Mossbauer Absorption Spectroscopy Research Articles

Local Structure and Anti-Structural Defects of Tin in Amorphous and Crystalline Ge2Sb2Te5 Films

The absorption Mossbauer spectroscopy on 119Sn impurity centers shows that germanium atoms in the structure of amorphous and polycrystalline Ge2Sb2Te5 films have different local symmetries (tetrahedral in the amorphous phase and octahedral in the crystalline). By emission Mossbauer spectroscopy on 119mSn impurity centers formed after the radioactive decay of 119Sb or 119mTe parent atoms, tin antisite defects at antimony and tellurium sites of crystalline Ge2Sb2Te5 films are identified.

Fe3 – δO4 Nanoparticles Synthesized in the Presence of Natural Polyelectrolytes

Mossbauer absorption spectroscopy and X-ray diffraction analysis were used to study Fe3 – δO4 nanoparticles formed in the presence of natural polyelectrolytes of two types: derivatives of humic materials and low polymer chitosan. By an example of samples synthesized using either the Massart method or by in situ oxidation of metal it was shown that polyelectrolytes differing in charge and hydrophility affect mainly the morphology of Fe3 – δO4 particles, whereas their composition and structure depend only slightly on the polyelectrolyte nature.

Synchrotron radiation based Mössbauer absorption spectroscopy of various nuclides

Synchrotron-radiation (SR) based Mossbauer absorption spectroscopy of various nuclides is reviewed. The details of the measuring system and analysis method are described. Especially, the following two advantages of the current system are described: the detection of internal conversion electrons and the close distance between the energy standard scatterer and the detector. Both of these advantages yield the enhancement of the counting rate and reduction of the measuring time. Furthermore, SR-based Mossbauer absorption spectroscopy of 40K, 151Eu, and 174Yb is introduced to show the wide applicability of this method. In addition to these three nuclides, SR-based Mossbauer absorption spectroscopy of 61Ni, 73Ge, 119Sn, 125Te, 127I, 149Sm, and 189Os has been performed. We continue to develop the method to increase available nuclides and to increase its ease of use. The complementary relation between the time-domain method using SR, such as nuclear forward scattering and the energy-domain methods such as SR-based Mossbauer absorption spectroscopy is also noted.

Cation ordering in Sr(Fe1 − x Co x )0.9Ga0.1O2.5 with brownmillerite structure

Samples of solid solutions of the series Sr(Fe1 − x Co x )0.9Ga0.1O2.5 with brownmillerite structure are produced from appropriate amounts of oxides Fe, Co, Ga, and SrCO3 in different gaseous media and temperature conditions using solid-phase synthesis. Features of the distribution of cations over nonequivalent positions of the crystalline structure of the obtained samples are studied using the 57Fe Mossbauer absorption and emission spectroscopy.

Formation of magnetic nanoparticles studied during the initial synthesis stage

The formation of iron oxide nanoparticles in course of a sol-gel preparation process was traced by UV/Vis and 57Fe Mossbauer absorption spectroscopy. Samples were extracted at different stages of the reaction. While spectra measured on samples extracted at low reactor temperatures showed the starting materials Fe(acac)3 diluted in benzyl alcohol undergoing slow paramagnetic relaxation, a sample extracted at a reactor temperature of 180 °C gave clear evidence for emerging iron oxide nanoparticles. A prolonged stay at 200 °C results in a complete transformation from Fe(acac)3 to maghemite nanoparticles.

Mössbauer studies of two-electron centers with negative correlation energy in crystalline and amorphous semiconductors

The results of the study of donor U −-centers of tin and germanium in lead chalcogenides by Mossbauer emission spectroscopy are discussed. The published data regarding the identification of amphoteric U −-centers of tin in glassy binary arsenic and germanium chalcogenides using Mossbauer emission spectroscopy, and in multicomponent chalcogenide glasses using Mossbauer absorption spectroscopy are considered. Published data concerning the identification of two-atom U −-centers of copper in lattices of semimetal copper oxides by Mossbauer emission spectroscopy are analyzed. The published data on the detection of spatial inhomogeneity of the Bose-Einstein condensate in superconducting semiconductors and semimetal compounds, and on the existence of the correlation between the electron density in lattice sites and the superconducting transition temperature are presented. The principal possibility of using Mossbauer U −-centers as a tool for studying the Bose-Einstein condensation of electron pairs during the superconducting phase transition in semiconductors and semimetals is considered.

High activities of iron-FSM-16 materials synthesized by a microwave-hydrothermal process in friedel-crafts alkylations

Iron-FSM-16 materials with different Si/Fe ratios (Si/Fe = 90, 60, and 10) have been synthesized by microwave-hydrothermal process and characterized by several spectroscopic techniques. X-ray diffraction, electron spin resonance, diffuse reflectance UV-visual and Mossbauer absorption spectroscopies allowed differentiation of several iron species. These species correspond to hematite particles, very small isolated Fe (III) species possibly incorporated in the mesoporous silica wall, and iron oxide clusters either isolated or agglomerated, forming “rafts” at the surface of the silica and exhibiting ferromagnetic ordering. Very importantly, catalytic data in benzylation of aromatic compounds such as benzene, toluene, and ethylbenzene with benzyl chloride shows that Fe-FSM-16 synthesized by microwave-hydrothermal process samples are very active and recycle catalysts.

Prospects for emission Mössbauer spectroscopy in chemical investigations

Emission Mossbauer 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 Mossbauer spectroscopy. The elements of Mossbauer 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 Mossbauer atoms in qualitatively different compounds, from elementary oxides, superconductors, insulators and magnetics to sophisticated bioorganic complexes, are considered.

Mössbauer study of matrix effects in formation of high oxidation states of iron and cobalt

To fix unstable states of pentavalent iron and cobalt, a matrix containing (according to the preparation conditions) potassium manganates (V) was simultaneously doped with iron and radioactive cobalt. The absorption and emission spectra of this matrix, with allowance for the reversal of the source and absorber functions, are similar, which is indicative of the identity of the oxidation states (IV, V, and VI) of iron and cobalt in it. Stabilization of 57Fe and 57Co in the β-MnO2 matrix is also investigated by Mossbauer absorption and emission spectroscopy.

Partitioning of calcium, magnesium, and transition metals between olivine and melt governed by the structure of the silicate melt at ambient pressure

Olivine/melt partitioning of the transition metal cations, Fe2+, Mn2+, Co2+, and Ni2+, together with Mg2+ and Ca2+, has been examined experimentally as a function of melt composition at ambient pressure. Melt structure was inferred from bulk-chemical composition, existing structural data, and 57Fe resonant absorption Mossbauer spectroscopy. Under isothermal conditions, K D(i-Mg)olivine/melt = ( C i/ C Mg)olivine/( C i/ C Mg)melt, is an exponential function of melt NBO/T for i = Ca2+, Mn2+, Co2+, and Ni2+. For i = Fe2+, the relationship is parabolic with maximum K D(Fe2+-Mg)olivine/melt -values at NBO/T near 1. At constant melt NBO/T, K D(i-Mg)olivine/melt increases systematically with decreasing cation radius, an effect that is more pronounced the more polymerized the melt. The K D(i-Mg)olivine/melt is also a positive and linear function of Na/(Na + Ca) of Al-free melts. This latter effect results from changes in Qn-species abundance governed by Na/(Na + Ca) of the melts. The enthalpy of the exchange equilibrium, iolivine + Mgmelt = imelt + Mgolivine, derived from the temperature-dependence of K D(i-Mg)olivine/melt, is also a positive function of the ionic radius of the cation. The relationship of enthalpy to melt polymerization also depends on cation radius. The K D(Fe2+-Mg)olivine/melt does not, however, follow this trend possibly because the bond distance, d Fe2+-O, in the melts depends on melt composition. The cations examined in this study are network-modifiers in silicate melts at ambient pressure. The solution behavior of network-modifying cations in melts is governed by the extent of steric hindrance near nonbridging oxygen, which in turn affects the energetics of metal-nonbridging oxygen bonds. Those structure effects, in turn, are related to the type of Qn-species, their abundance, and on the Al-distribution between the Qn-species. It is suggested, therefore, that the observed variations of mineral/melt partition coefficients with melt composition can be understood by considering bulk polymerization (NBO/T), the distribution of Al3+ among coexisting Qn-species, and the distribution of network-modifying cations among nonbridging in these Qn-species.

Magnetic collapse in yttrium iron garnet Y3Fe5O12 at high pressure

The effect of high pressures up to 70 GPa on single-and polycrystalline samples of yttrium iron garnet Y3 57Fe5O12 is studied by Mossbauer absorption spectroscopy (for the 57Fe nucleus) in a diamond-anvil cell. It is found that the hyperfine magnetic field H hf at 57Fe nuclei vanishes abruptly at a pressure of 48 ± 2 GPa, which indicates the transition of the crystal from the ferrimagnetic state to nonmagnetic one. The magnetic transition is irreversible. When the pressure decreases, the magnetic state is not recovered and the garnet remains nonmagnetic until zero pressure. The behavior of the quadrupole splitting and isomer shift shows that, simultaneously with the magnetic transition, irreversible electron and possibly spin transitions occur with changes in the local crystalline structure. The mechanisms of the magnetic collapse are discussed.

Rate of Iron Carbide Formation from Reduced Iron in CO-H2-H2S Mixtures under Pressurized Conditions

Under pressurized conditions up to 0.6 MPa, the rate of the iron carbide formation from completely reduced iron in CO-H 2 -H 2 S mixtures was measured by gravimetric technique. The fractional conversion for carbide formation, f θ , was defined as the weight gain divided by the weight of carbon for the complete conversion to cementite. First, the compositions of the partially reacted samples were analyzed by Mossbauer absorption spectroscopy. Then, the fractional conversion calculated from the composition, f M θ, was compared with f θ . The value of f θ agreed reasonably with that of f M θ in the range f θ <0.8, from which the gravimetric method is proved to be valid to the measurements for the rate of iron carbide formation. The carbide formation curves, f θ versus t relation, were not influenced by the weight of the sample and the flow rate of the reaction gas in the conditions; 50 mg, more than 3.33×10 -6 m 3 /s. Cross sectional view indicated the homogeneous distribution of cementite regions in the reduced iron particles partially converted to cementite. On the other hand, the rate was very sensitive to the conditions of the carburization reaction on the pore surface. The rate increased with increasing the content of carbon monoxide up to 80 to 90%, where the rate was the maximum, and then gradually decreased with further increasing the content. The rate increased with the total pressure, but decreased with the activity of sulfur. The rate controlling step of the iron carbide formation was discussed on the basis of the experimental results. The rate controlling step was assumed to be the carburization reactions on the pore surface of reduced iron particles.

Synthesis, Crystal Structure, Magnetic Susceptibility, and Mössbauer Spectroscopy of a Mixed-Valence Organic−Inorganic Hybrid Compound: (H3DETA)[Fe3(C2O4)2(HPO4)2(PO4)] (DETA = Diethylenetriamine)

A mixed-valence organic−inorganic hybrid compound, (H3DETA)[Fe3(C2O4)2(HPO4)2(PO4)] (DETA = diethylenetriamine), has been synthesized under hydrothermal conditions and characterized by single-crystal X-ray diffraction, magnetic susceptibility, 57Fe powder absorption Mossbauer spectroscopy, and electric field gradient calculations. It crystallizes in the triclinic space group P1 (No. 2) with a = 9.6367(7) A, b = 10.1760(7) A, c = 12.6164(9) A, α = 86.302(1)°, β = 68.880(1)°, γ = 61.800(1)°, and Z = 2 with R1 = 0.0397. The structure can be described as layers of trigonal bipyramidal FeIIO5, octahedral FeIIIO6, tetrahedral PO4 moieties, and monobidentate oxalate ligands, which are pillared through bisbidentate oxalate ligands to form a 3-D framework structure. The title compound is not only the first oxalatophosphate that contains two types of oxalate ligands but also the first example of mixed-valence transition metal oxalatophosphate.

Magnetic collapse and the change of electronic structure of FeBO3 antiferromagnet under high pressure

The effect of high pressures up to 60 GPa on single-crystal and polycrystalline samples of iron borate 57FeBO3 was studied by Mossbauer absorption spectroscopy (57Fe nuclei) in a diamond anvil cell. Magnetic field Hhf at the 57Fe nuclei increases with pressure but abruptly drops to zero at 46±2 GPa, indicating the crystal transition from the antiferromagnetic to nonmagnetic state. This is accompanied by an abrupt change in the isomer shift and quadrupole splitting. Their values in the high-pressure phase are evidence for the transition of Fe+3 ions from a high-spin (S=5/2, 6A1g) to low-spin (S=1/2, 6T2g) state (spin crossover). This correlates with an abrupt decrease in the unit-cell volume (by ∼9%) and optical gap. The change of the magnetic and electronic structures is explained by Mott’s transition with rupturing of strong d-d-electron correlations.

XPS and Mössbauer Characterization of Au/TiO2 Propene Epoxidation Catalysts

Gold catalysts supported on TiO2 and TiO2/SiO2 were used in gas-phase propene epoxidation with a hydrogen−oxygen mixture. The catalysts were characterized by 197Au Mossbauer absorption spectroscopy (MAS), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Gold particle sizes of 1 wt % Au catalysts calcined at 673 K ranged from 3 to 6 nm. Two Au contributions were found in Mossbauer spectra and assigned to bulk metallic Au atoms in the core of a gold particle and metallic gold on the outer surface of this particle. By MAS, no evidence for charge transfer from support to Au particle could be found. The Auger parameters confirmed that the surface layer of 3−5 nm gold particles is metallic. Deactivation is not due to a change in the active gold species but is related to the TiO2 support. In the preparation via deposition-precipitation, Au(OH)3 species are converted to metallic gold during calcination. Gold particles do not gradually grow during calcination, possibly due to the...

Monodisperse magnetic iron colloids grafted with polyisobutene: a model system for Fischer-Tropsch catalysts?

Iron colloids stabilised with polyisobutene with a radius ranging from 2.1 to 8.2 nm have been prepared and characterised by Mossbauer absorption spectroscopy. Comparison with results reported in the literature on sarcosyl and oleic acid stabilised colloids reveals that the stabilising ligand influences the properties of the colloids. By impregnating an alumina support with a solution of the r=2.1 nm Fe colloids a model catalyst has been prepared. The first results show that the impregnation was successful. However, due to the low Fe loading, the signal-to-noise ratio is too low to perform good measurements on this model system. Therefore, Fe-colloids will be prepared that are enriched in 57Fe.

Non-Langevin behaviour of the uncompensated magnetization in nanoparticles of artificial ferritin

The magnetic behaviour of nanoparticles of antiferromagnetic artificial ferritin has been investigated by 57Fe Mossbauer absorption spectroscopy and magnetization measurements, in the temperature range 2.5-250 K and with magnetic fields up to 7 T. Samples containing nanoparticles with an average number of 57Fe atoms ranging from 400 to 2 500 were studied. By analysing the magnetic susceptibility and zero field Mossbauer data, the anisotropy energy per unit volume is found, in agreement with some of the earlier studies, to have a value typical for ferric oxides, i.e. a few 105 ergs/cm3. By comparing the results of the two experimental methods at higher fields, we show that, contrary to what is currently assumed, the uncompensated magnetization of the ferritin cores in the superparamagnetic regime does not follow a Langevin law. For magnetic fields below the spin-flop field, we propose an approximate law for the field and temperature variation of the uncompensated magnetization, which was early evoked by Neel but has so far never been applied to real antiferromagnetic systems. More generally, this approach should apply to randomly oriented antiferromagnetic nanoparticles systems with weak uncompensated moments.

Low temperature Kondo reduction of quadrupolar and magnetic moments in the YbCu Al series

We present measurements in the YbCu5-xAlx series, down to the 50 mK range, using 170Yb Mossbauer absorption spectroscopy and magnetisation measurements. In this series, the hybridisation between the Yb 4f electrons and the conduction electrons is known to decrease as the Al content x increases. We apply the variational solution of the impurity Kondo problem to the interpretation of our data. We show that the Kondo temperature can be derived from the measured 4f quadrupole moment and, for the magnetically ordered compounds (\(\)), we obtain the exchange energy as a function of the Al content. Our findings are in general agreement with Doniach's model describing the onset of magnetic ordering according to the relative values of the Kondo and exchange energy scales.

IRRADIATION EFFECTS DETECTED BY MOSSBAUER SPECTROSCOPY IN IRON COMPLEXES

The nature and the extent of the Co-60 gamma radiolysis of several iron coordination compounds have been analysed by Mossbauer absorption spectroscopy. The comparison of the radiolytic yields with the after effects observed by Mossbauer emission spectroscopy in similar Co-57-doped compounds, supports the self-radiolysis model.

Gold and platinum molecular cluster compounds studied by $^{197}$ Au Mössbauer Spectroscopy

The technique of 197Au Mossbauer absorption spectroscopy has been used to study four varieties of the high nuclearity gold molecular cluster compounds abbreviated as Au55, all having a cuboctahedral structure, with 12 PPh3-ligands or modifications of PPh3. The technique of emission spectroscopy developed in our laboratory [1] has been applied to four different molecular platinum carbonyl cluster compounds of varying cluster nuclearity, abbreviated as Pt38, Pt26, Pt24, and Pt19 [2]. The four Au55 compounds were studied, both as dry materials and as frozen solutions. The M6ssbauer parameters of the chlorine ligated site of the water soluble version differ strongly due to the effect of Na+ on the Au-Cl distances. In the frozen solution, the effect of the ionic charge on this cluster can be clearly seen. The similarity of the spectra of all the Pt compounds, especially the occurrence of a substantial singlet contribution in all spectra, is explained by a coalescing mechanism for the low-nuclearity clusters, induced by the neutron irradiation damage inflicted while preparing the sample as M6ssbauer sources. The observed decrease of the absorption intensity with increasing temperature is evidence for a cluster character of the sample remaining after neutron irradiation.