57 Co; no after-effects were observed. The results suggest that the phases are formed during fast cooling and crystallization of the oxide from the high-temperature state of the thermal spike.

PHASES OF COBALT-IRON TERNARY DISILICIDES

Cobalt–iron transition-metal disilicides were investigated by Mossbauer effect and x-ray diffraction in order to determine the concentration range of their homogeneous and separate phase formation. Except at low Co or Fe concentrations, Co and Fe formed separate CoSi2 and FeSi2 phases. Up to 10 at % Co was found soluble in β-FeSi2; Fe dissolved in CoSi2 below 1.5 at % and was positioned at two different sites of cubic symmetry. The results obtained for the phase formation in thin layers of epitaxial CoSi2 on Si implanted with Fe were in agreement with the results obtained for the bulk samples.

Site location of Co in β-FeSi2

In order to reveal cationic site preference in β-FeSi2, Co-substituted samples synthesized by various techniques such as molecular beam epitaxy, ion implantation, and chemical vapor transport were investigated by Fe57 conversion electron Mossbauer (CEM) as well as Co57 Mossbauer emission (ME) spectroscopy. Literature on the structure of β-FeSi2 is somewhat contradictory, especially on the point of the population of the two iron sites in the orthorhombic structure. Co57 ME and Fe57 CEM spectra both showed two quadrupole split spectral components in the crystalline phase. Hyperfine parameters indicate that Co atoms substitute Fe in both Fe positions in the orthorhombic lattice. The aim of the present study was to get reliable results on the relative population of the two iron sites and determine the substitution of the iron sites by Co atoms in the β-FeSi2 lattice. The relative intensities of the two components in the absorption and emission Mossbauer spectra were found to be very similar for the samples pr...

The charge states of iron in insulators implanted with57Co and57Fe

Single crystals of α-Al2O3 and LiNbO3 were implanted with57Co (dose: up to 2×1015 atoms/cm2) and with57Fe (dose: 2×1015 atoms/cm2) ions. The Mössbauer spectra revealed the disordered atomic environment. Fe2+ and Fe3+ charge states were observed. The spectra were compared to the spectra of crystals doped with57Co. It was remarkable that in the doped α-Al2O3 Fe3+ states with slow spin-spin relaxation have appeared. The CEMS study of the samples implanted with57Fe resulted in Fe2+ ionic states indicating that a fraction of Co atoms can also be in Co2+ state.

Iron implantation in α-Al2O3

Single crystal and ceramic α-Al2O3 samples were implanted with F57e ions at different fluences, and different charge states of Fe were determined. The hyperfine interaction data showed that implanted ions are in Fe0, Fe2+, and Fe3+ states depending on the fluence. It is shown that after the annealing of the implanted sample in a 2.5CO 1CO2 atmosphere Fe spinel forms. The phase diagram of Fe–Al–O in a limited concentration range was studied by determining the hyperfine interaction parameters of the formed phases after annealing the samples in different atmospheres. The hyperfine interaction parameters of an ion implanted α-Al2O3 sample annealed in a 2.5CO 1CO2 atmosphere at different temperatures were determined. The hyperfine interaction parameters of the gradually forming Fe-spinel phase were compared with the values of synthesized Fe-spinel crystal.

Iron implantation in gadolinium gallium garnet studied by conversion-electron Mössbauer spectroscopy

Gadolinium gallium garnet single crystals were implanted with doses of ions in the range . Depending on the dose, iron with or charge states was found to have formed after the implantation. After a subsequent annealing in air, the iron oxidized to . The Mossbauer and channelling measurements showed lattice recrystallization taking place at . After recrystallization, the iron was found to have substituted for gallium ions both at the octahedral and at the tetrahedral positions. The relative concentration of the two types of iron at the two sites shifted towards the equilibrium distribution upon high-temperature annealing.