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 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 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.     

Study on chemical reactions of isolated Mössbauer probes in solid gas matrices using in-beam Mössbauer spectroscopy

In-beam Mössbauer spectra of ⁵⁷Fe after ⁵⁷Mn (T _1/2= 1.45 min) implantation into a CH ₄ matrix and mixture matrices of CH ₄ and Ar at 18 K were measured. The spectrum obtained in the CH ₄ matrix was analyzed well with a doublet and a singlet peaks. These components were assigned to two constitutional isomers of Fe(CH ₄)\(_{\mathrm {2}}^{\mathrm {+}}\) as derived from density functional theory calculations and the Mössbauer parameters. In the case of the low concentration of CH ₄ with an Ar matrix, the Fe ⁺ (3d ⁷4s) in the excited state atomic configuration and Fe ⁺ (3d ⁶4s ¹) in the ground state were observed, as observed in our previous implantation experiment into Ar and Xe matrices. The formation yields of Fe(CH ₄)\(_{\mathrm {2}}^{\mathrm {+}}\) are discussed in term of the number of first-nearest-neighbor CH ₄ molecules around an Fe ⁺ ion.     

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.     

Magnetism in some layer molecular magnets as revealed by Mössbauer spectroscopy

⁵⁷Fe Mössbauer spectroscopy was used to explore magnetic properties of two 2D molecular ferrimagnets. In NPn₄[Fe^IIFe^III(ox)₃] (Pn = n-C₅H₁₁, (ox) =(C₂O₄)^2?), the previously reported negative magnetization is shown here by external field studies to be due to a cross-over between Fe^III and Fe^II- magnetizations. The form and parameters of the magnetic Hamiltonian describing the temperature dependence of both the Fe^III hf-field and net magnetizations has been determined. In NBu₄[Mn^IIFe^III(ox)₃] (Bu = n-C₄H₉) soft XY-magnet the low temperature relaxation spectra are interpreted in terms of slowly varying classical magnetization-evolution at low temperature.     

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 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 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 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.     

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.     

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.     

Mössbauer spectroscopy in the investigation of new mineral–related materials

New materials based on the composition of the mineral schafarzikite, FeSb $_{2}\textit {O}_{4}$ , have been synthesised. $^{57}$ Fe- and $^{121}$ Sb- Mössbauer spectroscopy shows that iron is present as Fe $^{2+}$ and that antimony is present as Sb $^{3+}$ . The presence of Pb $^{2+}$ on the antimony sites in materials of composition FeSb $_{1.5}$ Pb $_{0.5}\textit {O}_{4}$ induces partial oxidation of Fe $^{2+}_{}$ to Fe $^{3+}$ . The quasi-one-dimensional magnetic structure of schafarzikite is retained in FeSb $_{1.5}$ Pb $_{0.5}\textit {O}_{4}$ and gives rise to weakly coupled non-magnetic Fe $^{2+}$ ions coexisting with Fe $^{3+}$ ions in a magnetically ordered state. A similar model can be applied to account for the spectra recorded from the compound Co $_{0.5}$ Fe $_{0.5}$ Sb $_{1.5}$ Pb $_{0.5}\textit {O}_{4}$ .     

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.     

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 study of the iron atom state in modified chromium dioxide

Powders of modified chromium dioxide produced by the hydrothermal method were studied using ⁵⁷Fe Mössbauer spectroscopy at a temperature of 298 K. The content of the modifier, i.e., ⁵⁷Fe compound, was varied from 2 to 10 mmol/mol Cr at a Sb content of 2.2 and 10 mmol/mol Cr. It was shown that, independently of concentrations, Fe³⁺ ions are distributed between three magnetic solid solutions (sextets): based on CrO₂ (bulk material and iron-enriched surface layer), based on Cr₂O₃, and surface β-CrOOH (doublet). In this case, chromium atoms were not substituted in the CrSbO₄ nucleation (12 nm in size) phase with an accuracy up to the Mössbauer factor. It was assumed that the powder coercivity, in addition to the size factor, is controlled by the iron concentration in the CrO₂ surface layer.