Mossbauer Measurements Research Articles

Mechanosynthesis of nanocrystalline MgFe2O4—neutron diffraction and Mössbauer spectroscopy

The evolution of nanocrystalline n-MgFe2O4 by high-energy milling a mixture of MgO and α-Fe2O3 for periods of between 0 h and 12 h has been investigated by neutron diffraction in addition to previous Mossbauer, XRD and HRTEM measurements. Complete transformation of the milled products to n-MgFe2O4 only occurs on milling to ∼8 h even though the average particle size decreases to < ∼10 nm after milling for 2 h. The applied field Mossbauer spectra of n-MgFe2O4 can be well described by two subspectra representing core and shell regions with different cation distributions and spin canting angles. The neutron pattern of nanocrystalline MgFe2O4 is described well by two components comprising nanoparticles of core and shell dimensions ∼7(1) nm and ∼0.7(1) nm, respectively, in support of the Mossbauer core-shell model.

Phase transformations of the FePO4 catalyst in the oxidative dehydrogenation to form an alkyl methacrylate

A phase specific iron orthophosphate catalyst, FePO4, was synthesized and subjected to oxidative dehydrogenation reactions to form an alkyl methacrylate. The phases of the catalyst, before and after the reactions, were characterized by Mossbauer spectroscopy. The Mossbauer spectra show the change of the catalyst precursor FePO4, tridymite-like phase (tdm), to the reduced form, iron(II) pyrophosphate, Fe2P2O7, and thereafter the phase change is governed by the temperatures of oxidation. X-ray diffraction and Mossbauer measurements on the spent catalyst, after using organic and water co-feeds, show a transformation of the catalyst to a mixture of phases which are condition specific.

Mössbauer spectroscopy and magnetic transition ofλ−(BETS)2FeCl4

The compound lambda-(BETS)2FeCl4 provides an effective demonstration of the interaction of pi-conduction electron and d-electron localized moment systems in molecular crystalline materials where antiferromagnetic insulating and magnetic field induced superconducting states can be realized. The metal-insulator transition has been thought to be cooperative, involving both the itinerant pi- electron and localized d-electron spins where antiferromagnetic order appears in both systems simultaneously. However, recent specific heat data has indicated otherwise [Akiba et al., J. Phys. Soc. Japan 78,033601(2009)]: although the pi-electron system orders antiferromagnetically and produces a metal-insulator transition, a "mysterious" paramagnetic d-electron state remains. We report 57Fe Mossbauer measurements that support the paramagnetic model, provided the d-electron spins remain in a fast relaxation state below the transition. From the measured hyperfine fields, we also determine the temperature dependence of the pi-d electron exchange field.

Multiple Bistability and Tristability with Dual Spin-State Conversions in [Fe(dpp)2][Ni(mnt)2]2·MeNO2

The multicomponent system of [Fe(dpp)(2)][Ni(mnt)(2)](2).MeNO(2) (1; dpp = 2,6-bis(pyrazol-1-yl)pyridine and mnt = maleonitriledithiolate) was prepared by the reaction of [Fe(dpp)(2)](BF(4))(2) with (Bu(4)N)[Ni(mnt)(2)] in MeNO(2). Variable-temperature X-ray structural analyses, magnetic susceptibility, and heat capacity measurements confirmed that 1 undergoes multiple spin-state conversions in both the cationic and anionic components. The asymmetric unit in the crystal contains one [Fe(dpp)(2)](2+) cation, two [Ni(mnt)(2)](-) anions ([Ni1](-) and [Ni2](-)), and one solvent molecule. Magnetic susceptibility measurements revealed that a paramagnetic state in the high-temperature region (HT phase) was abruptly converted to a diamagnetic low-temperature (LT) phase below 180 K as the temperature was lowered from 270 K. As the temperature was raised from 125 to 270 K, successive phase transitions occurred to the HT phase via intermediate phases (IM1, IM2, and IM3) at 175.5, 186.5, 194.0, and 244.0 K, respectively. In the HT phase [Fe(dpp)(2)](2+) is in the high-spin state, and each [Ni1](-) and [Ni2](-) moiety is arranged in monomeric form with an S = (1)/(2) spin ground state. In the LT phase [Fe(dpp)(2)](2+) is in the low-spin state and the nickel moieties are dimerized and diamagnetic. In the IM1 and IM2 phases the iron(II) sites are partially in the HS state and both [Ni](-) moieties are dimeric, as suggested by (57)Fe Mossbauer measurements. In the IM3 phase, [Fe(dpp)(2)](2+) is in the HS state and the anions exist in both their monomeric ([Ni1](-)) and dimeric ([Ni2](-)) forms. Rapid thermal quenching from 300 to 5 K yielded a metastable HS phase, which relaxed to the LT phase via the IM1 phase as the temperature was raised to 150 K. A partial light induced spin transition on the iron site was observed at 5 K.

Iron Oxide-Based Nanoparticles with Different Mean Sizes Obtained by the Laser Pyrolysis: Structural and Magnetic Properties

Nano-sized iron oxide-based particles have been directly synthesized by the laser induced pyrolysis of a mixture containing iron pentacarbonyl/air (as oxidizer)/ethylene (as sensitizer). In this paper we further demonstrate the possibility to vary the chemical composition and the nanoparticle dimensions of the iron oxide-based materials by handling the oxidation procedure in the frame of the laser pyrolysis process. Thus, nanoparticles with major maghemite/magnetite content may change composition into mixtures with variable amounts of three components: major gamma-Fe2O3/Fe3O4 iron oxide, metallic Fe and cementite Fe3C. By X-ray diffraction (XRD) it is found that the relative proportion of these phases differs in function of the reaction temperature (laser power). As revealed by transmission electron microscopy (TEM), mean particle sizes between about 4 nm and 6 nm and between about 9 and 11 nm may be prepared by varying the oxidation procedure and the laser power, respectively. By the controlled heating of samples (maximum temperature 185 degrees C), increased crystallinity for the gamma-Fe2O3/Fe3O4 oxide phase was found as well as an increase of the mean particle diameters. The examination of the magnetization curves for samples obtained for different laser powers indicates notable differences in the magnetic behavior and parameters. The temperature dependent Mossbauer measurements confirm the formation of larger particles at higher laser power densities as well as the presence of inter-particle magnetic interactions. On this basis, the estimation of phase composition for the different representative samples is given.

Processing of amplitude spectra in measurements with ionizing radiation by low energy resolution detectors

A method using intermediate processing of amplitude (energy) spectra is described. The method is an alternative to the known differential discrimination method, which is widely applied in measurements of EXAFS spectra and in Mossbauer and diffraction measurements. The method is efficient in cases in which the radiation spectrum contains lines poorly resolved in the used detector. For example, if weak fluorescence of a low-concentration impurity is detected in a sample against the background of intense fluorescence of the matrix, the described method allows one to noticeably improve the quality of the obtained information.

Microscopic Properties of γ-FeMn Studied by Mössbauer Spectroscopy

Detailed analysis of hyperfine interactions in γ-Mn 60 Fe 37 Cu 3 is presented. Isotropic three-dimensional Gaussian shape was introduced for modeling the distribution of hyperfine magnetic field derived from Mossbauer measurements performed in a broad temperature range. Electrical quadrupole interactions resulting from broken local symmetry, which cannot be treated as small perturbation in the vicinity of transition temperature, were treated by exact method of invariants. Obtained values of quadrupolar interactions are small and cause only spectra broadening. Small values of measured electric field gradient are confirmed by empirically found correlation between known quadrupolar splitting in selected class of systems and an estimation based on postulated pseudopotential. The effects of anharmonicity were considered in the interpretation of the temperature dependence of shift of the spectrum centre.

Reduction of hematite with ethanol to produce magnetic nanoparticles of Fe3O4, Fe1 − x O or Fe0 coated with carbon

The production of magnetic nanoparticles of Fe3O4 or Fe0 coated with carbon and carbon nanotubes was investigated by the reduction of hematite with ethanol in a Temperature Programmed Reaction up to 950°C. XRD and Mossbauer measurements showed after reaction at 350°C the partial reduction of hematite to magnetite. At 600°C the hematite is completely reduced to magnetite (59%), wustite (39%) and metallic iron (7%). At higher temperatures, carbide and metallic iron are the only phases present. TG weight losses suggested the formation of 3–56 wt.% carbon deposits after reaction with ethanol. It was observed by SEM images a high concentration of nanometric carbon filaments on the material surface. BET analyses showed a slight increase in the surface area after reaction. These materials have potential application as catalyst support and removal of spilled oil contaminants.

Hematite reaction with tar to produce carbon/iron composites for the reduction of Cr(VI) contaminant

In this work, highly reactive carbon–iron composites were prepared using a waste, i.e. tar, as carbon precursor and a simple iron oxide, i.e. hematite. Tar was impregnated on Fe2O3 with different tar/hematite weight ratios of 1:1; 2:1 and 4:1, and thermally treated under N2 atmosphere (400°C, 600°C and 800°C). Mossbauer, XRD and magnetization measurements suggested that treatment at 400°C and 600°C produces Fe3O4 but treatment at 800°C produced mainly Fe°. Raman and TG analyses of the different composites suggested the formation of carbon contents of 18, 24 and 32 wt.% as amorphous and graphitic highly dispersed on the Fe surface. The composites obtained at 800°C showed high efficiency to reduce Cr(VI) as CrO\(_{4}^{2-}\) in aqueous medium with much better results compared to finely ground commercial Fe°.

Structure and electrophysical properties of a lead iron niobate-based amorphous material

An amorphous material based on lead iron niobate (PFN) is studied by X-ray diffraction and dielectric and Mossbauer measurements over a wide temperature range. The atomic structure of amorphous PFN is found to be substantially disordered, which suppresses the transitions into ordered states in the electric and magnetic dipole subsystems that are inherent in crystalline PFN. The dependence of electrical conductivity σ on field frequency ω is shown to correspond to the law σ ∼ ω s , where parameter s decreases linearly with increasing temperature. This law corresponds to a hopping carrier transfer mechanism that is correlated due to the Coulomb interaction.

Characterization of Fe and Fe50Ni50 ultrafine nanoparticles synthesized by inert gas-condensation method

Nanoparticles of Fe and Fe50Ni50 were synthesized by inert gas-condensation method under pure helium atmosphere. The prepared nanoparticles samples were examined by high-resolution transmission electron microscopy, X-ray diffraction and Mossbauer spectroscopy. The synthesized nanoparticles consisted of core–shell type structure nearly spherical shape with a size comprised within the range 4–70 nm and they occur as clusters or chains. The Mossbauer measurements as well as X-ray diffraction showed, in both cases, the presence of iron-oxide phases.

Magnetic Properties of Iron Sulfides Doped With 3-D Transition-Metals

It is found by Mossbauer measurements on M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.025</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.975</sub> S (M=Sc , Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) that the 3-D-transition metal impurities profoundly affect both the structural phase and the spin rotation transitions of iron sulfide. It is noteworthy that both V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.025</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.975</sub> S and Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.025</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.975</sub> S have Morin transition temperatures T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</sub> which are distinctly different from that of FeS; furthermore, the directions of changes of T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</sub> are opposite for V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.025</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.975</sub> S and Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.025</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.975</sub> S . A vanadium impurity of 2.5% of the metal atoms in the iron sulfide makes the structure transition take place rapidly in a narrow temperature region of about 15 K, while the alpha transition in FeS takes place over a wide temperature range of about 200 K. It is also found that the alpha transition for V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.025</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.975</sub> S has a hysteresis width of 5 K. It is noteworthy that the structural transition is independent of the lattice parameters while the spin-rotation transition is dependent on them.

Mössbauer study of the magnetic phase composition of single-crystalline rutile (TiO2) implanted with iron ions

Depth-resolved Mossbauer measurements have been performed for four ferromagnetic samples obtained by the implantation of iron ions (enriched to ∼ 50% with 57Fe isotope) into single-crystalline rutile (TiO2) substrates with two crystallographic orientations [(100) and (001)] at different temperatures (300 and 900 K). It is established that the ferromagnetic properties of iron-implanted rutile samples at room temperature are determined by the presence of α-Fe and Fe3O4 phases. The phase composition of samples obtained by iron implantation into substrates heated to 900 K depends on the crystallographic orientation of the substrate, which is explained by a significant anisotropy of the diffusion of iron atoms in rutile.

Mössbauer Study of Iron Minerals Formed by Dissimilatory Bacterium

The kinetics of formation of iron-containing minerals by thermophilic dissimilatory iron-reducing bacteria Themincola ferriacetica (strain Z-0001) has been investigated by Mössbauer spectroscopy. The Mossbauer measurements were performed at room and low (77 and 4.2 K) temperatures and in a magnetic field of 6 T. Electron magnetic resonance (EМR) measurements were also carried out.

Gummic acid stabilized γ-Fe2O3 aqueous suspensions for biomedical applications

Biomedical applications of magnetic nanoparticles depend critically on their preparation as aqueous colloidal suspensions, or ferrofluids, with long term stability under physiological conditions. Dispersion of the magnetic nanoparticles is generally achieved by the use of protein cages, polysaccharide, polypeptide and charged macromolecular coatings, which minimize interparticle magnetic interactions, particle agglomeration and precipitation. The synthesis and characterization of gummic-acid stabilized maghemite ferrofluids is reported. X-ray diffraction, transmission electron microscope and dynamic light scattering measurements give a γ-Fe2O3 magnetic core diameter of 8 nm and a nanocomposite particle hydrodynamic diameter of 50 nm. Mossbauer and magnetization measurements indicate the presence of isolated, sterically stabilized superparamagnetic nanoparticles resistant to aging, and thus, promising agents for the production of novel magneto-pharmaceuticals.

Mössbauer diagnosis of the electronic structure and local environment of 119Sn in MgTiO3

Annealing of a hydroxide precursor containing equimolar amounts of Mg2+ and Ti4+ and small additions of Sn4+ (0.1 at %) in air at 900°C leads to titanate MgTiO3 with an ilmenite structure. The 119Sn Mossbauer spectrum of the sample (unresolved doublet with the isomer shift δ = 0.10 ± 0.01 mm/s and the quadrupole splitting Δ = 0.49 ± 0.02 mm/s) is evidence that the tin atoms are still in the oxidation state +4. Annealing of the precursor at the same temperature in a hydrogen atmosphere yields MgTiO3 containing Sn2+ ions (a doublet at δ = 2.82 ± 0.01 mm/s and Δ = 1.66 ± 0.03 mm/s) (the Sn2+/MgTiO3 sample). According to the spectral parameters, the 119Sn2+ ions have a low coordination number (CN ≪ 6) and are abnormally resistant to reduction to the metal. Analogous features of the crystal-chemical behavior of 119Sn2+ were previously observed during the Mossbauer study of the samples containing tin on the surface of Cr2O3, α-Al2O3, and MgO crystallites. The conclusion drawn from analysis of the 119Sn2+ Mossbauer parameters that tin in the Sn2+/MgTiO3 sample has surface localization was supported by X-ray photoelectron spectroscopy. Mossbauer measurements show that the tin of Sn2+/MgTiO3 when in contact with air is oxidized much more slowly than on the surface of Cr2O3, α-Al2O3, or MgO crystallites. The inhibition of the oxidation reaction is explained to be due to passivation of adsorbed O2 molecules caused by their interaction with mobile t2g electrons of Ti3+ forming in titanate during high-temperature annealing in H2. In addition to the Sn2+ doublet, the 119Sn spectrum shows a spectral component with parameters (δ ∼ 1.6 mm/s, Δ ≤ 0.2 mm/s) not fitting the known tin species that can form in MgTiO3. This component is explained by persistence in titanate of some Sn4+ ions immobilizing the mobile t2g electron at one of their neighboring Ti4+ cations.

Study of weak ferromagnetism in polycrystalline multiferroic Eu doped bismuth ferrite

Room temperature spontaneous magnetization and dielectric anomaly at Néel temperature are observed in 15% Eu doped bismuth ferrite indicating the multiferroic nature of the sample. With 15% Eu doping structural transformation from rhombohedral to triclinic is observed. Fe3+ and Eu3+ oxidation states are observed from F57e and E151u Mossbauer measurements, respectively. The high field F57e Mossbauer spectrum in longitudinal geometry shows an enhancement in the intensity of lines corresponding to Δm=0 transitions, i.e., second and fifth lines in six line pattern. This observation suggests that the origin of spontaneous magnetization is due to weak ferromagnetism of Dzyaloshinskii–Moriya type.

Force Spectroscopy of the Iron Atom in Heme Proteins

Nuclear resonance vibrational spectroscopy (NRVS) selectively reveals the complete vibrational density of states (VDOS) of a Mossbauer probe nucleus within a protein. Frequency moments of the VDOS determine effective force constants for 57Fe at the active sites of cytochrome c (cyt c) and deoxymyoglobin (Mb). The stiffness measures the force needed to displace the Fe with the other atoms fixed, and probes the nearest neighbor interactions with the Fe. The stiffness of the low spin Fe environment in cyt c greatly exceeds that for the high spin Fe in Mb, reflecting the shorter Fe–N bonds to the heme. Moreover, a significant stiffness decrease upon oxidation of cyt c tracks the longer Fe–S bond to Met 80 in the oxidized protein. Quantitative comparison with 57Fe/54Fe frequency shifts suggests that Fe-L vibrations contribute to the Raman signal of cyt c recorded in resonance with the heme Soret band. The resilience measures the force needed to displace the Fe with the surrounding atoms free to respond, and determines the magnitude of the thermal fluctuations of the Fe on a time scale determined by the experimental energy resolution (ca. 4 ps for the results reported here). Quantitative agreement with the temperature-dependent mean squared displacement determined from independent Mossbauer measurements confirms longstanding assumptions that vibrational motion dominates thermal fluctuations of the heme Fe below the well-known dynamical transition at ca. 200 K and identifies THz frequencies below 100 cm−1 as the dominant contribution. The resilience increases significantly for cyt c with respect to Mb, which we attribute to the increased number of covalent links between heme and peptide in the former protein. Molecular dynamics simulations reproduce the increased resilience of cyt c, but find no significant change with oxidation state.

Nb-containing hematites Fe2−xNbxO3: The role of Nb5+ on the reactivity in presence of the H2O2 or ultraviolet light

Abstract A series of Nb-containing hematites, Fe2−xNbxO3 (%Nb = 0.00, 1.49, 5.00 and 9.24) was prepared using the conventional co-precipitation method. Mossbauer and temperature-programmed reduction (TPR) measurements suggested the formation of the crystalline phase with partial substitution of Fe3+ by Nb5+ in the structure. N2 adsorption/desorption revealed that the presence of Nb has a remarkable effect on the textural properties of the material with an increase in the BET surface area. The reactivity of Fe2−xNbxO3 was investigated using the oxidation of the methylene blue dye used as a model pollutant. The obtained results showed that the presence of Nb seems not to act directly promoting the H2O2 decomposition, but improving the dye oxidation. The analysis using the ESI-MS technique showed partial oxidation observed through different intermediates before the mineralization. This suggests the use of Nb-doped hematite as an efficient catalyst in degradation reactions in the presence of H2O2 or ultraviolet light.

57Fe Hyperfine Interactions in the Sc(Fe1-xNix)2Laves Phases Synthesized under High Pressure

The structural and microscopic magnetic properties of the pseudobinary Sc(Fe1−xNix)2 system prepared under high pressure of 8 GPa were studied by X-ray diffraction and Mossbauer measurements in the range of 0.0 ≤ x ≤ 0.60. The samples for 0.0 ≤ x ≤ 0.10 have the hexagonal C14-type structure and those for 0.30 ≤ x ≤ 0.60 crystallize in the C15-type structure. Both the magnetic hyperfine field on Fe and the Curie temperature decrease with the increasing Ni content and the system becomes paramagnetic for x ≈ 0.60 at room temperature. The form and temperature dependences of the Mossbauer spectra for 0.40 ≤ x ≤ 0.50 indicate the coexistence of paramagnetic and ferromagnetic regions in the samples and occurrence of magnetic clusters with a wide distribution of the Curie temperatures. It is interesting that TC for the compounds obtained under high pressure are by about 100 K smaller in comparison with those for the isostructural samples produced under normal pressure even though interatomic distances are practically equal in the both types of compounds.