Structural Behavior of Pt–Sn Supported on MgO

Pt–Sn supported on magnesia and alumina were characterized, before and after treatment with hydrogen, by Mössbauer spectroscopy and X-ray diffraction. For the calcined samples on both supports tin is present as SnO₂ and platinum as metal. After reduction with hydrogen, platinum and tin diffuse into the magnesia lattice to form a solid solution. On alumina Sn(IV), Sn(II), Sn(0), Pt, Pt₃Sn, PtSn and PtSn₂ alloys are formed. The SnO interacts strongly with the alumina support. The catalytic activity of both Pt–Sn catalysts is strongly affected by the support. On alumina the dehydrogenation of cyclohexane is very high, whereas that on magnesia is almost non-active.     

Hyperfine Field and Isomer Shift Evolution in Hydrogenated Nd–Fe–B Alloy

RE₂Fe₁₄B (RE=rare earth) materials are capable of absorbing hydrogen to form a stable solid solution at room temperature. Hydrogenation produces a number of significant changes in the hyperfine interactions. In this work, ⁵⁷Fe Mössbauer effect spectroscopy and X-ray diffraction measurements were performed on Nd_14.01Hf_0.08Fe_78.91B_7.00 alloys submitted to thermal treatment in hydrogen atmosphere. A non-linear increase of the hyperfine fields and isomer shifts with hydrogen concentration was observed. The hyperfine parameters of the 8j₁ site exhibit a rather different evolution than those experienced by the other major sites (8j₂, 16k₁, 16k₂). The origin of the hyperfine field enhancement is analyzed in terms of volume expansion and H nearest neighbors to the Fe sites. A linear expression on these two effects to give account of isomer shift evolution for 8j₁ site is given.     

Phase Composition and Properties of Iron Nanocrystals and Clusters Embedded in MgO Matrix

Formation and stability of highly dispersed iron particles in crazed porous polymer matrices were studied. The iron–polymer composites obtained were characterized by different morphologies and dimensions of iron particles. The phase content of the iron constituent in a composite studied by Mössbauer spectroscopy was shown to depend on the type of the iron salt and the method of introduction of the initial reagents into a polymer.     

Structural transformations in lithiated Mn2Sb electrodes probed by Mössbauer spectroscopy and X-ray diffraction

The electrochemical reactions of Li with Mn₂Sb and small amounts of MnSb have been investigated by Mössbauer spectroscopy and X-ray diffraction. The lithiation transforms initially the MnSb part and later the Mn₂Sb part into amorphous LiMnSb. On further lithiation the LiMnSb phase transforms to rather well crystallized Li₃Sb. In the X-ray diffraction pattern for the fully lithiated sample extruded nanocrystalline Mn is seen for the first time in these type of electrodes. The first delithiation of Li₃Sb results in a mixture of three phases Mn₂Sb, MnSb and LiMnSb. After several cycles (charges and discharges) of the electrode the Mn₂Sb phase disappears completely.     

Phase coexistence in mechanicallly alloyed iron–manganese powders

Iron–manganese alloys with Mn concentration of 6–30 at.% were prepared by mechanical alloying. Structure and phase composition of the samples were investigated by X-ray diffraction and Mössbauer spectroscopy. Mechanical alloying results in the formation of b.c.c. and f.c.c. solid solutions with high concentration of structure defects and refined grains. A single b.c.c. phase was observed in Fe–Mn alloys a Mn concentration less than ≤8 at.% and, for higher Mn contents, a mixture of b.c.c. and f.c.c. phases was observed. The main features of phase composition of as-prepared alloys consisted in significant widening of single phase concentration range.     

NMR and Mössbauer Study of Al2O3–Eu2O3

Alumina–europia mixed oxides with 5 and 10 wt.% Eu₂O₃ were studied by Mössbauer spectroscopy, ²⁷Al MAS-NMR and X-ray diffraction (XRD). The samples were prepared by the sol–gel technique. The XRD patterns for the calcined samples show a broad peak around 2θ = 30° which is assigned to the Eu₂O₃; after treatment with hydrogen at 1073 K no reduction to Eu⁺² or Eu⁰ was observed. The NMR spectra show three peaks, which are assigned to the octahedral, pentahedral and tetrahedral aluminum sites; the intensity of each peak depends on the concentration of europium ions. The Mössbauer spectra of the calcined samples show a single peak near zero velocity which is attributed to the Eu⁺³; after H₂ treatment at 1073 K similar spectra were obtained, suggesting Eu⁺³ is not reducibly at this temperature.     

Phase Transitions of Ruthenium-Doped Iron Oxide Studied by 57Fe Mössbauer Spectroscopy at Elevated Temperatures

Hyperfine Interactions - The 57Fe Mössbauer spectra recorded in situ from 5% ruthenium-doped maghemite show parameters typical for maghemite up to 600 K and a hyperfine field distribution...     

Phase transformation of strontium hexagonal ferrite

The phase transformation of strontium hexagonal ferrite (SrFe₁₂O₁₉) to magnetite (Fe₃O₄) as main phase and strontium carbonate (SrCO₃) as secondary phase is reported here. SrFe₁₂O₁₉ powder was obtained by a heat treatment at 250 °C under controlled oxygen flow. It was observed that the phase transformation occurred when the SrFe₁₂O₁₉ ferrite was heated up to 625 °C in confinement conditions. This transformation took place by a combination of three factors: the presence of stresses in the crystal lattice of SrFe₁₂O₁₉ due to a low synthesis temperature, the reduction of Fe³⁺ to Fe²⁺ during the heating up to 625 °C, and the similarity of the coordination spheres of the iron atoms present in the S-block of SrFe₁₂O₁₉ and Fe₃O₄. X-ray diffraction analysis confirmed the existence of strain and crystal deformation in SrFe₁₂O₁₉ and the absence of them in the material after the phase transformation. Dispersive X-ray absorption spectroscopy and Fe⁵⁷ Mössbauer spectroscopy provided evidences of the reduction of Fe³⁺ to Fe²⁺ in the SrFe₁₂O₁₉ crystal.     

Comparative Study by MS and XRD of Fe50Al50 Alloys Produced by Mechanical Alloying, Using Different Ball Mills

In this work we report a comparative study of the magnetic and structural properties of Fe₅₀Al₅₀ alloys produced by mechanical alloying using two different planetary ball mills with the same ball mass to powder mass relation. The Fe₅₀Al₅₀ sample milled during 48 h using the Fritsch planetary ball mill pulverisette 5 and balls of 20 mm, presents only a bcc alloy phase with a majority of paramagnetic sites, whereas that sample milled during the same time using the Fritsch planetary ball mill pulverisette 7 with balls of 15 mm, presents a bcc alloy phase with paramagnetic site (doublet) and a majority of ferromagnetic sites which include pure Fe. However for 72 h of milling this sample presents a bcc paramagnetic phase, very similar to that prepared with the first system during 48 h. These results show that the conditions used in the first ball mill equipment make more efficient the milling process.     

Lattice Dynamics and Inelastic Nuclear Resonant X-Ray Scattering

Measurement of thermal and elastic properties of materials, like phonon density of states, specific heat or speed of sound, by a new X-ray scattering technique is presented. Inelastic nuclear resonant scattering of X-rays produced from new electron storage rings, coupled with advances in high-energy-resolution crystal optics and fast detectors has enabled the development of a new method of analyzing the energy loss in a scattering process with a resolution of 10⁷ or better in the X-ray region of 6–30 keV. Some unique aspects like element (isotope) selectivity, the amount of material needed for analysis (nanograms) and physical size that X-rays can be focused (5 micrometer or better) favors this approach over more established techniques of neutron scattering, Mössbauer, and Raman spectroscopy. Applications to several unique cases (e.g., multilayers and high pressure) are discussed.     

Structural and magnetic phase formation in nanophase brass—iron electron compounds

Starting with Cu0.65Zn0.35 with an e/a ratio of 1.35 we studied the phase formation in nanophase (Cu_0.65Zn_0.35)_1?x Fe _x alloys in the concentration range 0.1 ≤x ≤0.7 to see the effect of altering the electron concentration. The evolution of bcc phase from the fcc phase as a function of Fe concentration was investigated by Mössbauer spectroscopy and X-ray diffraction. The grain size, lattice parameters, and average hyperfine magnetic field distributions were estimated for the nanophase alloys. The fcc phase was observed to persist up to 40 atomic per cent Fe substitutions, a mixed (fcc + bcc) phase region up to 70 atomic per cent Fe and bcc phase beyond 70 atomic per cent Fe. The magnetic state of the alloys changed from nonmagnetic forx ≤0.3 to magnetically ordered state at room temperature forx ≤0.33, which lies in the fcc phase region. The fcc phase alloys of Fe with non-magnetic metals have very low magnetic transition temperatures. However, in this system the room temperature state is unusually magnetic     

Mössbauer and X-Ray Diffraction Investigations of a Series of B-Doped Ferrihydrites

X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy are used to characterize the influence of borate on two-line ferrihydrite's structure and develop likely models for its attachment. Particle sizes were in the 2–4 nm range, and as borate sorption increased, the ferrihydrite particle size decreased. The d-spacings of two-line ferrihydrite increased with increased borate adsorption. Isomer shift and quadrupole splitting exhibit slight increasing trends as well. Also, the phase transformation temperature of ferrihydrite to hematite is significantly raised due to borate coating of the surface. We suggest borate is sorbed onto the surface by attachment to the oxygen corners of the iron octahedra that are on the surface of the nanoparticles, placing boron in a tetrahedral molecular geometry.     

X-ray and Mössbauer study of structural changes in K3Na(FeO4)2

Mixed potassium–sodium ferrate(VI), K₃Na(FeO₄)₂, has been synthesized by precipitation from alkaline solution. At room temperature it decomposes spontaneously giving Fe(III) compounds and ferrate(VI) with a structure similar to that of K₂FeO₄, which is confirmed by X-ray diffraction and Mössbauer spectroscopy.     

Effect of Precipitation Temperature and Number of Iron Atoms per Molecule on the Structure of Hydrated Iron(III) Oxyhydroxide Ferritin Cores Synthesised in vitro

Hyperfine Interactions - Horse spleen apoferritin was reconstituted with varying numbers of iron atoms per protein shell at 25 and 50°C. Samples with mean particle sizes ranging from...     

Temperature dependent Mössbauer and neutron diffraction studies of Cu x Fe1−x Cr2S4 compounds

The cation distribution and magnetic structure of Cu _x Fe_1?x Cr₂S₄ (x?=?0.1, 0.2, 0.3, 0.4, and 0.5) has been studied by X-ray and neutron diffraction, vibrating sample magnetometer (VSM), and Mössbauer spectroscopy. The charge state of Fe is found to be ferrous (Fe²⁺) for the x?=?0.1 sample; ferric (Fe³⁺) for the x?=?0.5 sample; mixed state (Fe²⁺, Fe³⁺) for the x?=?0.2, 0.3, and 0.4 samples. The Mössbauer spectra of the x?=?0.1 sample show asymmetric line broadening, which is considered to be due to the Jahn–Teller effect of Cu²⁺ ions, and a symmetrical six-line pattern is shown for the x?=?0.5 sample. The valence state of the Cu ions for the x?=?0.1 and 0.5 samples is found to be divalent and monovalent, respectively. The magnetic structure of the samples was determined to be a ferrimagnetic structure with antiparallel alignment of the Fe and Cr ion magnetic moments.     

Phase Composition of Oxidised Layers Grown on Steel Exposed to Liquid Lead at 749 K

The corrosion resistance of AISI 316 LN stainless steel was studied in contact with stagnant, oxygen-saturated liquid lead at 749 K for times up to 1200 h. The reaction products were analysed by means of optical and scanning electron microscopy, electron probe microanalysis, X-ray diffraction analysis and Mössbauer spectroscopy. Thin layers of largely variable thickness formed on the surface, mainly consisting of Fe₃O₄, with small amounts of FeO in the inner regions, Fe₂O₃ and an Fe-Pb-O ternary product in the outermost regions. The alloying elements Cr and Ni diffused to a different extent into Fe₃O₄. A mechanism is proposed to explain formation and growth of the reaction products.

     

Mössbauer study of nanodimensional nickel ferrite – mechanochemical synthesis and catalytic properties

Iron–nickel spinel oxide NiFe₂O₄ nanoparticles have been prepared by the combination of chemical precipitation and subsequent mechanical milling. For comparison, their analogue obtained by thermal synthesis is also studied. Phase composition and structural properties of iron–nickel oxides are investigated by X-ray diffraction and Mössbauer spectroscopy. Their catalytic behavior in methanol decomposition to CO and methane is tested. An influence of the preparation method on the reduction and catalytic properties of iron–nickel samples is established.     

Iron in Parkinson's Disease Revisited

Hyperfine Interactions - Mössbauer studies of fresh frozen samples taken at autopsy from different parts of the human brain (globus pallidus (GP), substantia nigra (NS), and hippocamp (Hip))...     

What Oxidation State of Iron Determines the Amethyst Colour?

A colourless quartz crystal doped with ⁵⁷Fe³⁺ was obtained by hydrothermal synthesis in an NH₄F solution. The crystal was transformed into violet amethyst by gamma-irradiation. The change in colour was accompanied by changes in the Mössbauer spectrum that can be interpreted as the conversion of trivalent iron into the tetravalent state: Fe³⁺→Fe⁴⁺.

     

High-Energy Ball Milling of Some Intermetallics

High-energy ball milling of metallic powders has been used in recent years for the synthesis of alloys through reactions mainly occurring in solid state. The diffusive phenomena accompanying and promoting the reactions of formation are related to the microstructure acquired by the powders as a consequence of the intense mechanical deformations. The process induces a remarkable comminution of powder particles, inside of which domains of nanometric size are formed and compositional variations often occur. Several analytical techniques are suitable for following the structural evolution of the powders during milling. Among them, Mössbauer spectroscopy is suitable for obtaining detailed local information on the atomic arrangement of the treated materials, if one of the constituents is a Mössbauer isotope, and for detecting little changes occurring at an atomic scale. For these reasons Mössbauer spectroscopy is more sensitive than other analytical techniques especially in the early stages of the process. Some recent results are presented regarding in particular the Fe–Cu, Fe–Al, Fe–Al–Cu, NiAl(Fe) and Fe–Mn systems.