Mössbauer effect study of the Β-FeOOH to α-Fe2O3 phase transformation

-FeOOH was precipitated from a chloride solution. Mössbauer spectra were taken at room temperature (RT) and at 4.2 K in zero field and in external magnetic fields. Samples aged for successively longer time periods were studied. They contain -FeOOH and -Fe₂O₃, the Mössbauer spectra of which show superparamagnetic behavior. The transformation into -Fe₂O₃ is already induced by aging in the mother solution at 100 C. A final heat treatment at 350 C leads to the complete transformation into -Fe₂O₃ and to an increase of the crystallite size.     

Investigation of Rust Formed on Steel Surfaces and Related Oxyhydroxides

In order to indentity the corrosion products formed on steel surfaces from ⁵⁷Fe Mössbauer spectroscopy, detailed Mössbauer parameters have been determined for various kinds of iron-oxyhydroxides: -FeOOH, -FeOOH, -FeOOH and -FeOOH. ⁵⁷Fe Mössbauer measurements of the iron oxyhydroxides indicate the following results. Fe occupies a single site in -FeOOH, but below the Néel temperature as at e.g., 300 K the Mössbauer spectrum is always broad, showing a distribution of the strength of the magnetic exchange interactions. Its shape depends on the grain-size and synthetic methods of the specimen. Fe occupies 3 sites in -FeOOH. High-purity reagents of -FeOOH always contain small amounts of -FeOOH and their Néel temperatures depend on the synthetic methods of the specimen. Mössbauer spectroscopy of the synthetic -FeOOH shows very broad distribution of the hyperfine magnetic fields.     

An unambiguous procedure for discovering relaxation influence on Mössbauer spectra

A mathematical procedure which gives an opportunity to distinguish between relaxation and distribution mechanisms of the Mössbauer line broadening is proposed. It is based on the method of Mössbauer line sharpening developed recently [1] and allows one to answer unambiguously the question whether or not relaxation manifests itself in Mössbauer measurements by examining the wings of the spectrum. The procedure is applied to the spectra of high spin ferric metmyoglobin and (Fe_0.65Ni_0.35)_1–x Mn _x alloys. Analysis of the metmyoglobin spectra manifests the presence of the relaxation influence in the temperature range 10 KT100 K. Analysis of the alloy spectra shows that for the temperatures 4 KT300 K and Mn concentrations 0x0.245 no relaxation is observed and the line broadening is mainly caused by the distributions of magnetic hyperfine fields. A possible explanation of this result is given.     

Mössbauer Spectroscopy on Nuclear Power Plant Materials

Experimental results of original irradiated reactor pressure vessel surveillance specimens are presented and discussed in the paper. In 1994, the new Extended Surveillance Specimen Program for Nuclear Reactor Material Study was started in collaboration with the nuclear power plant (NPP) Bohunice and NPP Research Institute Trnava (Slovakia). Three batches of special prepared Mössbauer samples (after 1, 2 and 3 years stay in irradiation channels) were measured and interpreted using the new four components approach of Mössbauer spectra evaluation with the aim to observe microstructural changes due to thermal and neutron treatment resulting from operating conditions in NPP. The systematic changes in the relative areas of Mössbauer spectra components were observed and discussed.     

Mössbauer Study of Oxygenated Iron-Phthalocyanines,a Precursor of Magnetic Storage Material

⁵⁷Fe transmission Mössbauer spectroscopy and X-ray diffractometry were used to study - and -polymorphs of iron phthalocyanines (FePc) to get information about their oxygenation in connection with controlled pyrolysis of flat layered phthalocyanines initiated by radicals at relatively low temperatures, to form acicular carbide particles encaged in carbon. Mössbauer spectroscopy and XRD revealed that the oxygenation of both - and -polymorphs of FePc was successfully achieved. New Mössbauer spectral components (doublets) appeared upon the oxygenation in both and FePc. This can be attributed to iron microenvironments containing oxygen between the layers of iron phthalocyanines.     

Dynamical property of YFeMnO4

In order to interpret abrupt changes of distributions of orientations andmagnitudes of hyperfine fields in Mössbauer measurements of⁵⁷Fe in YFeMnO₄, a model is proposed. The origin is assumed to be not static but dynamic. Fluctuating fields are caused by spins in a fully frustrated spin system, namely, a Heisenberg spin system on a triangular lattice in a two-dimensional antiferromagnet. Extending the stochastic theory for the fluctuation between two states to that among many states with a Gaussian distribution, we fit the spectra with three parameters, hyperfine fieldH _hf, correlation time _c, andwidth of fluctuating field , within a limited condition of =_c=1. Obtained results represent the characteristic features of Mössbauer spectra well, in spite of a simplified model.     

Resonant-Detector Mössbauer Spectroscopic Studies of Sn Doped SiO2 Analysed Using Quantum Mechanical Theory

It has already been established that, by using a resonant detector in Mössbauer spectroscopy, the minimum spectral linewidth is 1.46. Here is the linewidth of the Mössbauer excited-state nuclear level. It is well known that the minimum linewidth obtained in conventional Mössbauer experiments is 2. The quantum mechanical calculation using a nuclear-resonant detector, which predicts this result, is summarized. The fundamental equations describing the system are solved in the frequency domain and applied to the experimental results. The experimental results using the resonant-detector Mössbauer technique and an Sn-doped SiO₂ sample are presented. The best fit to the data is obtained using the resonant-detector quantum mechanical theory.     

Synchrotron Mössbauer source of57Fe radiation

A nonradioactive source of Mössbauer radiation is described for use in Mössbauer absorption and scattering spectroscopy. The radiation is generated by synchrotron Xrays in an iron borate single crystal set in diffraction conditions at the Néel temperature (75.3°C). Like a conventional Mössbauer source the new Synchrotron Mössbauer (SM) source emits singleline radiation of about natural linewidth, but in addition the emitted radiation is fully recoilless, highly directed and of pure linear polarization. An extremely high suppression of the electronic scattering is achieved. The latter circumstance allows one to perform Mössbauer experiments using pulsed synchrotron radiation in a steady state mode as in a normal Mössbauer measurement.The theory of the SM source is developed. First Mössbauer spectra obtained with the SM source are shown. Applications of the SM source are discussed.     

The effect of iron doping in La0.8Sr0.2Fe0.05Co0.95O3-δ perovskite

La0.8Sr0.257Fe0.05Co0.95O3- perovskite is investigated by 57Fe transmission and emission Mössbauer spectroscopy, X-ray diffraction, AC magnetic susceptibility and magnetotransport measurements. Temperature dependence of the 57Fe Mössbauer isomer shift, quadrupole splitting, magnetic hyperfine field, line broadening, and relative spectral area is presented in a detailed manner for La0.8Sr0.257Fe0.05Co0.95O3- . The oxidation state of iron is determined to be Fe3+, and the presence of preferential electronic charge compensation Fe3+ Fe4+ over that of Co3+ Co4+ is excluded. Relaxation of iron magnetic moments reflected by the 57Fe Mössbauer spectra of La0.8Sr0.257Fe0.05Co0.95O3- are interpreted as evidence for the existence of superparamagnetic like Co clusters and a corresponding cluster glass magnetic phase formed below T 65 K.     

Preparation and Properties of FeNi-Mg/MgO Nanocomposite

The FeNi-Mg/MgO nanocomposite synthesized by spark erosion of pure Mg and Fe₇₀Ni₃₀ electrodes was investigated by X-ray diffraction, Mössbauer spectroscopy and magnetic measurements. From the X-ray diffraction measurements Mg, MgO, and -FeNi were recognized. According to the Mössbauer spectra analysis, iron atoms were identified in -FeNi, Fe_1-x O, and FeNi in Mg/MgO phases. The FeNi in Mg/MgO phase consists of fcc -FeNi phase which is antiferromagnetic below T _N 40 K and weakly ferromagnetic bcc -FeNi clusters. The magnetic measurements confirmed the presence of the -FeNi phase and the composite model in the form of isolated magnetic particles in Mg/MgO matrix.     

Electronic structure and spin relaxation of Fe(III) in LiNbO3

Crystal field parameter have been determined from EPR spectra (77K) of LiNbO₃Fe(III). Anomalous line shapes are described phenomenologically and saturation studies are compared with calculations using a second order dynamic spin-hamiltonian H. Mössbauer spectra of LiNbO₃Fe(III) have been interpreted by means of simulations using Liouville superoperators including a static fine-and a hyperfine spin-hamiltonian and H. Calculations have been performed using an effective spin of 1/2, the supermatrix with dimension 288 and a reduced formalism (supermatrix with dimension 96).     

Mössbauer Study of Iron-Containing Carbon Nanotubes

⁵⁷Fe transmission Mössbauer at temperatures between 18 and 298 K and magnetic measurements have been used to characterize Fe-filled carbon nanotubes which were prepared by pyrolisis of Ferrocene + C₆₀ at atmospheric pressure under an Ar atmosphere at 1050°C. The Mössbauer data have shown that the Fe phases encapsulated within the carbon nanotubes are -Fe, Fe₃C and -Fe. The magnetic results are compatible with the Mössbauer data. Taken together the results allow us to propose a simple picture of the distribution of iron phases within the carbon nanotubes which would consist of an -Fe core surrounded by an -Fe shell, finally covered by an Fe₃C layer.     

Mössbauer Study of the Thermal Behaviour of Garnets Used in High-Energy Water Jet Technologies

A high-energy water jet combined with silicate garnets as abrasives has been proven to be a powerful tool for disintegration of hard materials. Thermal heating of the garnets is one way for structure improvement of the abrasive material. Room-temperature Mössbauer spectra of initial powdered almandine samples are characterised by one doublet corresponding to Fe²⁺ in dodecahedral position 24c. Almandine garnet, industrial product Barton HP 80 as reference material in all experiments, has a second doublet corresponding to Fe³⁺ in octahedral position 16a. In room-temperature spectra of heated almandine garnet samples from locality Ktí and Mdnec (heated under temperatures 200–1000°C by 100 degrees for 1 hour in air) a new doublet originating from -Fe₂O₃ nanoparticles appeared. Under a heating temperature of higher than 800°C, the broad sextets of -Fe₂O₃ and -Fe₂O₃ in spectra were discovered. No additional doublets or sextets appeared in room-temperature Mössbauer spectra of reference material of almandine garnet Barton HP 80 heated under each temperature.     

Mössbauer spectroscopy study on the hydrothermal transformation α-FeOOH→α-Fe2O3

The reaction kinetics of the hydrothermal transformation -FeOOH-Fe₂O₃ was studied by means of Mössbauer spectroscopy. From the reaction isotherms, a monomolecular, first order reaction was found to characterise the hydrothermal transformation of alpha oxihydroxide to the alpha iron oxide. The rate constant as well as the activation energy of this process were determined. No intermediate phases were identified in the hydrothermal samples. The thermodynamic properties of the hydrothermal system -FeOOH-Fe₂O₃ in correlation with Mössbauer spectroscopy data are discussed.     

Mössbauer hyperfine interactions in thermally treated iron-yttrium oxide systems

Iron-yttrium mixed oxides in varying ratios (YFe 91, 31, 11, 12, 13, and 19) annealed to different temperatures (850, 1000 and 1250 C) have been investigated by Mössbauer spectroscopy down to 85 K. The Mössbauer spectra are complex in nature, showing up to three magnetically split hyperfine patterns in iron-rich (Y Fe 11, 12, 13 and 19) samples; these three components are attributed to characteristic YFeO₃, Y₃Fe₅O₁₂ and hematite. The yttrium-rich samples (YFe 91, 31) show only one six-line spectrum of YFeO₃.     

Mössbauer study of a novel binuclear Fe(II/III) delocalized-valence compound

In this paper, we briefly summarize the main conclusions of the Mössbauer analysis of [L₂Fe₂(-OH)₃] (ClO₄)₂·2CH₃OH·2H₂O with L=N,N',N"-trimethyl-1,4,7-triazacyclononane, a novel dimeric iron compound, which possesses a central exchange-coupled delocalized-valence Fe(II/III) unit. The complete delocalization of the excess electron in the dimeric iron center is concluded from the indistinguishability of the two iron sites in Mössbauer spectroscopy. The magnetic Mössbauer spectra imply a system spinS _t=9/2 for the dimer in its ground state. The values for hyperfine and spin-Hamiltonian parameters, obtained from simulations of the Mössbauer spectra, are =0.74 mms^–1, E _Q=–2.14 mms^–1,A =–10.6 T,A =–13.5 T andD=1.8 cm^–1. The system spinS _t=9/2 is interpreted to be a consequence of double-exchange coupling.     

Evaluation of Mössbauer line diffusion broadening by exponential fit

The Mössbauer line broadening measured by R.Lindsey in -iron stabilized with vanadium is evaluated by an exponential fit described in the APPENDIX of this paper. By this procedure the low temperature Mössbauer line width _m ⁰ , the diffusion frequency factorD ₀ and the activation enthalpyH are computed.Knauer's andSørensen-Trumpy's definitions of the time correlation factorf _t , necessary for theD ₀ calculation from Mössbauer line broadening, are applied to the Fe-V diluted BCC solid solutions.     

Mössbauer spectroscopy of supported bimetallic catalysts: 1∶5 FeM/SiO2 (M=Ru,Rh, Pd,Ir, Pt)

Mössbauer spectra of SiO₂-supported bimetallic FeM (M=Ru, Rh, Pd, Ir, and Pt) with FeM=15 arter treatments such as reduction, exposure to CO and passivation in air are described and compared with previous results obtained on 11 FeM/SiO₂ catalysts.     

A study of a fullyc-axis oriented dc-sputtered thin film of YBa2(Cu0.98 57Fe0.02)3O6.8 by means of CEMS,XRD and SEM

A fullyc-axis oriented thin film of YBa₂(Cu_0.98 ⁵⁷Fe_0.02)₃O_6.8 prepared by planar dcsputtering has been investigated by means of Mössbauer spectroscopy. Room temperature⁵⁷Fe conversion electron Mössbauer spectra taken at different angles between the -ray direction and the normal (=c-axis) of the film show four subspectra: A (quadrupole splitting E _Q1.9 mm/s), B (E _Q1 mm/s), C (E _Q0.5 m/s) and D (E _Q1.6 mm/s). For subspectra A, B and C, we found the same hyperfine parameters as already published on other samples. The hyperfine parameters for subspectrum D are determined for the first time using a fully oriented sample. For D, we found the asymmetry parameter 0.6 andV _zz (the main component of the electric field gradient) lying in the a-b-plane.     

Electrostatic hyperfine interactions in Y(Fe,Al)2

Room temperature⁵⁷Fe Mössbauer and X-ray diffraction measurements are reported for Y(Fe_xAl^1–x)₂, 0.1x 0.9. For x0.65 broadened and on the Al-rich side structurized Bragg-peaks are observed. The asymmetry of the quadrupole split Mössbauer spectra depends on x. Whereas two different slopes are obtaind for the concentration dependence of the mean quadrupole splitting on the Al- and the Fe-rich side, the isomer shift exhibits a smooth dependence on x, which can be correlated to the change in volume caused by the Al substitution.