On the application of selective-excitation double Mössbauer spectroscopy to problems in materials science

Results showing the influence of optical thickness of a scatterer in selective-excitation double M?ssbauer spectroscopy are presented on the example of α-Fe and FeBO₃. Significant transformation of the spectral γ-radiation structure predicted theoretically is demonstrated for single-layered FeBO₃.     

Evaluation of the detection limit in electron-recording Mössbauer spectroscopy

A semiempirical formula is derived for evaluating the lowest concentration of Mössbauer atoms whose signal can be reliably recorded by electron Mössbauer spectroscopy. The concentration is calculated from the resonance effect magnitude in a sample of known chemical composition, on the basis of the cross sections of photoelectric absorption of source gamma photons by the sample atoms, the energy and probability of production of photoelectrons and their associated Auger electrons, and the probability of recoilless gamma-photon absorption in the sample. Experimental testing showed that the calculated and measured values of differ by not more than 20%.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 20–23, September, 1987.     

Mössbauer spectroscopy in physical metallurgy

In this anniversary contribution the natural and intimate match making which occurs between the two star performers-⁵⁷Fe in Mössbauer spectroscopy and iron in physical metallurgy—is described by selecting typical examples reflecting the author's interest: phases, f.c.c. -Fe, defects, diffusion and amorphous metals.     

Mössbauer spectroscopy in molecular magnetism

The aim of this paper is to highlight some selected research activities on molecular magnetic materials using Mössbauer spectroscopy as a technique carried out in our laboratory in recent years. The first part of the present article is devoted to the studies of the various magnetic interactions, metal-to-metal electron-transfer phenomenon, glass transition occurring in molecular magnetic materials, whereas the second part deals with the iron(II) high spin (S = 2)–low spin (S = 0) transition phenomenon occurring in some isoxazole ligand based iron(II) compounds as examples with unusually complicated spin transition behaviour. Also, an example of a dinuclear a spin crossover compound of iron(II) is described, where Mössbauer spectroscopy has most convincingly unraveled the mechanism of the spin transition process. Finally, an example from our most recent studies of spin crossover materials exhibiting both thermal spin crossover and liquid crystalline properties in the same temperature interval near room temperature will be presented.     

Mössbauer spectroscopy of zirconium alloys

Mössbauer investigations of zirconium alloys were examined. Data about the chemical state of iron atoms in the zirconium alloys of different composition has been provided. Mössbauer spectroscopy revealed that small quantities of iron in binary zirconium alloy are in the solid solution α-Zr (up to 0.02 wt.%). Different iron atoms concentration and thermo-mechanical treatments may lead to formation the intermetallic compounds Zr₃Fe, Zr₂Fe, ZrFe₂. Adding tin atoms does not affect the formation and shape of Mössbauer spectra of these compounds. Adding Cr and Nb atoms makes significant changes in the shape of Mössbauer spectra and leads to the formation of complex intermetallic compounds. Adding Cu and W atoms, the shape of the binary alloys spectra (Zr-Fe) remains unchanged, but a change in the temperature dependence behavior of the spectral parameters occurs and also, changes to the properties of the alloys.     

Mössbauer spectroscopy in determining the gas molecular state

A possibility of conversion electron Mössbauer spectroscopy for determining the gas molecular state is shown. For acceleration of gas interaction with active surface the thin iron layer enriched with ⁵⁷Fe was applied on aluminum foil and gas discharge is used.     

Mössbauer spectroscopy of grain boundaries in ultrafine-grained materials produced by severe plastic deformation

The applicability of Mössbauer spectroscopy in studying grain boundaries in ultrafine-grained materials produced by severe plastic deformation (SPD) is analyzed. It is shown that grain boundaries after SPD are in a nonequilibrium state that is characterized by excess free volume.     

Mössbauer investigation of temperature transformations in bacterial ferrihydrite

Ferrihydrite nanoparticles formed as a result of the microorganism activity have been studied using Mössbauer spectroscopy, X-ray powder diffraction analysis, and X-ray fluorescence analysis. Three positions of trivalent iron with nonoverlapping ranges of quadrupole splittings have been revealed in bacterial ferrihydrite: QS{Fe³⁺(1)} = 0.49–0.83 mm/s, QS{Fe³⁺(2)} = 0.84–1.10 mm/s, and QS{Fe³⁺(3)} = 1.25–1.73 mm/s. It has been experimentally demonstrated that the Fe³⁺(3) positions are the centers of nucleation of the hematite phase in the course of heat treatment.     

Using Mössbauer spectroscopy as key technique in the investigation of nanosized magnetic particles for drug delivery

This paper describes how cobalt ferrite nanoparticles, suspended as ionic or biocompatible magnetic fluids, can be used as a platform to built complex nanosized magnetic materials, more specifically magnetic drug delivery systems. In particular, the paper is addressed to the discussion of the use of the Mössbauer spectroscopy as an extremely useful technique in supporting the investigation of key aspects related to the properties of the hosted magnetic nanosized particle. Example of the use of the Mössbauer spectroscopy in accessing information regarding the nanoparticle modification due to the empirical process which provides long term chemical stability is included in the paper.     

Investigation of the cation distribution in perovskite-like oxides with Mössbauer spectroscopy

Complex perovskite-like oxides, such as LnFe_2/3Mo_1/3O₃ orthoferrites, Ln_8?ySr_yCu_8?xFe_xO₂₀ (8-8-20), Pr₄BaCu₄FeO_13-δ(4-1-5), YBa_2-yLa _y Cu_3-xFe _x O_7-δ and Y_1-yCa _y Ba _y La _y Cu_3-xFe _x O_7-δ (1-2-3), are studied by means of Mössbauer spectroscopy. At room temperature, the spectra of the orthoferrites contain only magnetic components. The spectra of the 1-2-3 compounds contain only magnetically disordered components: iron atoms substitute for copper at Cu(1) sites, taking various configurations: planar squares, quadratic pyramids, and octahedra. Cuprates 8-8-20 and 4-1-5 have a wide diversity of spectra. In the 8-8-20 oxides, a phase related to the pyramidal environment of the iron cations is present at any iron concentration. In all the perovskites, iron cations become magnetically ordered only at octahedral sites of the structure.     

Mössbauer spectroscopy of 151 europium dicarboxylates

We have previously reported the stability of europium oxalate compared to ammonium europium bis-salen and europium benzoate. We now extend the dicoarboxylic acid chain of the oxalate by introducing additional-CH₂-groups in the dicarboxylate ligands by using malonate, succinate, glutarate and adipate. Additionally, we have examined the effect of alterations in the succinate dianion by introducing functional groups such as [C = C]in the case of the maleide and-OH group in the case of the malide. This study is an attempt to further characterize these compounds. Infrared spectra were used to characterize bridging and chelating dicarboxylates while Mössbauer spectroscopy measurements were used to gain better insight into the structure of heterocyclic “cages” containing two Eu^3?+? ions and two dianions.     

Practical aspects of Mössbauer spectroscopy instrumentation

The setting up and updating of a Mössbauer laboratory imply the acquisition and assembling of different units. Guidelines concerning either the construction or the acquisition of the various parts which compose a Mössbauer spectrometer are given in this paper.     

Mössbauer spectroscopy with the 93 keV-resonance in67Zn

The procedures and results of high resolution Mössbauer experiments performed with the 93.3 keV resonance in absorbers of⁶⁷ZnO,⁶⁷ZnS (both wurtzite and sphalerite structures),⁶⁷ZnSe,⁶⁷ZnTe and⁶⁷ZnF₂ are reported. An essentially linear dependence between the isomer shift and the Pauling electronegativity of the ligands was found. A value of r ²=+11×10^–3 fm² is estimated. The Mössbauer parameters for both ZnS structures are equal within present limits of error. In particular, no quadrupole interaction was observed in the wurtzite modification. This indicates a nearly identical local symmetry at the Zn site in the two ZnS structures.Work supported by the Bundesministerium für Forschung und Technologie, Federal Republic of Germany and the Kernfor-schungszentrum Karlsruhe     

Mössbauer spectroscopy studies of the magnetic properties of ferrite nanoparticles

Studies of the ferrite nanoparticles prepared by the chemical decomposition of iron chlorides with a various ratio ξ = Fe³⁺/Fe²⁺ are herein presented. The microstructure and the magnetic properties have been studied by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Mössbauer spectroscopy (MS). The TEM studies show that the nanoparticles have almost a spherical shape with the diameter of (12 ± 2) nm for all samples. The measured XRD pattern was mainly composed of lines which were indexed with a cubic spinel structure. The analysis of the Mössbauer data shows that the microstructure of the nanoparticles consists of the core formed by nonstoichiometric magnetite and maghemite shell. A small amount of hematite, probably on the surface of the nanoparticles with ξ = 1.75, 2.0, was detected. At temperatures T ≤ 150 K the spin canting of surface maghemite with ξ = 2.25 was observed while for the samples with ξ = 1.75, 2.0 such effect was suppressed by the presence of hematite on the surface of the nanoparticles. Infield Mössbauer spectra with ξ = 1.75, 2.0 show that magnetic moments of the magnetite/maghemite core are parallel while magnetic moments of the surface hematite are perpendicular to the direction of the external magnetic field.     

Mössbauer spectroscopic investigation of tin-doped thiospinels

The tin-doped sulphur-containing compound of composition Fe_1.05Cr_1.90Sn_0.05S₄, in which tin is located on the octahedral sites as Sn^4?+? in the spinel-related structure, has been examined by ¹¹⁹Sn Mössbauer spectroscopy. The data complement results obtained by ⁵⁷Fe Mössbauer spectroscopy and show that tin increases the magnetic ordering temperature of pure FeCr₂S₄.     

A new study on the catalytic mechanism of Fe-based alloys in diamond formation by Mössbauer spectroscopy

M?ssbauer spectroscopy has been used to systemically study the catalytic mechanism of Fe-based alloys in diamond formation at high temperature–high pressure (HTHP) for the first time. M?ssbauer spectra reveal the magnetic state of the 3d electrons of a Fe atom in the Fe-based alloy catalyst during diamond formation at HTHP. During carburization at lower temperatures than that required for diamond formation and diamond formation in the diamond-stability region using Fe-based alloys as a catalyst, both the quadrupole splitting QS and the isomer shift IS change from negative to positive, especially reaching a state in which they are zero. It was indicated that the state of the 3d-shell electrons of the iron atom changes greatly during carburization and diamond formation and that the incomplete 3d sub-bands of Fe atoms in the catalyst alloys could be filled up in proper order by electrons of interstitial carbon atoms. During diamond formation, the unpaired 3d-shell electrons of an iron atom in the Fe-based alloy absorb and interact with 2P_z electrons of the carbon atoms. There exist a Fe–C bonding and an electron charge transfer stage. The 2P_z electrons of the carbon atoms could be dragged into the metal atoms in the catalyst alloy and would make a transition of triangular (sp²π) hybridization of valence electrons to tetrahedral (sp³) hybridization of valence electrons (a transition of sp²π bonds of graphite to sp³ bonds of diamond), resulting in a transition of graphite structure to diamond. Although the conclusion of this study is strictly applicable only to Fe-based alloy catalysts, it could be considered more general because of the chemical similarities between the transition elements used as solvent catalysts for diamond synthesis. Received: 2 March 2001 / Accepted: 20 August 2001 / Published online: 2 October 2001     

Pervasiveness of cluster excitations as seen in the Mössbauer spectra of magnetic materials

Magnetic cluster excitations in various physical systems (e.g., soliton bearing one-dimensional solids, metallic alloys, amorphous materials, small particle aggregates, magnetically ordered substances near T_C, transition metal di-chloride graphite intercalation compounds, etc.) are described. Use of Fe-57 Mössbauer effect spectroscopy as a probe of the spin dynamics for inverse autocorrelation times between 10⁷Hz to 10¹⁰ Hz is emphasized. Particular attention is given to systems which exhibit local or long range magnetic order and whose Mössbauer spectra must therefore be described by more than one autocorrelation (or dwell) time for fluctuations between different allowed hyperfine field directions on a given site.     

Mössbauer spectroscopy of the Zr-rich region in Zr–Nb–Fe alloys with low Nb content

Intermetallic phases and solid solutions in the Zr-rich region of the Zr–Nb–Fe system with low Nb content are studied by Mössbauer spectroscopy complemented with X-ray diffraction, optical and scanning electron microscopy and electron microprobe analysis. The phases found in each sample were those expected from the corresponding binary Zr–Fe system. Furthermore, one of the samples showed a ternary cubic Ti₂Ni type phase with a similar stoichiometry to the tetragonal Zr₂Fe compound. Mössbauer parameters were suggested to this phase (IS: - 0.12 mm/s, QS: 0.30 mm/s), to the bcc Zr(β) phase (IS: (-0.11 α 0.01) mm/s, QS: (0.23 α 0.02) mm/s), and to the hcp Zr(β^T) phase (IS: (-0.24 α 0.02) mm/s, QS: (0.45 α 0.02) mm/s).