Stanislaw Dubiel

The kinetics of the s -phase formation promoted by an isothermal anneal at T 700ë Chasbeenstudiedinapolycrystallinealloyof (Fe 53.8Cr46.2)± 0.1at.%Ti using the 57Fe MoÈ ssbauer eÄ ect. It was found that the addition of 0 .1at.%Ti to an Fe53.8Cr46.2 alloy drastically accelerates the a ± s transformation. The fraction of thes phaseprecipitatedfollowstheAvrami± Johnson± Mehl equation. Thebest ® tstothedatayieldedthetimeconstants, fromwhichaTi-induceddecreaseinthe activation energy

Ca,Fea-,O4 samples, with z 5 0.28, prepared from a- Fez03 and CaO by the Bridgeman method were investigated by means of 57Fe Miissbauer spectroscopy. Room temperature spectra gave evidence that the samples were multi-phase with 73% to 99% contribution of the calcjomagnetite, depending on the sample. Analysis of the subspectra associated with the calciomagnetite enabled to condnde that Ca-ions reside

Scales resulted from a sulphidation of Fe 100−x Cr x alloys (x ≤ 26) at 1073K in the atmosphere of H 2/H 2 S with the partial pressure of sulphur of 10−8 atm were investigated by means of transmission and conversion electron Mssbauer spectroscopies, X-ray diffraction, electron microscopy and microprobe analysis. Although crystallographically single-phase, the scales were revealed to be

Page 1. The Debye temperature of the Fe–Cr sigma-phase alloys This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2005 J. Phys.: Condens. Matter 17 6889 (http://iopscience.iop.org/0953-8984/17/43/008) ...

The influence of sulphidation on the metallic phase in a series of Fe100- x Cr x alloys ( x<15) was investigated by means of57Fe Mössbauer spectroscopy. The results obtained gave evidence that the sulphidation at 1073 K is selective in the atmosphere of H2/H2S, with the partial pressure of sulphur ranging from 10-7 to 10-9 atm. At x cr=5.0(1) at.%, a change of the sulphidation preference occurs. For x 5.0 at.%, in iron.

A series of four r-Fe100xMox samples was investigated with ac magnetic susceptibility measurements.
An evidence was found that the ground magnetic state of the samples is constituted by a spin glass
(SG) with the spin glass temperature ranging between ca. 34 K for x = 45 and ca. 16 K for x = 53. The SG state
is heterogeneous and it can be divided into a weak-irreversibility and a strong-irreversibility domains. Its
figures of merit are typical of metallic (canonical) spin glasses

A series on nanocrystalline Fe(100-x)Cr(x) alloys prepared by mechanical alloying was investigated with Xray
diffraction (XRD), scanning electron microscopy (SEM) and M€ossbauer spectroscopy (MS) techniques.
XRD was used to structurally characterize the samples whereas MS permitted phase analysis as well as
determination of the Debye temperature, TD. Concerning the latter, an enhancement relative to bulk qDvalues
was revealed in the range of ~40 < x <~50. In a sample of Fe55.5Cr44.5 two phases were detected
viz. (1) crystalline and magnetic with TD = 572 (56) K and (2) amorphous and paramagnetic with TD = 405 (26) K.

Electronic structure Korringa-Kohn-Rostoker calculations for the s-phase in Fe(100-x)V(x) were performed in compositional range of its occurrence (34 < x < 65). Fe and V magnetic moments and hyperfine fields were determined for five inequivalent lattice sites in two models of magnetic structure, namely ferromagnetic FM and so-called anti-parallel one, APM, dominated by antiferromagnetic coupling. The average magnetic moments calculated for FM state overestimate the experimental data, whereas the
corresponding quantities computed for APM state, underestimate them. Such a behavior remains in line
with total energy values being similar for both models. The calculations showed that both average magnetic moments and hyperfine fields (on Fe and V atoms) vary with a number of Fe atoms in the nearest neighbor shell, NNFe, starting from critical values of NNFe, characteristic of each site.
The calculated hyperfine fields for Fe and V showed an important role of valence contributions, being strongly dependent on local magnetic moments arrangements. In the case of Fe, the computed hyperfine fields are overestimated with respect to the Mossbauer data, while the corresponding values for V are underestimated with respect to the NMR results. However, the linear correlation between average magnetic moment and hyperfine fields, observed experimentally in FeV sigma-phase, can be well reproduced when combining theoretical results for the two above-mentioned magnetic structure models.

A series of nine samples of sigmaFe(100-x)Mo(x) with 44 < x < 57 were synthesized by a sintering method. The samples were investigated experimentally and theoretically. Using X-ray diffraction techniques structural parameters such as lattice constants, atomic positions within the unit cell and populations of atoms over five different sublattices were determined. An information on charge-densities and electric field gradients at particular lattice sites was obtained by application of the Korringa-Kohn-Rostoker (KKR) method for electronic structure calculations. Hyperfine quantities calculated with KKR were successfully applied to analyze Mössbauer spectra measured at room temperature.

The magnetization of the sigma-phase Fe0.53Cr0.47 and Fe0.52V0.48 alloys was studied as a function of temperature and field. The experiments show that both materials behave
magnetically as re-entrant spin glass systems. Field versus temperature diagrams were obtained where the locations of the paramagnetic phase, the intermediate ferromagnetic-like
phase and the spin glass fundamental state were displayed. These diagrams are in qualitative agreement with the predictions of the mean field theory for the interplay between the ferromagnetic and spin glass orderings. The critical phenomenology near the para–ferromagnetic transition could be investigated. It was found that the paramagnetic susceptibility is quite well described by the extended scaling scheme, where the reduced temperature is written as tau=(T-Tc)/T. The value obtained for the susceptibility critical exponent gamma is intermediate between the prediction of the 3D Heisenberg universality class and the large values observed in spin glasses, as previously found in other re-entrant systems. The data do not confirm the validity of the extended scaling in the ferromagnetic-like phase. Using either the conventional or extended scaling protocols, the exponents betha and delta were found to have values close to those reported for spin glass transitions. Despite the relevance of disorder and the anomalous values determined for betha, gamma  and delta, the Widom scaling relation holds as an equality.

Neutron powder diffraction was used to study the distribution
of Co and Cr atoms over different lattice sites as well as the
lattice parameters of sigma-phase compounds Co(100 - x)Cr(x) with x = 57.0, 62.7 and 65.8. From the diffractograms recorded
in the temperature range of 4.2–300 K it was found for the five crystallographically independent sites that A (2a) and D (8i) are predominantly occupied by Co atoms, while sites B (4f), C (8i) and E (8j) mainly accommodate Cr atoms. The lattice parameters a and c exhibit linear temperature dependencies, with different expansion coefficients in the temperature ranges of 4.2–100 and 100–300 K.

Concentration of Cr in the Fe-rich a-phase, x, resulted from a phase decomposition caused by an
isothermal annealing at T = 415 and 450 C of a non-irradiated (NR) Fe-Cr14 EFDA sample and that of a He-ions irradiated one (IR) annealed at 415 C was determined with Mossbauer spectroscopy. The x-value in the latter was by ca. 3 at% higher than the one in the NR-counterpart. The activation energy for the phase decomposition in the NR-sample was 122 kJ mol1. In the IR-sample its value was by 12 kJ/ mol
lower. Avrami exponents for the NR-samples were close to 0.5, and that the IR-sample had a value of about 1.

Application of Mössbauer spectroscopy with a high velocity resolution (4096 channels) for comparative
analysis of iron cores in a human liver ferritin and its pharmaceutically important models Imferon,
Maltofer® and Ferrum Lek as well as in iron storage proteins in chicken liver and spleen tissues allowed
to reveal small variations in the 57Fe hyperfine parameters related to differences in the iron core structure.
Moreover, it was shown that the best fit of Mössbauer spectra of these samples required different
number of components. The latter may indicate that the real iron core structure is more complex than
that following from a simple core–shell model. The effect of different living conditions and age on the
iron core in chicken liver was also considered.

Effect of 25 keV He ion irradi ation on three model Fe(100-x)Cr(x) alloys (x = 5.8, 10.75 and 15.15) was investigated
by means of the conversion electron Mössbauer spectroscopy. A short-range ordering (SRO)
parameter concerning a distribution of Cr atoms wit hin the first-two neighbour shells, <a12>, for all three
non-irradiate d samples was positive, hence indicative of ordering, yet its amplitude , hence the degree of
ordering, was decreasing with x. For the samples irradiated to the dose of 1.2 10E17 He+/cm2 (7.5 dpa)
<a12 > was unaffe cted for x = 5.8 and 10.75 but its value was negative for x = 15.15 i.e. an irradiationinduced
inversion of the SRO-parameter occurred.

Distribution of Cr atoms in Fe(1−x)Cr(x) studied with the Mössbauer spectroscopy was shown to be
characteristic of neigbor shell, and, to lesser extent, of heat treatment. SRO-parameter averaged over the first-two neighbor shells, 〈α12〉, in quenched samples was slightly positive o rclose to zero for x<∼0.14, and more positive at higher x-values. In isochronally annealed samples 〈α12〉 followed the trend
predictedbyMCsimulations[PRB77,134206(2008)].Clear-cutsegregationinducedbyanisothermal
annealing at415 1C wasfoundforFe0.85Cr0.15

X-ray diffraction (XRD) and Mössbauer spectroscopy techniques combined with theoretical calculations
based on the Korringa-Kohn-Rostoker (KKR) electronic structure calculation method were used to
investigate sigma-phase Fe(100-x)Re(x) alloys (x = 43, 45, 47, 49 and 53). Structural data such as site occupancies
and lattice constants were derived from the XRD patters, while the average isomer shift and distribution
curves of the quadrupole splitting were obtained from the Mössbauer spectra. Fe-site charge-densities
and the quadrupole splittings were computed with the KKR method for each lattice site. The calculated
quantities combined with the experimentally determined site occupancies were successfully used to
decompose the measured Mössbauer spectra into five components corresponding to the five sublattices.

Formation energy of the sigma-phase in he Fe–V alloy system, ΔE, was computed in the full compositional
range of its occurrence (∼34 ≤x≤∼60) using the electronic band structure calculations by means of the KKR method. ΔE-values were found to strongly depend on the Fe concentration,also its variation with different site occupancies was characteristic of a given lattice site. Calculated magnetic, S_m, and configuration, S_c, entropy contributions were used to determine sublattice occupancies for various compositions and temperatures. The results agree well with those obtained from neutron diffraction
measurements.

Effect of He ions irradiation on three model Fe(100-x)Cr(x) alloys (x = 5.8, 10.75 and 15.15) was investigated
with the conversion electron Mössbauer spectroscopy. The study of the alloys irradiated with 25 keV ions
revealed that the strongest effect occured in the Fe84.85Cr15.15 sample where an inversion of a short range-
order (SRO) parameter was found. Consequently, the investigation of the influence of the irradiation
dose, D, was carried out on the chromium – most concentrated sample showing that the average
hyperfine field, <B>, the average angle between the normal to the sample’s surface and the magnetization
vector, <theta>, as well as the actual distribution of Fe/Cr atoms, as expressed by SRO parameters, strongly
depend on D. In particular: (a) <B> increases with D, and its maximum increase corresponds to a decrease
of Cr content within the two-shell volume around the probe 57Fe nuclei by 2.3 at%, <theta> decreases by
13 deg at maximum, (c) SRO-parameter averaged over the two-shell volume increases with D from weakly
negative value (indicative of Cr atoms ordering) to weakly positive value (indicative of Cr atoms clustering).
The inversion takes place at D = 7 dpa.

The Debye temperature, Q_D, of sigma-phase Fe(100-x)Mo(x) compounds with 47 < x < 56.7 was determined from
the temperature dependence of the centre shift of Mössbauer spectra recorded in the temperature range
of 80-300 K. Its compositional dependence shows a weak increase with x the rate of which, in a linear
approximation, is equal to 3.1 K/at%. The results are compared with the corresponding ones found
previously for the s-phase in FeeCr and FeeV compounds showing that Q_D-values are characteristic of
a given alloy.

Anomalies in the temperature dependences of the recoil-free factor, f, and the average
center shift, <CS>, measured by 57Fe Mossbauer spectroscopy, were observed for the first time
in the archetype of the σ-phase alloys system, FeCr. In both cases the anomaly started at the
temperature close to the magnetic ordering temperature, and in both cases it was indicative of
lattice vibrations hardening. As no magnetostrictive effects were found, the anomalies seem to be
entirely due to a spin-phonon coupling. The observed changes in f and in <CS> were expressed in
terms of the underlying changes in the potential, ΔEp, and the kinetic energy, ΔEk, respectively.
The former, with the maximum value larger by a factor of six than the latter, decreases, while the
latter increases with T. The total mechanical-energy change, ΔE, was, in general, not constant,
but it resembled that of ΔEp. Only in the range of 4–15 K, ΔE was hardly dependent on T. The
decrease of ΔE on lowering T can be interpreted as an increase of the spin-phonon coupling.

A comparative study of ferrous gluconate as well as fresh and outdated tablets
of Ascofer® was carried out usingM¨ossbauer spectroscopy with a high velocity resolution.
The obtained results revealed the presence of three ferrous and one ferric component in
all investigated samples which may be related to ferrous gluconate molecules and ferric
contamination and/or aging effect

Structural (lattice parameters and sub lattice occupancies) and electronic (charge-density and electric
field gradient) properties in a series of mu-Fe(100-x)Mo(x) (37.5 <x< 44.5) compounds were studied
experimentally (X-ray diffraction and Mössbauer spectroscopy) and theoretically (charge and spin
self-consistent Korringa–Kohn–Rostoker Green’s function method). The lattice parameters a and c
showed a linear increase with x while all five lattice sites were found to be populated by both alloying
elements: A(1a) and B(6h) predominantly by Fe atoms whereas C(2c) and D(2c') by Mo atoms hardly
depending on the composition. The population of Fe atoms on the site E(2c") was ranging between
50% at x = 37.5 and 20% at x = 44.5. Fe-site charge-density (isomer shift) and the electric field gradient
(quadrupole splitting, QS) were revealed to be characteristic of the lattice site and both of them were
almost x-independent. The difference in the charge-density at Fe-atoms at the sites B (the highest value)
and those at the sites D (the lowest value) was estimated as high as 0.18 e. The average charge-density
increases linearly with x. The largest QS-values were those at the sites A and C, while the smallest ones at
the site D. The average QS-value was 0.25 mm/s.