Mössbauer spectroscopy in catalysis

Application of in situ Mössbauer spectroscopy for studying catalysts and catalytic processes is discussed. Examples are presented to illustrate the potentials of the method by describing studies on supported heterogeneous catalysts performed with ¹¹⁹Sn and ⁵⁷Fe spectroscopies in cases with certain metals and alloys, oxides and porous substances. The results are interpreted in comparison to the catalytic performance.     

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 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.     

Spin conversion detected by Mössbauer spectroscopy and μSR on a 1D FeII paramagnetic chain

While magnetic properties of the 1D chain [Fe(hyetrz)₃](4-bromophenylsulfonate)₂ investigated over the temperature range from 300 K to 2 K show paramagnetic behavior, detailed ⁵⁷Fe Mössbauer and muon spin relaxation measurements reveal an unexpected spin conversion. Approximately ~14 % of the high-spin ions are found to convert to the low-spin state with a transition temperature T _1/2?~?120 K.     

Mössbauer spectroscopy and quality control in ferrate technology

This paper describes some prospective reactants for the ferrate technology of water treatment, the ways of their industrial production and the application of the Mössbauer spectroscopy for their quality control.     

Digital pulse processing in Mössbauer spectroscopy

In this work we present some advances towards full digitization of the detection subsystem of a Mössbauer transmission spectrometer. We show how, using adequate instrumentation, preamplifier output of a proportional counter can be digitized with no deterioration in spectrum quality, avoiding the need of a shaping amplifier. A pipelined architecture is proposed for a digital processor, which constitutes a versatile platform for the development of pulse processing techniques. Requirements for minimization of the analog processing are considered and experimental results are presented.     

Pressure-induced magnetization in FeO: evidence from elasticity and Mössbauer spectroscopy

The complete elastic tensor of Fe0.94O (wüstite) has been determined to 10 GPa using acoustic interferometry at GHz frequencies inside a diamond-anvil cell. The soft mode (C44) elastic constant of FeO is reduced by 20% over the experimental pressure range. An unusual discontinuity in the pressure derivatives of C11 and C12 at 4.7+/-0.2 GPa corresponds to the pressure at which the onset of a magnetic ordering transition is observed by high-pressure M?ssbauer spectroscopy and neutron powder diffraction. Our new results combined with literature structural high P-T data suggest that there is a magnetic, although still cubic, phase of FeO between approximately 5 and approximately 17 GPa.     

Mössbauer spectroscopy at ISOLDE

Applications of radioactive ion beams produced at the ISOLDE facility for Mössbauer studies of probe atoms in solids are presented. Examples are given for a site-selective incorporation on different substitutional sites in compound semiconductors by ion implantation and thermal annealing of the radiation damage resulting from the implantation. The interactions of the probe atoms with lattice defects created in the implantation process have been studied to elucidate likely causes for the site-selective implantation mechanism. The technique has enabled to determine the electronic densities at electrically active substitutional probe atoms, having shallow donor or acceptor states as well as states deeper in the band gap. The results are in good agreement with theoretical results from local density calculations. Methodological aspects of the Mössbauer emission techniques employed at ISOLDE are compared to alternative accelerator based techniques and the consequences of the application of different precursor isotopes to the ⁵⁷Fe Mössbauer isotope are treated in detail for ⁵⁷Fe in silicon. Finally, results obtained for the magnetic hyperfine interactions of 5 sp impurities associated with vacancies in ferromagnetic metals are discussed.     

67Zn Mössbauer spectroscopy

The 93.31-keV Mössbauer resonance in⁶⁷Zn is used to determine tiny changes of the transition energy. We first report on a gravitational redshift (GRS) experiment with⁶⁷GaZnO single crystal source and⁶⁷ZnO powder as absorber. The results on the GRS show that solid state effects which are difficult to control experimentally, in particular the extreme sensitivity of the electric field gradient tensor (efg) in⁶⁷ZnO to small external pressure, finally limit the accuracy in determining the GRS. The resonance has been further employed to investigate solid state properties of⁶⁷ZnO with high precision. At large pressures⁶⁷ZnO exhibits a phase transition from hep (wurtzite) to fcc (NaCl) structure. The changes of the efg and of the s-electron density θ(0) at the⁶⁷Zn nucleus with reduced unit cell volume show that covalency of the Zn-O bond plays and essential role. We have performed self-consistent Hartree-Pock cluster calculations to describe the experimental changes of θ(0) in detail.     

Mössbauer spectroscopy of lanthanum and holmium ferrites

PrRh₂Si₂ is highly anisotropic Ising-type antiferromagnetic system. The ordering temperature (T _N~ 68 K) of PrRh₂Si₂ is exceptionally high on the de-Gennes scale in the family of RRh₂Si₂ (R = rare earths). The reason for this anomalous behaviour is not clear. It is believed that the presence of uniaxial anisotropy assists in enhancing the T _N. The Mössbauer study was performed on a 10% Fe-doped PrRh₂Si₂ sample to understand the magnetic coupling between different sites of PrRh₂Si₂. The Mössbauer spectra together with the magnetic susceptibility data suggest that the magnetic coupling in PrRh₂Si₂ is provided mainly by the RKKY interaction between Pr-moments.     

High temperature Mössbauer spectroscopy of titanomagnetite and maghemite in basalts

Mössbauer spectroscopy of basalt lava samples, exhibiting reversible thermal magnetization (J _s -T) curves with Curie temperatures of about 840 K, has revealed considerable amounts of maghemite (-Fe₂O₃) in many samples. In view of the expected instability of maghemite at temperatures above 620 K, this reversibility came as a surprise. For further studies of the magnetization-temperature relationship of these minerals, we have constructed an elliptical radiation-heated furnace in which Mössbauer spectra can be acquired at temperatures between 300 and 900 K. Measurements at different temperatures have been obtained for two types of basalts, one in which the magnetic minerals are nearly pure magnetite and the other where the room temperature spectrum indicates a mixture of maghemite and magnetite. The two series show different features of the collapse of the internal magnetic hyperfine field, and the composition of minerals in the samples changes during the treatment, showing maghemite.     

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 with actinide elements

Although formally equivalent to the lanthanide (4f) elements, the light actinides show a much more varied behaviour due to the larger spatial extent and ionizability of the 5f electrons. The application of Mössbauer spectroscopy for the determination of electronic properties of the actinides is outlined. Emphasis is put on high pressure Mössbauer experiments using the 60 keV transition in²³⁷Np to study questions of delocalization of 5f electrons.Work performed under the auspices of the Bundesministerium für Forschung und Technologie, Federal Republic of Germany.     

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.     

Fast detectors for Mössbauer spectroscopy

The methods to increase the productivity (statistical quality) of Mössbauer measurements have been considered. Some fast detectors for gamma- and secondary radiation have been described. These detectors allow in many cases to essentially reduce the time for the Mössbauer spectra accumulation with a given productivity.     

Iron in typical and atypical parkinsonism – Mössbauer spectroscopy and MRI studies

Iron may play important role in neurodegeneration. The results of comparative studies of human brain areas (control and pathological) performed by Mössbauer spectroscopy (MS) and magnetic resonance imaging (MRI) techniques are presented. Mössbauer spectroscopy demonstrated a higher concentration of iron in atypical parkinsonism (progressive supranuclear palsy PSP) in the brain areas Substantia Nigra (SN) and Globus Pallidus (GP) involved in this pathological process, compared to control, while the concentration of iron in pathological tissues in typical parkinsonism (Parkinson’s disease - PD) did not differ from that in control. These results were compared with the changes in 1/T1 and 1/T2 (T1 and T2 being the relaxation times determined by MRI). A good linear correlation curve was found between the concentration of iron as determined by MS in different areas of control human brains and between 1/T1 and 1/T2. Whereas the finding in PSP-GP (the brain area involved in PSP) also fitted to such a correlation, this was not so for the correlation between pathological SN – the brain area involved in both diseases – and 1/T2, indicating a dependence of T2 on other factors than just the concentration of iron.     

Mineral identification in Colombian coals using Mössbauer spectroscopy and X-ray diffraction

Minerals were identified in three Colombian coal samples from the Southwest of the country using M?ssbauer spectroscopy and X-ray diffraction. Original and sink separated coal fractions of specific gravity 1.40 and 1.60 with particle size less than 600 μm were used in the study. Using M?ssbauer spectroscopy, the minerals identified in the original coal samples were pyrite jarosite, ankerite, illite and ferrous sulfate, whereas by means of X-ray diffraction, minerals identified were kaolinite, quartz, pyrite, and jarosite. Differences in mineral composition were found in the original and sink separated fractions using both techniques. M?ssbauer spectra show that the mineral phases in low concentrations such as illite, ankerite and ferrous sulfate do not always appear in the spectra of sink coals, despite of those minerals occurring in the original coal, due to the fact that they are associated with the organic matter and not liberated in the grinding process. X-ray results show that the peak intensity grows as the specific gravity is increased indicating that the density separation method could be an effective process to clean coal. This revised version was published online in July 2006 with corrections to the Cover Date.     

Prospects for emission Mössbauer spectroscopy in chemical investigations

Emission Mössbauer spectroscopy is a radiochemical method for investigating materials and the consequences of nuclear transformations taking place in them. Isotopes are traditionally used as structural probes, and the sensitivity of the method is 2–3 orders of magnitude higher than that of absorption Mössbauer spectroscopy. The elements of Mössbauer isotopes with parent nuclei that undergo electron capture or a converted isomeric transition (i.e., lead to high Auger ionization) are the best-studied elements. The electron processes that accompany ionization and their effect on the state of daughter Mössbauer atoms in qualitatively different compounds, from elementary oxides, superconductors, insulators and magnetics to sophisticated bioorganic complexes, are considered.