57Fe-Mössbauer study of electrically conductive alkaline iron vanadate glasses

A relationship between local structure, thermal stability and electrical conductivity (σ) of xR₂O·10Fe₂O₃·(90 ? x)V₂O₅ glasses (abbreviated as xRFV glasses, where R = Li, Na, K; x = 20 and 40 in mol %) was investigated by ⁵⁷Fe-Mössbauer spectroscopy, X-ray diffractometry, differential thermal analysis (DTA) and DC two- and four-probe method. From DTA study, thermal stability of 20RFV glasses is lower than that of 40RFV glasses by evaluating Hruby parameter (K _gl). Constant activation energy for crystallization (E _a) of 2.5 eV obtained from both 20RFV and 40RFV glasses indicate that the crystallization proceeds with the cleavage of Fe–O bond having the energy of 2.6 eV. Isochronally annealed 20RFV glass at 400–450 °C resulted in the increase in electrical conductivity (σ) from the order of 10^?3 to 10^?1 S cm^?1, whereas slight decrease in σ was observed for 20RFV glass annealed above 460 °C. A paramagnetic doublet with an identical isomer shift (δ) of 0.39 mm s^?1 was observed in the ⁵⁷Fe-Mössbauer spectra of 20RFV glass after isothermal annealing conducted at 400–450 °C for 100 min, which caused a decrease of quadruple splitting (Δ) from 0.67 to 0.52 mm s^?1 for 20LiFV glass and from 0.66 to 0.53 mm s^?1 for 20NaFV glass. On the other hand, three paramagnetic doublets with δ and Δ of 0.40 and 0.25, 0.38 and 0.60, and 0.31 and 1.11 mm s^?1 respectively were observed for 20RFV glass annealed at 460–550 °C, reflecting precipitation of semiconducting FeVO₄ phase having σ of 6.0 × 10^?7 S cm^?1. It can be concluded that isochronal annealing of 20RFV glass below 450 °C resulted in increase in σ due to the structural relaxation, while annealing above 500 °C resulted in the decrease of σ due to the precipitation of FeVO₄ phase.     

The relationship between SnII fraction and visible light activated photocatalytic activity of SnOx·SiO2 glass studied by Mössbauer spectroscopy

The relationship between local structure and visible-light photocatalytic ability of tin silicate glass prepared by sol–gel method was investigated. ¹¹⁹Sn Mössbauer spectrum of SnO_x·SiO₂ glass prepared from SnCl₂ showed a small peak of Sn^II component besides the major amount of Sn^IV. The smallest bandgap energy of 2.5 ± 0.5 eV was estimated from Tauc plot, and the largest first order rate constant (k) of (13.8 ± 0.1) 10⁻³ min⁻¹ was recorded from the methylene blue degradation test under visible-light irradiation. It is concluded that Sn^II shows remarkable photocatalytic ability when it is incorporated into silica glass matrix.

     

Degradation of passive layers of iron studied by conversion electron Mössbauer spectroscopy

Integral electron Mössbauer spectroscopy (ICEMS) and additionally some electrochemical methods were used to characterize the passivation process of iron (low carbon steel) in sulfate, sulfate+sulfite (a possible model solution of acid rain) solutions and in phospate buffer. The phase compositions and thicknesses of the passive layers formed due to the electrochemical polarizations were analyzed in dependence on the duration of the anodic passivations and on the pH of the used electrolytes. The passive layer, as determined from the Mössbauer spectra, consists mainly of -FeOOH, however in sulfite containing sulfate aqueous solution at pH 3.5 Fe₃C and despite ex-situ circumstances FeSO₄·H₂O was detected after the shortest polarization time. The film thickness, which was found to grow nearly linearly with polarization time in pure sulfate solution and in phospate buffer, reached a maximum of 60–160 nm (depending on pH) in sulfate+sulfite solution after a passivation time of about 4 hours. It has been proved, that HSO₃ ^–-ion, which is contained by acid rain, initiate pit formation under acid conditions and so enforces the corrosion of iron. The experimental results furthermore suggest, that not the whole oxidic layer is responsible for the passivity but only a very thin intermediate layer formed between an inner oxide layer of a cubic structure and the rhombic oxide (-FeOOH) cover.     

Vibrational and 57Fe-Mössbauer spectra of FeTbGe2O7

The infrared, Raman and 57Fe-M?ssbauer spectra of FeTbGe2O7 were recorded and analyzed on the basis of its crystallographic data. For comparative purposes, similar measurements were also performed with FeYGe2O7 and with other isostructural compounds containing different lanthanide cations.     

Study of the relationship of small variations of the molecular structure and the iron state in iron containing proteins by Mössbauer spectroscopy: biomedical approach

This review considers the results of experimental M?ssbauer studies and theoretical calculations of the effect of small variations of protein molecular structure on the iron electronic structure and stereochemistry in order to understand the proteins structural heterogeneity and functional variety. Structural changes in iron containing proteins during various diseases are also considered. These results show the relationship of the small structural variations and M?ssbauer parameters of iron containing proteins and demonstrate the possibilities of M?ssbauer spectroscopy to obtain new information at the molecular level in biomedical research.     

Mössbauer study of the effect of boron and iron contents on the dynamic behavior of iron in sodium phosphate glass system

Mössbauer measurements were performed at different temperatures in order to examine the dynamic behavior of iron in the glass system: 42.5% P₂O₅, 42.5% Na₂O, 15% Fe₂O₃. Variation of the dynamic behavior was traced by substituting B₂O₃ for P₂O₅ [30 P₂O₅, 12.5 B₂O₃, 42.5 Na₂O, 15 Fe₂O₃] and by increasing the amount of iron at the expense of Na₂O [42.5 P₂O₃, 15 Na₂O, 42.5 Fe₂O₃]. The Mössbauer measurements gave the values of Debye temperature (_D), mean square displacement <²>, mean square velocity <v ² > of oscillation, the lattice time () and the strength parameter (B) for each glass. These values were discussed with the results of DTA, density, hardness and D. C. conductivity.     

Distinct reaction pathways followed upon reduction of oxy-heme oxygenase and oxy-myoglobin as characterized by Mössbauer spectroscopy

Activation of O(2) by heme-containing monooxygenases generally commences with the common initial steps of reduction to the ferrous heme and binding of O(2) followed by a one-electron reduction of the O(2)-bound heme. Subsequent steps that generate reactive oxygen intermediates diverge and reflect the effects of protein control on the reaction pathway. In this study, M?ssbauer and EPR spectroscopies were used to characterize the electronic states and reaction pathways of reactive oxygen intermediates generated by 77 K radiolytic cryoreduction and subsequent annealing of oxy-heme oxygenase (HO) and oxy-myoglobin (Mb). The results confirm that one-electron reduction of (Fe(II)-O(2))HO is accompanied by protonation of the bound O(2) to generate a low-spin (Fe(III)-O(2)H(-))HO that undergoes self-hydroxylation to form the alpha-meso-hydroxyhemin-HO product. In contrast, one-electron reduction of (Fe(II)-O(2))Mb yields a low-spin (Fe(III)-O(2)(2-))Mb. Protonation of this intermediate generates (Fe(III)-O(2)H(-))Mb, which then decays to a ferryl complex, (Fe(IV)=O(2-))Mb, that exhibits magnetic properties characteristic of the compound II species generated in the reactions of peroxide with heme peroxidases and with Mb. Generation of reactive high-valent states with ferryl species via hydroperoxo intermediates is believed to be the key oxygen-activation steps involved in the catalytic cycles of P450-type monooxygenases. The M?ssbauer data presented here provide direct spectroscopic evidence supporting the idea that ferric-hydroperoxo hemes are indeed the precursors of the reactive ferryl intermediates. The fact that a ferryl intermediate does not accumulate in HO underscores the determining role played by protein structure in controlling the reactivity of reaction intermediates.     

57Fe- and119Sn-Mössbauer effect of oxide glasses

Coordination number of network-former (NWF) and formation of nonbridging oxygen (NBO) at the site neighboring to NWF can be estimated from Mössbauer measurements, since small amounts of Fe³⁺ and Sn⁴⁺ substitute NWF in several oxide glasses. Gamma-ray or thermal neutron irradiation of oxide glasses causes electron or charge transfer from oxygen to the Mössbauer ions, and the probability depends on the fraction of NBO. On the contrary, -ray irradiation of phosphate glasses results in oxidation of Fe²⁺ to Fe³⁺ since iron plays a role of network modifier (NWM) at interstitial sites. Debye temperature _D obtained from low-temperature Mössbauer measurements reflects the site occupation of Mössbauer ions in glasses. A linear relationship between glass transition temperature (T _g ) and quadrupole splitting () of Fe³⁺,T _g — rule, is also effective for determining the site occupation of Mössbauer ions.     

Coprecipitation of iron and platinum(IV) hydroxo complexes as probed by Mössbauer spectroscopy

Stabilization of ⁵⁷Fe compounds in matrices of solid solutions of platinum(IV) superoxo- and hydroxo complexes was probed by Mössbauer spectroscopy. The ratio Fe^III/Fe^IV in these matrices is 20/1.     

Characterization of iron promoter in tungstated zirconia catalysts by Mössbauer spectroscopy at very low temperatures

A tungstated zirconia (WZ) catalyst with iron promoter used for the conversion of n-pentane into isopentane has been characterized by M?ssbauer spectroscopy. The M?ssbauer spectra have been recorded in zero magnetic field in the temperature range 0.05-295 K and with a magnetic field up to 7 T between 4.2 and 50 K. Both the recording of M?ssbauer spectra with an applied magnetic field and at extremely low temperature allowed for the demonstration that iron is present in the catalysts as (i) hematite (alpha-Fe2O3) particles a few 10 nm in size, (ii) very small oligomeric Fe(III) species, probably in solid solution in zirconia, and (iii) Fe(III) oxide clusters showing magnetic ordering, probably embedded in the first surface layer and thus forming "rafts". These latter clusters form two ensembles with quite different sizes: one with diameters of about 3 nm, the other with diameters larger than 30 nm. These results are in agreement with those recently obtained by X-ray absorption spectroscopy and electron paramagnetic resonance.     

Mössbauer study of films produced by laser deposition of iron oxides

Iron oxides, magnetite Fe₃O₄ and hematite Fe₂O₃ were laser-deposited onto Al substrates at various temperatures, and the Mössbauer spectra of the films were measured. The compositions of the films changed depending on the formation temperature of the substrate, oxide deficiency in the lattice structures and the formation process of the iron oxides. The films were composed of Fe_3?x O₄ and Fe_1?x O independent of the laser-evaporation source (magnetite or hematite). Fe_3?x O₄ was seen to be dominant at higher temperatures and Fe_1?x O was dominant at lower temperatures. The compositions of the films were confirmed by X-ray diffraction (XRD) measurements, and the surfaces of the deposited films were examined using scanning electron microscopy (SEM).     

Surface oxidation of tin chalcogenide nanocrystals revealed by 119Sn-Mössbauer spectroscopy

Narrow band gap tin(II) chalcogenide (SnS, SnSe, SnTe) nanocrystals are of high interest for optoelectronic applications such as thin film solar cells or photodetectors. However, charge transfer and charge transport processes strongly depend on nanocrystals' surface quality. Using (119)Sn-M?ssbauer spectroscopy, which is the most sensitive tool for probing the Sn oxidation state, we show that SnS nanocrystals exhibit a Sn((IV))/Sn((II)) ratio of around 20:80 before and 40:60 after five minutes exposure to air. Regardless of the tin or sulfur precursors used, similar results are obtained using six different synthesis protocols. The Sn((IV)) content before air exposure arises from surface related SnS(2) and Sn(2)S(3) species as well as from surface Sn atoms bound to oleic acid ligands. The increase of the Sn((IV)) content upon air exposure results from surface oxidation. Full oxidation of the SnS nanocrystals without size change is achieved by annealing at 500 °C in air. With the goal to prevent surface oxidation, SnS nanocrystals are capped with a cadmium-phosphonate complex. A broad photoluminescence signal centered at 600 nm indicates successful capping, which however does not reduce the air sensitivity. Finally we demonstrate that SnSe nanocrystals exhibit a very similar behavior with a Sn((IV))/Sn((II)) ratio of 43:57 after air exposure. In the case of SnTe nanocrystals, the ratio of 55:45 is evidence of a more pronounced tendency for oxidation. These results demonstrate that prior to their use in optoelectronics further surface engineering of tin chalcogenide nanocrystals is required, which otherwise have to be stored and processed under inert atmosphere.     

Investigation on the magnetic and Mössbauer spectroscopy of 57Fe doped LiMnPO4

Journal of Radioanalytical and Nuclear Chemistry - The 57Fe doped LiMnPO4 cathode with potential applications in Li-ion batteries was prepared by solid-state reaction. The magnetic susceptibility...     

Self-assembly of dinitrosyl iron units into imidazolate-edge-bridged molecular squares: characterization including Mössbauer spectroscopy

Imidazolate-containing {Fe(NO)(2)}(9) molecular squares have been synthesized by oxidative CO displacement from the reduced Fe(CO)(2)(NO)(2) precursor. The structures of complex 1 [(imidazole)Fe(NO)(2)](4), (Ford, Li, et al.; Chem. Commun.2005, 477-479), 2 [(2-isopropylimidazole)Fe(NO)(2)](4), and 3 [(benzimidazole)Fe(NO)(2)](4), as determined by X-ray diffraction analysis, find precise square planes of irons with imidazolates bridging the edges and nitrosyl ligands capping the irons at the corners. The orientation of the imidazolate ligands in each of the complexes results in variations of the overall structures, and molecular recognition features in the available cavities of 1 and 3. Computational studies show multiple low energy structural isomers and confirm that the isomers found in the crystallographic structures arise from intermolecular interactions. EPR and IR spectroscopic studies and electrochemical results suggest that the tetramers remain intact in solution in the presence of weakly coordinating (THF) and noncoordinating (CH(2)Cl(2)) solvents. M?ssbauer spectroscopic data for a set of reference dinitrosyl iron complexes, reduced {Fe(NO)(2)}(10) compounds A ((NHC-iPr)(2)Fe(NO)(2)), and C ((NHC-iPr)(CO)Fe(NO)(2)), and oxidized {Fe(NO)(2)}(9) compounds B ([(NHC-iPr)(2)Fe(NO)(2)][BF(4)]), and D ((NHC-iPr)(SPh)Fe(NO)(2)) (NHC-iPr = 1,3-diisopropylimidazol-2-ylidene) demonstrate distinct differences of the isomer shifts and quadrupole splittings between the oxidized and reduced forms. The reduced compounds have smaller positive isomer shifts as compared to the oxidized compounds ascribed to the greater π-backbonding to the NO ligands. M?ssbauer data for the tetrameric complexes 1-3 demonstrate larger isomer shifts, most comparable to compound D; all four complexes contain cationic {Fe(NO)(2)}(9) units bound to one anionic ligand and one neutral ligand. At room temperature, the paramagnetic, S = (1)/(2) per iron, centers are not coupled.     

High-Tc superconductors studied by57Co,57Fe,119Sn,and151Eu mössbauer spectroscopy

¹⁵¹Eu,¹¹⁹Sn,⁵⁷Fe, and⁵⁷Co Mössbauer spectroscopy was used to study YBa₂Cu₃O_7– high-T_c superconductors in which sites were replaced by various Mössbauer nuclides in order to get information about site preference and structural changes. By decomposition of the Mössbauer spectra, the Y site substituted by Eu and the Cu sites occupied by Sn, Fe, and Co substituents were identified. It was found that Fe and Co prefer the Cu(1) site while Sn prefers the Cu(2) site in the orthorhombic perovskite lattice of these superconductors. The site preference is enhanced in the presence of additional substituents. A sequence of thermal heat treatments permits reversible cycling of⁵⁷Co between the two copper sites in the YBa₂[Cu(⁵⁷Co)]₃O_7– perovskite. In all cases Eu(III) and Sn(IV) states are determined, but the valence state of Fe and Co can be both three and four. BelowT _c, low-temperature phase transformation and phonon softening were shown from the anomalous temperature dependence of isomer shifts and the area fractions of¹⁵¹Eu and¹¹⁹Sn Mössbauer spectra of EuBa₂(Cu_1-xSn_x)₃O_7- cuprates.