B. Sepiol

Diffusion mechanism and defect concentrations in β′-FeAl, an intermetallic compound with B2 structure

Abstract A statistical thermodynamic model of the Wagner-Schottky type is applied in order to determine the defect pattern in the β′-FeAl phase. Model calculations based on the concentration dependence of the thermodynamic activity of Al yield the following defect concentrations for the composition Fe50.5Al49.5 at 1338 K: 1.76 ± 0.20% of all lattice sites are vacant, 3.36 ± 0.40% of the Al-sublattice sites are occupied by Fe atoms, and 0.84 ± 0.40% of the Fe-sublattice sites are occupied by Al atoms. The obtained concentration of anti-structure Fe atoms is close to the number derived from Mossbauer spectroscopy studies which were carried out to determine the diffusion jump mechanism of Fe atoms in the ordered alloy. Experimental results and model calculations imply vacancies and anti-structure atoms on both sublattices. Therefore, although β′-FeAl in principle belongs to the triple-defect B2-type, a certain degree of ‘hybrid behaviour’ must be taken into consideration.

Chemical and structural properties of the system Fe2O3-Nd2O3

Chemical and structural properties of the system (1−x)Fe2O3 + xNd2O3, 0≤x≤1, were investigated using X-ray diffraction, 57Fe Mössbauer spectroscopy and Fourier transform-infrared (FT-IR) spectroscopy. The samples were prepared by the chemical coprecipitation and thermal treatment of Fe(OH)3/Nd(OH)3 coprecipitates. X-ray diffraction showed the presence of oxide phases α-Fe2O3 + NdFeO3 in the Fe2O3-rich region, and the oxide phases Nd2O3 + NdFeO3 in the Nd2O3-rich region. 57Fe Mössbauer spectra were characterized with one sextet of spectral lines at room temperature. Mathematical evaluation of the Mössbauer spectra showed distinct regularities in the changes of Mössbauer parameters, thus indicating the presence of two subspectra with very similar spectral behaviour. High sensitivity of the Nd2O3 phase to the moisture and atmosphere CO2 was demonstrated by FT-IR spectroscopy.

Mössbauer-effect study of the phase separation in the Fe-Cr system

In situ and conventional room temperature 57Fe-site Mossbauer spectroscopy (MS) were used to study the phase separation process in the Fe-Cr alloy system. It was shown that MS is well able to distinguish between (i) nucleation and growth and (ii) spinodal decomposition, the two mechanisms held responsible for the separation. The Fe-rich branch of the spinodal line was located at 80.8≤x≤84 at.% Fe at T = 415 °C. It was also shown that the saturation of the average hyperfine field cannot be taken as an indicator of the termination of the separation process. The kinetics of the process dramatically depends on the aging temperature, being faster by a factor of 1.4 at T = 440 °C than at T = 415 °C.

Lattice dynamics and related diffusion properties of intermetallics: I. Fe3Si

The phonon dispersion of iron-rich Fe-Si alloys of D03 structure (space group Fm-3m) has been studied by inelastic neutron scattering. The measurements were carried out on two crystals of different composition: Fe75Si25 at 20 and 930 degrees C, and Fe80Si20 at 20, 930 and 1100 degrees C. The respective degree of order was determined by powder diffraction measurements. A general decrease of phonon frequencies with increasing temperature is found, the temperature dependence being strongest for transverse phonons with ( xi xi 0) propagation. With decreasing order, phonon gaps close and the highest optical band degenerates into a broad distribution of inelastic intensity due to disorder scattering. The dependence of the dispersion on the alloy composition is not very pronounced. The migration enthalpies as well as several other thermodynamic quantities have been calculated from the densities of states. Although the low frequencies of the transverse phonons indicate particularly low migration barriers, they cannot explain the pronounced change of the diffusivity with composition. This composition dependence is tentatively explained by high vacancy concentrations.

Double electron exchange in Fe1−xO : A Mössbauer study

Abstract Mossbauer spectroscopy investigations have been performed at room temperature upon Fe 1−x O samples with 0.05 ⩽ x ⩽ 0.094 which have been previously characterized by X-ray diffraction and thermomagnetic analysis and the composition of which have been determined by thermogravimetric oxidation to Fe 2 O 3 . A model allowing to calculate the amount of Fe 3+ ions has been proposed; it assumes a fast electron exchange over iron atoms in octahedral sites between Fe 2+ and Fe 3+ . The amount of Fe 3+ ions calculated from the Mossbauer data agrees very well with those obtained directly from the thermogravimetric analysis.

Interplay between structural and magnetic properties of L10-FePt(001) thin films directly grown on MgO(001)

We present a detailed study of the magnetic and structural properties of L10-FePt thin films. The films are prepared via molecular beam epitaxy directly onto MgO(001) substrates, i.e., without buffer layer. Despite the large lattice misfit between the in-plane lattice parameters of L10 FePt and MgO, highly ordered thin films are obtained with the easy magnetization c axis perpendicular to the film plane. Via high resolution transmission electron microscopy and Rutherford backscattering measurements we focus on the FePt/MgO interface to study the misfit relaxation and the defect density. Further, the influence of elevated substrate temperatures and of postgrowth high temperature annealing on the structural and magnetic properties is discussed.

High temperature Mössbauer effect study of Fe90Zr7B3 nanocrystalline alloy

Fe90Zr7B3 NANOPERM alloy is investigated in as-quenched and nanocrystalline forms by means of high temperature (up to 1040 K) Mossbauer spectroscopy. These studies are aimed at revealing the relationship of microstructure to magnetic properties for57Fe phases and their temperature dependences in NANOPERM-type ternary alloy at temperatures exceeding the onset of the second crystallization. For this purpose the nanocrystalline sample was prepared by annealing an amorphous precursor at 893 K for 1 h providing 54% of bcc α-Fe nanocrystalline grains. At this stage the first crystallization is almost completed. Because of the progress of the crystallization process during the acquisition of Mossbauer spectra beyond the temperature of the first crystallization, the results obtained are discussed for three temperature intervals: below the first crystallization (782 K), between the first and the second crystallization, and above the second crystallization temperature (931 K). Conclusions related to the evolution of the crystalline fraction, interfacial regions and the amorphous residual phase are derived by comparing spectral parameters obtained from the in situ high temperature Mossbauer effect measurements with those from room temperature Mossbauer spectra acquired immediately after each high temperature experiment. The latter revealed structural modifications imposed during Mossbauer spectroscopy at high temperatures, whereas the in situ experiments identify thermally induced dynamic processes.

An ultrahigh vacuum system for in situ studies of thin films and nanostructures by nuclear resonance scattering of synchrotron radiation.

A multifunctional ultrahigh vacuum (UHV) system has been set up at the nuclear resonance beamline ID18 of the European Synchrotron Radiation Facility (ESRF). Thin and ultrathin films, nanoislands and -wires, multilayers, and stoichiometric oxides can be prepared by molecular beam epitaxy and characterized by low-energy electron diffraction, Auger electron spectroscopy, and reflection high-energy electron diffraction. Upon characterization the sample is transferred under UHV conditions to the chamber for experiments with the synchrotron beam. Electronic and magnetic properties, vibrational dynamics, and diffusion phenomena can be investigated by several synchrotron radiation based techniques, such as nuclear forward scattering, nuclear inelastic and quasielastic scattering, synchrotron radiation based perturbed angular correlations, and nuclear and electronic reflectivity. In addition, two portable UHV chambers serve to transfer the sample to other beamlines profiting from the available experimental techniques at the ESRF.

Diffusion in ordered Fe-Si alloys

The measurement of the diffusional Mössbauer line broadening in single crystalline samples at high temperatures provides microscopic information about atomic jumps. We can separate jumps of iron atoms between the various sublattices of Fe-Si intermetallic alloys (D03 structure) and measure their frequencies. The diffusion of iron in Fe-Si samples with Fe concentrations between 75 and 82 at% shows a drastic composition dependence: the jump frequency and the proportion between jumps on Fe sublattices and into antistructure (Si) sublattice positions change greatly. Close to Fe3Si stoichiometry iron diffusion is extremely fast and jumps are performed exclusively between the three Fe sublattices. The change in the diffusion process when changing the alloy composition from stoichiometric Fe3Si to the iron-rich side is discussed.

Studies of atomic diffusion in Ni-Pt solid solution by x-ray photon correlation spectroscopy

Atomic scale x-ray photon correlation spectroscopy (aXPCS) was used to study atomic diffusion in the Ni(97)Pt(3) solid solution with both a single crystal and a polycrystalline sample. Different jump diffusion models are discussed using experimental results and Monte Carlo simulations. The sensitivity of aXPCS experiments to short-range order (in this case governed by a strong Pt-Pt repulsive force) is demonstrated. The activation energy of 2.93(10) eV as well as diffusivities in the range of 10(-23) m(2) s(-1) at 830 K agree very well with the results of tracer diffusion studies at much higher temperatures.