Study of the structure and electronic state of thiolate-protected gold clusters by means of 197Au Mössbauer spectroscopy

We have investigated the structures and electronic states of a series of glutathionate-protected Au clusters, Au _n (SG) _m with n = 10 ? ～55, using ¹⁹⁷Au Mössbauer spectroscopy, which allows us to probe the local environment of the constituent Au atoms via isomer shift (IS) and quadrupole splitting (QS). The spectral profile abruptly changes on going from Au₂₂(SG)₁₇ to Au₂₅(SG)₁₈, then it smoothly changes to that of Au～₅₅(SG)_m. However, the spectral profile dramatically changes on going from Au～₅₅(SG)_m to the dodecanethiolate-protected Au cluster with average diameter of 2 nm. The ¹⁹⁷Au Mössbauer spectra of glutathionate-protected Au clusters and dodecanethiolate-protected Au clusters were successfully analyzed on the basis of the structure and electronic state of Au₂₅(SG)₁₈.     

Magnetism of ErAgSn studied by 119Sn Mössbauer spectroscopy

Antiferromagnetic ErAgSn compound was investigated in detail by ¹¹⁹Sn Mössbauer spectroscopy in a temperature range between 2.2 and 300 K. The ¹¹⁹Sn spectra recorded below 4.2 K can be well fitted with a single main magnetic component in agreement with recent neutron diffraction studies [1]. A broad distribution of magnetic hyperfine fields observed above 4.2 K and enhanced spin correlations among Er³⁺ ions at T > T _N = 5.6 K are the remarkable features of the investigated system.     

Local order of icosahedral quasicrystals studied by Mössbauer spectroscopy

Thermodynamically stable Al-Cu-Fe and Fe-doped ferromagnetic Al-Cu-Ge-Mn icosahedral quasicrystals are studied by⁵⁷Fe transmission Mössbauer spectroscopy and X-ray diffraction experiments. Al₆₅Cu₂₀Fe₁₅ quasicrystalline alloy was subjected to a mechanical grinding (MG) for up to 800 hours in a ball mill. Presence of the amorphous phase which co-exists with the quasicrystalline one is revealed in the early stage of MG. Mössbauer measurements were performed on icosahedral Al₄₀Cu_10–x Ge₂₅Mn₂₅Fe _x quasicrystal (x0.1; 3) in a temperature range from 10 K to 548 K. It was found that a magnetic transition occurs at about 30 K which is far belowT _c reported in the literature. It is concluded that AlGeMn ferromagnet which is present in the samples does not affect the magnetic transition observed and the transition is an intrinsic property of the Al-Cu-Ge-Mn host alloy.Samples of icosahedral quasicrystals were kindly provided by Profs. A. Inoue, T. Masumoto and P. H. Shingu. Ball milling was performed in Kyoto University by a courtesy of Prof. P. H. Shingu. This work was supported by the project for priority areas on properties of quasicrystals (No. 01630003) from the Japanese Ministry of Education, Science and Culture.     

Weathering of Martian and Earth surface studied by Mössbauer spectroscopy

Martian regolith and Earth’s basaltoid samples have been investigated by means of Mössbauer spectroscopy. The identification of the same minerals: olivine, pyroxene, magnetite, hematite and confrontation of the Fe^3?+?/Fe^2?+?, Fe^3?+?/Fe_tot, Fe^2?+?/Fe_tot ratios are presented. Co-existence of olivine and hematite in Martian regolith, absent in presented by authors terrestrial samples has been tentatively explained.     

Gold-surface binding of molecular switches studied by M?ssbauer spectroscopy

The nonanuclear coordination compound [Mo^IV{(CN)Fe^III(3-methyl-saldptn)}₈]Cl₄ exhibits multiple spin transitions (3-methyl-saldptn = N,N′-bis(3′′-methyl-2′′-hydroxy-benzyliden)-1,7-diamino-4-azaheptane). This spin crossover cluster is bound via a self-assembled monolayer onto a two dimensional array gold surface. M?ssbauer spectroscopy indicates that the thermally and optically induced spin crossover of the compound is maintained. Thereby, the foundation for its potential practical application (e.g. in the field of information storage) was laid.     

Fast relaxation of polycrystalline Fe(III) complexes studied by Mössbauer spectroscopy

Mössbauer spectra of polycrystalline samples of Fe(Ox)₃, Fe(BPHA)₃ and K₃[Fe(malonate)₃] · 3H₂O exhibit fast relaxation patterns in the temperature range 4.2–300 K. The magnetic hyperfine splitting has nearly completely collapsed due to rapid electronic spin-spin relaxation of the Fe ions. We use a static Hamiltonian and describe the spin-spin interaction by an effective field Hamiltonian . From this the relaxation supermatrix and the Mössbauer spectra are calculated. The random texture of the samples is taken into account by averaging the radiation dipole operators over the whole sphere. Least-squares fits of the spectra for longitudinal, isotropic and transverse spin relaxation are presented. From the fits the temperature dependence of the relaxation is obtained. We conclude that in these materials the iron is present as high-spin Fe(III) and that the crystal field splitting constantD is greater than zero. The sign ofV _zz is found to be positive.     

Dynamics of iron in Fe-porphyrin aggregates studied by X-ray absorption and Mössbauer spectroscopy

The iron-porphyrin aggregates were studied by optical absorption and fluorescence method, infrared spectroscopy, X-ray absorption and Mössbauer spectroscopy. The aggregation of porphyrin molecules strengthens the Fe-ligands bonds and accelerates the spin-spin relaxations. A significant speeding-up of relaxation was observed with lowering the temperature down to 25 K. The comparison of the EXAFS (Extended X-ray Absorption Fine Structure) and Mössbauer spectroscopy results enabled some separation of the individual Fe vibration from its collective movement with ligands.     

Carrier mobility of iron oxide nanoparticles supported on ferroelectrics studied by Mössbauer spectroscopy

⁵⁷Fe Mössbauer spectroscopy was performed on two types of Fe oxide nanoparticles supported on a typical ferroelectric, BaTiO3. It was found that the valence state of FeO nanoparticles changed to a mixed 2+/3+ state at high temperature where BaTiO₃ shows paraelectric behaviour. We attribute this phenomenon to the fluctuation of electric dipoles which realizes carrier injection into the Fe oxides. This is the first report which discusses a dynamical valence state of transition metal oxides supported on ferroelectrics.     

Iron-metal multilayers studied by Mössbauer spectroscopy,RBS and X-ray diffraction

Fe/M (M = Ag, Zn and Sn) multilayers prepared by a vacuum evaporation method are studied by Mössbauer spectroscopy (MS), Rutherford backscattering spectroscopy (RBS) and X-ray diffraction (XRD). For the case of an M = Ag multilayer, MS reveals that Fe in the multilayer remains as an-phase down to the layer thickness of 10 nm. This result is in agreement with the RBS result that Fe and Ag form a completely discrete layer structure without any mutual mixing. For the case of M = Zn and Sn, RBS reveals that a considerable mixing has taken place between Fe and Sn during the specimen preparation. MS on Fe/Sn specimens with different layer thickness shows that an alloy phase of about 5 nm thickness is formed at the interface. Structural as well as magnetic properties of the alloy phase are discussed based on MS at different temperatures and on reported results of the intermetallic compound FeSn.     

Studying track electrons in frozen aqueous solutions by means of emission Mössbauer spectroscopy

It is shown by means of emission Mössbauer spectroscopy that the reactivity of NO ₃ ^? cations with quasi-free track electrons exceeds those of H₃O⁺, ClO ₄ ^? , and HSO ₄ ^? ions by a factor of only three. This is in drastic contradiction to published assertions that these values differ by two orders of magnitude. An explanation for this discrepancy is proposed.     

Corrosion studies of iron and its alloys by means of57Fe Mössbauer spectroscopy

Some of the advantages and limitations of Mössbauer spectroscopy when used in corrosion research are shown by using three examples taken from the work of the authors on (i) the passive layer of iron, (ii) the corrosion of weathering steels by SO₂-polluted atmospheres and (iii) the performance of rust converters.     

Characterisation of synthetic trioctahedral micas by Mössbauer spectroscopy

Trioctahedral potassium micas |K}[M₃]〈T₄〉O₁₀(OH)₂ have been synthesized by hydrothermal techniques with various cationic substitutions in the octahedral and the tetrahedral sheet. Taking annite |K}[Fe ₃ ²⁺ ]〈AlSi₃〉O₁₀(OH)₂ as the reference mineral, [Fe²] was replaced by [Mg²] and [Ni²], 〈Al³⁺〉 by 〈Fe³⁺〉 and finally [Fe²⁺] + 〈Si⁴⁺〉 by [Al³⁺] + 〈Al³⁺〉. Mössbauer spectra were evaluated in terms of quadrupole splitting distributions (QSDs) using three generalized sites for 〈Fe³⁺〉, [Fe³⁺] and [Fe²]. Annites, nominally free of 〈Fe³⁺〉, show a lower limit of [Fe³⁺]/Fe _tot of 0.10, which stabilizes the structure. The ferrous iron, [Fe²], QSD consists of two main components. In some of the solid solution series, there is strong experimental evidence for a third ferrous component, particularly at higher [Al³⁺] contents. This third component is centered at low quadrupole splittings and may be assigned to a defect [Fe²] site, forming 1:2 structures with two neighbouring trivalent octahedral cations. For charge compensation one OH^? is replaced by O^2? for each [M³⁺] cation. The ferrous QSDs vary systematically with chemical composition. Compared to those of annite, the QSD parameters (mean quadrupole splitting 〈QS〉 and quadrupole splitting with maximum probability, QS _peak ) are shifted towards higher values with increasing [Mg²] and [Ni²] contents, and decrease slightly with increasing content of trivalent cations. These trends can be interpreted in terms of changes in the local environment around the Fe probe nucleus, i.e., in terms of decreasing or increasing distortions from the ideal octahedral configurations.     

Fe/Dy artificial multilayered films studied by57Fe Mössbauer spectroscopy

Magnetic properties of multilayered Fe/Dy films with artificial superstructures have been investigated by⁵⁷Fe Mössbauer spectroscopy. Doublet peaks are observed at room temperature when the Fe layer is thinner than 20Å. Mössbauer spectra for thicker Fe layers correspond to α-Fe spectra. In certain samples, i.e. [Fe(44Å)/Dy(6Å)], a gradual spin reorientation takes place, which is evidenced from the change of relative intensities of Δm=0 lines with decreasing temperature.     

Structural transformations in NANOPERM-type alloys studied by Mössbauer spectrometry and diffraction of synchrotron radiation

As-quenched Fe_91???x Mo₈Cu₁B _x NANOPERM-type alloys (x?=?12, 15, 17, 20) were exposed to continuous heat treatment up to 800°C. During the temperature increase (ramp of 10 K/min), an in situ acquisition of diffracted synchrotron radiation intensities was performed. The obtained data are correlated with those derived from the volume and surface Mössbauer effect experiments. The onset of individual crystallization steps as well as identification of the crystalline phases created is discussed.     

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.     

Hydrogenation of FeCoZr–Al2O3 nanocomposites studied by Mössbauer spectroscopy and magnetometry

Hydrogenation effects on crystalline and magnetic structure of nanocomposites (FeCoZr) _x (Al₂O₃)_100???x , 38?≤?x?≤?63 at.% are studied by ⁵⁷Fe Mössbauer spectroscopy and magnetometry. Variations of local structure, blocking temperature and mean FeCoZr nanoparticles’ volume are discussed with respect to (i) composition and (ii) two competing processes—H₂ incorporation and annealing—occurred during treatment in H₂ plasma.     

Effect of pseudohalides in pentadentate-iron(III) complexes studied by DFT and Mössbauer spectroscopy

Mononuclear iron complexes in which the iron(III) ion is coordinated by a pentadentate Schiff base ligand L⁵ with two phenolate, two imino and one amino group can exhibit a spin crossover. In this contribution experimental results are presented for complexes with cyanate and thiocyanate as co-ligands. Furthermore, theoretical results of quantum chemical calculations of energies and entropies for the low-spin and high-spin state are shown and compared with Mössbauer results. We also demonstrate how the ligand field of the monodentate co-ligand influences the spin crossover energies and entropies in [Fe^IIIL⁵NCY] complexes.     

Mössbauer study of197Au in Zn and Cd single crystals

The 77.3 keV Mössbauer transition of¹⁹⁷Au was used to study the hyperfine interactions and recoilfree fractions of dilute Au impurities in Zn and Cd single crystals at 4 K. Mössbauer sources were prepared by ion implantation of^197mHg/¹⁹⁷Hg at ambient temperature. From the quadrupole splittings the electric field gradients $$\begin{gathered} eq(Au\underline {Cd} ) = + 11.7(6) \times 10^{17} v/cm^2 and \hfill \\ eq(Au\underline {Zn} ) = ( + )15.0(2.5) \times 10^{17} v/cm \hfill \\ \end{gathered} $$ were determined. The electric field gradients as well as the isomer shifts are in good agreement with the systematics of other impurity host systems. The recoilfree fractions agree with estimates using the mass corrected Debye temperatures of the host lattice.