Mössbauer study of phase separation in Ni80 57Fe1P19 amorphous alloys

Mössbauer spectroscopy, X-ray diffractometry, scanning calorimetry and electrochemical measurements were used to study the crystallization process of Ni₈₀ ⁵⁷Fe₁P₁₉ amorphous alloys kept in melt at different temperatures before quenching. Samples were heated up to 430°C and 720°C at a rate of 20°C/min, in order to reach characteristically different stages of crystallization. Even at the same crystallization, stage the room temperature Mössbauer spectra and the X-ray differactograms were different depending on the temperature (1050°C or 1400°C) at which the samples were kept before quenching the melt. The Mössbauer spectra showed a paramagnetic component and two sextets (H=267 kOe and H=245 kOe) at 430°C while at 720°C there was only one sextet (H=267 kOe) besides the paramagnetic component. The changes in the Mössbauer spectra of different samples due to crystallization are consistent with the explanation that phase separation occurs in Ni₈₀ ⁵⁷Fe₁P₁₉ rapidly quenched from the melting temperature of 1050°C.     

Low temperature route to the multiferroic FeAlO3: XRD and Mössbauer characterizations

A low temperature route to the multiferroic FeAlO₃ is being reported. Four different synthesis routes namely, the solid-state route, the co-precipitation route, the sol–gel route and the high-energy ball-milling route have been carried out. The as-prepared samples of all the four routes have been subjected to sintering at 1,000°C. The sintered samples were characterized by X-ray diffraction, thermo-gravimetric analysis and Mössbauer spectroscopy. X-ray diffractograms revealed the coexistence of the orthorhombic multiferroic phase along with some unreacted impurities. Mössbauer spectroscopy confirmed the presence of the desired multiferroic phase along with residual hematite. A discussion on the various aspects of the different synthesis procedures is presented.     

Crystallization study of amorphous system Fe84−x W x B16 (x=3; 5)

A complex study of the crystallization process of the amorphous system was carried out by Mössbauer spectroscopy, X-ray diffraction and DTA. Alloy samples crystallized at 600 °C contain the following phases: α-Fe, Fe₃B and solid solution (Fe, W)₃B.     

Magnetic Study of Nanocrystalline Ferrites and the Effect of Swift Heavy Ion Irradiation

100 MeV Si⁺⁷ irradiation induced modifications in the structural and magnetic properties of Mg_0.95Mn_0.05Fe₂O₄ nanoparticles have been studied by using X-ray diffraction, Mössbauer spectroscopy and a SQUID magnetometer. The X-ray diffraction patterns indicate the presence of single-phase cubic spinel structure of the samples. The particle size was estimated from the broadened (311) X-ray diffraction peak using the well-known Scherrer equation. The milling process reduced the average particle size to the nanometer range. After irradiation a slight increase in the particle size was observed. With the room temperature Mössbauer spectroscopy, superparamagnetic relaxation effects were observed in the pristine as well as in the irradiated samples. No appreciable changes were observed in the room temperature Mössbauer spectra after ion irradiation. Mössbauer spectroscopy performed on a 12 h milled pristine sample (6 nm) confirmed the transition to a magnetically ordered state for temperatures less than 140 K. All the samples showed well-defined magnetic ordering at 5 K, whereas, at room temperature they were in a superparamagnetic state. From the magnetization studies performed on the irradiated samples, it was concluded that the saturation magnetization was enhanced. This was explained on the basis of SHI irradiation induced modifications in surface states of the nanoparticles.     

Characterization of Prussian blue and its thermal decomposition products

The mixed-valence state of Prussian blue and its thermal decomposition products has been studied by Mössbauer and infrared spectra, and X-ray powder diffraction and conductivity measurements. The⁵⁷Fe Mössbauer spectra have revealed that the coordination environment of Prussian blue is not changed by the heat-treatment at lower than 200°C while the flipping of the cyano ligands takes place when the heat-treatment temperature exceeds 250°C. The electrical conductivity of the Prussian blue samples heat-treated in vacuo at 300 and 350°C is higher than that of the samples heat-treated at lower temperatures. All the spectral measurements have demonstrated that a new mixed-valence state is produced in Prussian blue by thermally flipping the cyano ligand and quenching the flipped cyano ligand to liquid nitrogen temperature.     

High Temperature Transformation of Iron Minerals in Bauxite

Mössbauer hyperfine and X-ray diffraction studies have been carried on original bauxite sample and thermally treated (between 200 and 1200°C) samples. Mössbauer spectrum of original bauxite shows the dominance of hematite (α-Fe₂O₃) along with Al-substituted hematite and goethite (α-FeOOH) of fine particle size. Mössbauer parameters for thermally treated samples indicate transformation of almost all of iron oxy-hydroxides into hematite up to 350°C. Around 800°C the formation of a new iron phase, assigned as iron substituted mullite (3Al₂O₃ 2SiO₃), occurs and its intensity increases with the rise in temperature and reaches a saturation around 1200°C. Furthermore, our studies confirm that HCl leaching is quite effective for the iron oxide removal.     

Investigation of the transition of acrylate cation exchangers from solidlike into highly elastic state under hydration

The transition from solid-like (glassy) state into the (elastic) state with high intramolecular mobility under hydration was investigated for two⁵⁷Fe³⁺-substituted cation exchangers (SGK ?7 and SG ?1 m with acrylic and methacrylic nature of chains). The transition was observed as sharp change of Mössbauer lineshape (appearance of quasielastic components) and decrease of Mössbauer effect probability. The region of hydration degrees in which the transition occurs in both systems varies from 1 to 3–4 H₂O molecules per ?COOH group depending on cross-linkage concentration in a nonmonotonous way.     

Mössbauer investigation of Fe−Zr amorphous alloys hydrogenated after annealing

Transmission and conversion electron Mössbauer spectroscopy were used in order to get further information bout thé effect of hydrogenation in Fe₈₉Zr₁₁ amorphous alloys in connection with the observation that the magnetically split spectrum appearing at room temperature due to a moderate hydrogenation gradually collapses with increasing hydrogen content. The Mössbauer measurements were performed on differently hydrogenated amorphous samples aged at 150 °C 300 °C and 600 °C before the hydrogenation. The time dependence of hydrogenation has also been measured. From the changes of Mössbauer parameters depending on the annealing temperature as well as on the cathodic potential of hydrogenation (compared with the hydrogen concentration measured by gas chromatography) we can conclude that relaxation processes and structural changes, taking place in these amorphous alloys, influence the hydrogen uptake, already at relatively low temperatures.     

Observation of iron impurity diffusion in silicon under bending stress by Mössbauer spectroscopy

In the present work, the formation of the Al₇₀Cu₂₀Fe₁₀ icosahedral phase by mechanical alloying the elemental powders in a high-energy planetary mill was investigated by X-ray diffraction and Mössbauer spectroscopy. It was verified that the sample milled for 80 h produces an icosahedral phase besides Al(Cu, Fe) solid solution (β-phase) and Al₂Cu intermetallic phase. The Mössbauer spectrum for this sample was fitted with a distribution of quadrupole splitting, a doublet and a sextet, revealing the presence of the icosahedral phase, β-phase and α-Fe, respectively. This compound is not a good hydrogen storage. The results of the X-ray diffraction and Mössbauer spectroscopy of the sample milled for 40 h and annealed at 623°C for 16 h shows essentially single i-phase and tetragonal Al₇Cu₂ Fe phase.     

Mössbauer spectroscopical investigation of amorphous Fe–Y alloy ribbons prepared by melt spinning

Fe–Y amorphous alloy ribbons were prepared by the melt spinning method and characterized by X-ray diffraction, Mössbauer spectroscopy and inelastic neutron scattering. X-ray diffraction demonstrates that the Fe_0.7Y_0.3 ribbons are completely amorphous, whereas the Fe_0.3Y_0.7 ribbons contain a small fraction of crystalline Y precipitates in the amorphous Fe–Y matrix. Mössbauer spectroscopy between 4.2 to 300 K reveals the amorphous nature of the Fe–Y matrix and the Fe_0.7Y_0.3 ribbons. The preliminary neutron scattering results S(Q, ω) show excess low energy vibrational modes which gives rise to the so called “boson peak” in this amorphous material.     

Magnetic and Mössbauer spectroscopic studies of NiZn ferrite nanoparticles synthesized by a combustion method

The properties of nanocrystalline Ni_0.5Zn_0.5Fe₂O₄ synthesized by an auto-combustion method have been investigated by magnetic measurements and Mössbauer spectroscopy. The as-synthesized single phase nanosized ferrite powder is annealed at different temperatures in the range 673–1,273 K to obtain nanoparticles of different sizes. The powders are characterized by powder X-ray diffraction, vibrating sample magnetometer, transmission electron microscopy and Mössbauer spectroscopy. The as-synthesized powder with average particle size of ~9 nm is superparamagnetic. Magnetic transition temperature increases up to 665 K for the nanosized powder as compared to the transition temperature of 548 K for the bulk ferrite. This has been confirmed as due to the abnormal cation distribution, as evidenced from room temperature Mössbauer spectroscopic studies.     

Mössbauer study of phase transition caused by oxidation in the temperature region from 20 to 1000°C in almandine garnets

A Mössbauer investigation has been carried out on garnets from Měděnec and Kti? (Czech Republic). Changes of Fe²⁺ and Fe³⁺ crystallographic sites were observed in these silicate garnets after temperature processing under oxidising atmosphere. The temperature processes were realised from 200 to 1000°C by 100 degrees. The results of Mössbauer experiments are compared with the results of the chemical analysis, X-ray diffraction, infrared spectroscopy, and magnetic susceptibility measurements.     

Synthesis and properties of Fe-W powder

This paper deals with the study of Fe-W powders, which were prepared by electrolyzing, and their stabilized properties. The synthesized Fe-W powders were studied using X-ray diffraction, thermal analysis, Mössbauer spectroscopy and thermomagnetic measurement. A poor hydrogen absorption occurred. After the heat treatment at 800°C, the amorphous and α-Fe(W) phases were decomposed into prevailing α-Fe with approx. 3 at.% W accompanied by the W(Fe), λ-Fe₂W, and Fe²⁺ in oxide phases.     

Synthesis and characterization of hollow ��-Fe2O3 sub-micron spheres prepared by sol�Cgel

In this work we report the preparation of magnetic hematite hollow sub-micron spheres (??-Fe₂O₃) by colloidal suspensions of ferric nitrate nine-hydrate (Fe(NO₃)₃·9H₂O) particles in citric acid solution by following the sol?Cgel method. After the gel formation, the samples were annealed at different temperatures in an oxidizing atmosphere. Annealing at 180°C resulted in an amorphous phase, without iron oxide formation. Annealing at 250°C resulted in coexisting phases of hematite, maghemite and magnetite, whereas at 400°C, only hematite and maghemite were found. Pure hematite hollow sub-micron spheres with porous shells were formed after annealing at 600°C. The characterization was performed by X-ray diffraction (XRD), Mössbauer spectroscopy (MS) and scanning electron microscopy (SEM).     

Mössbauer studies on ultraporous Fe-Oxide/SiO2 aerogel

Magnetic aerogels with very low volume density of ～0.2 g/cm³ were prepared by sol-gel method and supercritical drying. The resulting materials were monolithic and displayed high surface area. By X-ray diffraction and Mössbauer spectroscopy the crystalline phase formed inside the mesopores of the SiO₂ matrix was identified as a spinel iron oxide. Comparison of the magnetic measurements with Mössbauer spectra at various temperatures contributed to the elucidation of the magnetic state of this nanocomposite system with restricted magnetic interactions, in particular its transition to a superparamagnetic state.     

Determination of inelastic critical scattering at a first order phase transition in the CDW structure of 1TTaS2 using Mössbauer diffraction

We report Mössbauer diffraction measurements of the temperature dependence of the elastic and inelastic intensities at the (100) Bragg reflection in 1TTaS₂. These measurements use a newly developed microfoil conversion electron (MICE) spectrometry. They cover the temperature range from 19°C to 100°C, bracketing the first order 1T₁ to 1T₂ phase transition in the charge density wave structure at 79°C. The elastic Bragg peak shows a discontinuity at the phase transition as reported by Moret and Colella. The inelastic scattering shows a significant peak near the phase transition. This peak is interpreted as inelastic critical scattering at this first order phase transition.     

Mössbauer study of illitic clays containing iron-rich impurities

The iron mineralogy of nineteen illitic clays from eastern Bavaria was studied by Mössbauer spectroscopy and X-ray powder diffraction. Mössbauer spectra of the <2 μm fraction were taken at RT, 120 K and 4.2 K. The clays contain both paramagnetic Fe³⁺ and Fe²⁺. Superparamagnetic oxides are frequently present. The Fe²⁺ quadrupole splitting and the ratio of Fe³⁺ at 4.2 K to Fe²⁺ at 120 K are correlated and define two groups distinguished by their mineral content. The samples were heated systematically for 48 h up to 1250°C in steps of 50°C. One clay which is rich in chlorite and Fe(II) was studied in detail after firing in air and following a reduction for 3 h at 800°C with charcoal. The transformations of the mineral phases with temperature as shown by X-ray diffraction are also evident in the Mössbauer spectra.     

Mössbauer study of SnO<Subscript>2</Subscript> powders doped with dilute <Superscript>57</Superscript>Fe prepared by a sol-gel method

SnO₂ powders, doped with various ⁵⁷Fe contents were prepared by a sol-gel method, and annealed finally at 500 °C and 650 °C. These samples were characterized by Mössbauer spectroscopy, vibrating sample magnetometer (VSM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) to investigate the relationship of magnetic properties, grain sizes, annealing temperatures and Mössbauer parameters. The particle sizes of SnO₂ powders reduced to less than 100 nm with the increase of Fe contents up to 5%. Rutile SnO₂ was the only phase obtained for all samples. Room temperature Mössbauer spectra suggest the presence of two different paramagnetic iron sites for all samples and one magnetically relaxed species for those samples with the lowest iron concentrations. The magnetization increased with the Fe content, but was reduced for the samples annealed at 650 °C perhaps due to a segregation of α-Fe₂O₃ doped with tin.     

Structural transformation from amorphous to hexagonal tungsten bronze FeF3, studied by Mössabauer spectroscopy

By annealing vapor-deposited amorphous FeF₃ at 180°C for 2 days, a crystalline phase has been obtained; it was identified by Mössbauer spectroscopy as hexagonal tungsten bronze FeF₃. the structural quality of this phase was good enough to provide the hyperfine parameters and notably the electric field gradient tensor. The role of Fe²⁺ impurities in this new synthesis method is discussed.     

Mössbauer study of MnZn ferrite formation under low oxygen pressure conditions

The solid state reaction of ferrite formation was studied under low oxygen partial pressure (10^-3% O₂) by Mössbauer spectroscopy, X-ray diffraction and density measurements. At temperatures below 500°C, a Mn ferrite was formed, whereas the Zn ferrite formation starts later at the same temperature than in air sintering. The appearance of an intermediate Mn ferrite is caused by the low valency state of Mn ions under such strong reducing conditions. The MnZn ferrite formation is completed by 800°C.No direct correlation was found between ferrite formation and densification.