arXiv subjects

S. M. Dubiel

Publications and source records attributed to S. M. Dubiel.

At least 37 records · Page 2Linked to original sources

arXiv2014

Discovery and characterization of magnetism in sigma-phase intermetallic Fe-Re compounds

Systematic experimental (vibrating sample magnetometry) and theoretical (electronic structure calculations using charge and spin self-consistent Korringa-Kohn-Rostoker Green function method) studies were performed on a series of intermetallic sigma-phase Fe(100-x)Re(x) (x = 43-53) compounds. Clear evidence was found that all investigated samples exhibit magnetism with an ordering temperature ranging between 65 K for x = 43 and 23 K for x = 53. The magnetism was revealed to be itinerant and identified as a spin-glass (SG) possibly having a re-entrant character. The SG was found to be heterogeneous viz. two regimes could be distinguished as far as irreversibility in temperature dependence of magnetization is concerned: (1) of a weak irreversibility and (2) of a strong one. According to the theoretical calculations the main contribution to the magnetism comes from Fe atoms occupying all five sub lattices. Re atoms have rather small moments. However, the calculated average magnetic moments are highly (ferromagnetic ordering model) or moderately (antiparallel ordering model) overestimated relative to the experimental data.

cond-mat.str-el↗

arXiv2014

Structural and electronic properties of the mu-phase Fe-Mo compounds

Structural (lattice parameters and sublattice occupancies) and electronic (charge-density and electric field gradient) properties in a series of compounds mu-Fe_(100-x)Mo_x were studied experimentally (X-ray diffraction and Mossbauer spectroscopy) and theoretically (charge and spin self-consistent KorringaKohnRostoker Green 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 and B predominantly by Fe atoms whereas C and D by Mo atoms hardly depending on the composition. The population of Fe atoms on the site E was ranging between ca. 50% at x=37.5 and ca. 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 equivalent to 0.18 electrons. 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.

cond-mat.str-el↗

arXiv2014

Evaluation of the Debye temperature for iron cores in human liver ferritin and its pharmaceutical analogue Ferrum Lek using Mossbauer spectroscopy

An iron polymaltose complex Ferrum Lek used as antianemic drug and considered as a ferritin analogue and human liver ferritin were investigated in the temperature range from 295K to 90K by means of 57Fe Mossbauer spectroscopy with a high velocity resolution i.e. in 4096 channels. The Debye temperatures equal to 502K for Ferrum Lek and to 461K for human liver ferritin were determined from the temperature dependence of the center shift obtained using two different fitting procedures.

physics.bio-ph↗

arXiv2012

Effect of thermal history on the short-range order in Fe-Cr alloys

Effect of a thermal history of Fe(100-x)Cr(x) (x < 20) samples on a Cr atoms distribution within the first (1NN) and the second (2NN) neighbor-shells was studied with the Mössbauer spectroscopy. The distribution was expressed in terms of the Cowley-Warren short-range order (SRO) parameters: for 1NN, for 2NN and for 1NN-2NN. It was shown to be characteristic of the thermal treatment and of the neigbor shell. For quenched samples, is positive for all x-values, while shows inversion at x about 8 from positive to weakly negative. Similar character has , but the degree of ordering in 1NN-2NN is lower than that in 2NN. Isochronally annealed samples exhibit similar behvior for x > ~8, but significantly different for x < ~8 where the inversion both in and occurs at x about 3, yet in the opposite direction. The follows the trend predicted by Erhart et. al. [PRB 77, 134206 (2008)]. A clear-cut inversion induced by an isothermal annealing at 415 C was found for the Fe85Cr15 sample.

cond-mat.mtrl-sci↗

arXiv2012

Fe-rich border of miscibility gap and activation energy of phase decomposition in a Fe-Cr alloy

Concentration of Cr in the Fe-rich alpha-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 (IR) one annealed at 415 C was determined with Mössbauer spectroscopy. The x-value in the latter was by ~3 at% higher than the one in the NR-counterpart. The activation energy for the phase decomposition in the NR-sample was 122 kJ/mol. 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 for the IR-sample had a value of about 1.

cond-mat.mtrl-sci↗

arXiv2012

Sigma-phase in the Fe-Re alloy system: experimental and theoretical studies

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 σ-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.

cond-mat.mtrl-sci↗

arXiv2012

Structural and hyperfine characterization of σ-phase Fe-Mo alloys

A series of nine samples of σ-Fe_{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 Mossbauer spectra measured at room temperature.

cond-mat.mtrl-sci↗

arXiv2012

Formation energy in σ-phase Fe-V alloys

Formation energy of the σ-phase in the 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 and configuration entropy contributions were used to determine sublattice occupancies for various compositions and temperatures. The results agree well with those obtained from neutron diffraction measurements.

cond-mat.mtrl-sci↗

arXiv2012

Effect of He-ion irradiation on Fe-Cr alloys: Mössbauer - effect study

Effect of He ion 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 Fe(84.85)Cr(15.15) 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, , the average angle between the normal to the sample's surface and the magnetization vector, , as well as the actual distribution of Fe/Cr atoms, as expressed by SRO parameters, strongly depend on D. In particular: (a) 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%, decreases by ~13 degree at maximum, (c) SRO-parameter averaged over the two-shell volume increases with D from weakly negative value (indicative of Cr atoms clustering) to weakly positive value (indicative of Cr atoms ordering). The inversion takes place at D ca. 7 dpa.

cond-mat.mtrl-sci↗

arXiv2012

Effect of magnetism on the lattice dynamics in the sigma-phase Fe-Cr alloys

Anomalies in the temperature dependences of the recoil-free factor, f, and the average center shift, , measured by 57-Fe Mossbauer Spectroscopy, were observed for the first time in the archetype of the σ-phase alloys system, Fe-Cr. 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 were expressed in terms of the underlying changes in the potential, ΔE_p, and the kinetic energy, ΔE_k, 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, as expected for the Debye-like vibrations, but it resembled that of ΔE_p. Only in the range of 4-15 K, ΔE was hardly dependent on T.

cond-mat.mtrl-sci↗

arXiv2012

The Debye temperature of sigma-phase Fe-Mo compounds as determined with Mössbauer spectroscopy

The Debye temperature, T_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 whose rate, in a linear approximation, is equal to 3.1 K/at%. The results are compared with the corresponding ones found previously for the sigma-phase in Fe-Cr and Fe-V compounds.

cond-mat.mtrl-sci↗

arXiv2011

Short-Range Order in Fe-Rich Fe-Cr Alloys as Revealed by Mössbauer Spectroscopy

Distribution of Cr atoms in Fe_{100-x}Cr_x alloys with x \le 25 within the first two coordination shells, 1NN-2NN, around probe 57Fe atoms was studied by means of the Mössbauer Spectroscopy. Clear evidence was found that the distribution is characteristic of a given atomic configuration. Only the configurations with Cr atoms situated in 2NN exhibit a quiasi-random distribution, while those with Cr atoms present in 1NN shell and those with Cr atoms in both shells is evidently not random showing clastering or anticlastering features. These findings are novel and much more complete than the up-to-date knowledge according to which there is an inversion of the short-range order parameter at x = 11. The inversion in the present study was found only for the configuration with one Cr atom in 1NN. On average, Cr atoms are distributed almost randomly within the 1NN-2NN volume for x \eq ~10, and do cluster for larger x.

cond-mat.mtrl-sci↗

arXiv2011

Study of phase stability in the $σ$-FeCr system

Formation energy of the $σ$-phase in the Fe-Cr alloy system, $ΔE$, was computed versus the occupancy changes on each of the five possible lattice sites. Its dependence on a number of Fe-atoms per unit cell, $N_{Fe}$, was either monotonically increasing or decreasing function of $N_{Fe}$, depending on the site on which Fe-occupancy was changed. Based on the calculated $ΔE$ - values, the average formation energy, $<ΔE>$, was determined as a weighted over probabilities of different atomic configurations. The latter has a minimum in the concentration range where the $σ$-phase exists. The minimum in that range of composition was also found for the free energy calculated for 2000 K and taking only the configurational entropy into account.

cond-mat.mtrl-sci↗

arXiv2010

Magnetic properties of $σ$-FeCr alloy as calculated with the charge and spin self-consistent KKR(CPA) method

Magnetic properties of a $σ-$Fe$_{16}$Cr$_{14}$ alloy calculated with the charge and spin self- consistent Korringa-Kohn-Rostoker (KKR) and combined with coherent potential approximation (KKR-CPA) methods are reported. Non-magnetic state as well as various magnetic orderings were considered, i.e. ferromagnetic (FM) and more complex anti-parallel (called APM) arrangements for selected sublattices, as follows from the symmetry analysis. It has been shown that the Stoner criterion applied to non-magnetic density of states at the Fermi energy, $E_F$ is satisfied for Fe atoms situated on all five lattice sites, while it is not fulfilled for all Cr atoms. In FM and APM states, the values of magnetic moments on Fe atoms occupying various sites are dispersed between 0 and 2.5 $μ_B$, and they are proportional to the number of Fe atoms in the nearest-neighbor shell. Magnetic moments of Cr atoms havin much smaller values were found to be coupled antiparallel to those of Fe atoms. The average value of the magnetic moment per atom was found to be $<μ>=0.55 μ_B$ that is by a factor of 4 larger than the experimental value found for a $σ-$Fe$_{0.538}$Cr$_{0.462}$ sample. Conversely, admitting an anti- parallel ordering (APM model) on atoms situated on C and D sites, according to the group theory and symmetry analysis results, yielded a substantial reduction of $<μ>$ to 0.20 $μ_B$. Further diminution of $<μ>$ to 0.15 $μ_B$, which is very close to the experimental value of 0.14 $μ_B$, has been achieved with the KKR-CPA calculations by considering a chemical disorder on sites B, C and D.

cond-mat.mtrl-sci↗

arXiv2010

Relationship between Colours of Ochre from Roussillon and Content of Iron-Bearing Minerals

Nine samples of ochre originating from the Sentier des Ocres near Roussillon, France, were studied with Mossbauer spectroscopy and Spectro-photo-colorimetry. The former yielded a quantitative phase analysis of iron containing minerals (goethite, hematite and kaolin), and the latter enabled determination of the CIE-L*a*b* colorimetric coefficients. Based on the results obtained it is shown that both a* and b* coordinates are responsible for the colour of the investigated ochres.

cond-mat.mtrl-sci↗

arXiv2010

Detection of sub-lattice magnetism in sigma-phase Fe-V compounds by zero-field NMR

The first successful measurements of a sub-lattice magnetism with 51V NMR techniques in the sigma-phase Fe(100-x)Vx alloys with x = 34.4, 39.9 and 47.9 are reported. Vanadium atoms present on all five crystallographic sites are magnetic. Their magnetic properties are characteristic of a given site, which strongly depend on the composition. The strongest magnetism exhibit sites A and the weakest one sites D. The estimated average magnetic moment per V atom decreases from 0.36 muB for x = 34.4 to 0.20 muB for x = 47.9. The magnetism revealed at V atoms is linearly correlated with the magnetic moment of Fe atoms, which implies that the former is induced by the latter.

cond-mat.mtrl-sci↗

arXiv2010

Effect of time and storing conditions on iron forms in ferrous gluconate and Ascofer

Antianemic medicament Ascofer and ferrous gluconate, its basic iron bearing ingredient, were studied with the use of Mossbauer spectroscopy. Room temperature spectra gave a clear evidence that two phases of iron were present viz. ferrous (Fe2+) as a major one with a contribution of 85+-5%, and ferric (Fe3+) whose contribution was found to be 15+-5%. However, the actual values of the contributions of the two kind of the iron ions in Ascofer depend on sample's age: the abundance of Fe2+ ions increases with time by 10% after 51 months, while that of Fe3+ decreases by the same amount. This means that an internal reduction of Fe3+ ions takes place. Ferrous ions were shown to occupy at least two different sites. In Ascofer, the relative abundance of the two sites does not depend on the age of sample, while in the gluconate the population of site 1 increases and that of site 2 decreases with the age of the sample.

physics.med-ph↗

arXiv2010

Vibrational properties of alpha- and sigma-phase Fe-Cr alloy

Experimental investigation as well as theoretical calculations, of the Fe-partial phonon density-of-states (DOS) for nominally Fe_52.5Cr_47.5 alloy having (a) alpha- and (b) sigma-phase structure were carried out. The former at sector 3-ID of the Advanced Photon Source, using the method of nuclear resonant inelastic X-ray scattering, and the latter with the direct method [K. Parlinski et al., Phys. Rev. Lett. {78, 4063 (1997)]. The characteristic features of phonon DOS, which differentiate one phase from the other, were revealed and successfully reproduced by the theory. Various data pertinent to the dynamics such as Lamb-Mossbauer factor, f, kinetic energy per atom, E_k, and the mean force constant, D, were directly derived from the experiment and the theoretical calculations, while vibrational specific heat at constant volume, C_V, and vibrational entropy, S were calculated using the Fe-partial DOS. Using the values of f and C_V, we determined values for Debye temperatures, T_D. An excellent agreement for some quantities derived from experiment and first-principles theory, like C_V and quite good one for others like D and S was obtained.

cond-mat.mtrl-sci↗