Oxidation of iron studied by PAC and CEMS

A metallic Fe specimen, implanted with ¹¹¹In, was oxidized and subsequently annealed in a high vacuum for PAC spectroscopy. This treatment gave rise to a huge PAC signal. The magnitude of the hyperfine field was found to be one third of that in metallic Fe. CEMS on an enriched Fe foil given exactly the same treatment has revealed that a maghemite phase (γ-Fe₂O₃) is formed right after the oxidation treatment and a magnetite phase (Fe₃O₄) after the vacuum annealing. ¹¹¹In in the magnetite phase was found to give rise to a PAC signal with large amplitude. PAC spectroscopy in an external magnetic field has revealed that the site of ¹¹¹In is the tetrahedral site of the magnetite with the hyperfine field of +12 T, which is in excellent agreement with those in the ferrites. The present method of oxidation of metallic Fe with nuclear probes in it is quite useful for the study of oxidation processes. Also, it provides us with a simple means to prepare ferrite specimens incorporated with nuclear probes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Vacancy ordering in γ-Fe2O3 nanocrystals observed by Fe NMR

The identification by ⁵⁷Fe internal field nuclear magnetic resonance (NMR) of hyperfine fields at four Fe sites in the (average) tetragonal unit cell of vacancy-ordered γ-Fe₂O₃ (maghemite) is reported. The effects of vacancy redistribution due to annealing the partially vacancy-ordered form has been observed in the ⁵⁷Fe lineshape. In addition, the reduction of the particle size of the vacancy-ordered form has been observed to gradually eliminate the vacancy ordering and then to cause a transition from ferrimagnetism to superparamagnetism.     

Mössbauer studies of magnetite and Al-substituted maghemites

This paper reviews a systematic Mössbauer study of maghemite γ-Fe₂O₃ and Al-substituted maghemites γ-(Fe_1?y Al _y )₂O₃. Three series of samples prepared from different methods and having different morphological characteristics and aluminum contents were investigated. It was found that both the cation distribution and the solubility limit depend on the preparation method, and no general conclusion in that respect could be inferred. From the temperature dependence of the hyperfine fields the exchange integrals could be calculated, and were found as: J _AB= ?25 K, J_AA= ?18 K, and J_BB= ?3 K. The hyperfine fields show a crossing in the vicinity of 300 K, for both substituted and unsubstituted samples, as a result of the relatively strong A–A interaction. The Curie temperatures were found to be in the range of 948–730 K, the lower value referring to the sample containing 22 mole% Al. The influence of maghemite on the Mössbauer spectra (MS) of magnetite was explored in some detail. It was demonstrated on the basis of the MS recorded for a variety of reference mixtures, that it is not possible to resolve the ferric A-site components due to maghemite and magnetite, even with the absorber subjected to a strong external field.     

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).     

Characterization of Iron Oxides Commonly Formed as Corrosion Products on Steel

For fundamental studies of the atmospheric corrosion of steel, it is useful to identify the iron oxide phases present in rust layers. The nine iron oxide phases, iron hydroxide (Fe(OH)₂), iron trihydroxide (Fe(OH)₃), goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH), feroxyhite (δ-FeOOH), hematite (α-Fe₂O₃), maghemite (γ-Fe₂O₃) and magnetite (Fe₃O₄) are among those which have been reported to be present in the corrosion coatings on steel. Each iron oxide phase is uniquely characterized by different hyperfine parameters from M?ssbauer analysis, at temperatures of 300K, 77K and 4K. Many of these oxide phases can also be identified by use of Raman spectroscopy. The relative fraction of each iron oxide can be accurately determined from the M?ssbauer subspectral area and recoil-free fraction of each phase. The different M?ssbauer geometries also provide some depth dependent phase identification for corrosion layers present on the steel substrate. Micro-Raman spectroscopy can be used to uniquely identify each iron oxide phase to a high spatial resolution of about 1 μm. This revised version was published online in August 2006 with corrections to the Cover Date.     

Superparamagnetic properties of <Emphasis Type="Italic">γ</Emphasis>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> particles: Mössbauer spectroscopy and d.c. magnetic measurements

Particles of Fe oxide were prepared by chemical coprecipitation and their sizes were shown by TEM and confirmed by XRD to be in the range of 5 nm. The Mössbauer spectra at 120K clearly indicated the absence of magnetite and presence of the maghemite (γ-Fe₂O₃) phase.We studied the transition of the system to superparamagnetic behaviour, which strongly depends on the relevant time window amounting to ～ 10^?7 s for Mössbauer spectroscopy of ⁵⁷Fe and units of seconds for d.c. magnetic measurements. From the temperature dependences of magnetic moments of zero-field-cooled (ZFC)) and field-cooled (FC) samples, the distributions of blocking temperatures were determined. The comparison of the transition temperatures derived from these two types of measurements gave an independent estimate for the pre-exponential factor and the energy barrier and thus magnetocrystalline anisotropy in an order-of-magnitude agreement with the published data for bulk γ-Fe₂O₃.     

High temperature Mössbauer spectroscopy of titanomagnetite and maghemite in basalts

Mössbauer spectroscopy of basalt lava samples, exhibiting reversible thermal magnetization (J _s -T) curves with Curie temperatures of about 840 K, has revealed considerable amounts of maghemite (-Fe₂O₃) in many samples. In view of the expected instability of maghemite at temperatures above 620 K, this reversibility came as a surprise. For further studies of the magnetization-temperature relationship of these minerals, we have constructed an elliptical radiation-heated furnace in which Mössbauer spectra can be acquired at temperatures between 300 and 900 K. Measurements at different temperatures have been obtained for two types of basalts, one in which the magnetic minerals are nearly pure magnetite and the other where the room temperature spectrum indicates a mixture of maghemite and magnetite. The two series show different features of the collapse of the internal magnetic hyperfine field, and the composition of minerals in the samples changes during the treatment, showing maghemite.     

Local structural order in nanostructured hematite

Nanostructured α-Fe₂O₃ powders were prepared by high-energy ball milling. The milling process spans grinding times from 30 min to 24 h. The as-milled samples were characterized by means of ⁵⁷Fe Mössbauer spectrometry, Rietveld analysis of X-ray diffraction data and particle size analysis. The obtained results evidence the presence of disordered hematite characterized by a hyperfine field distribution with a well-behaved dependence on the mean crystallite size for which the mean hyperfine field decreases asymptotically as the grain size decreases. A new relationship is proposed in order to describe such behavior. Finally the presence of superparamagnetic grains, the occurrence of a partial topotactic phase transformation into a spinel phase and tool induced contamination are also presented and discussed.     

57Fe Mössbauer study of Fe nitrides

Summary Magnetic properties of Fe nitrides have been re-examined by⁵⁷Fe M?ssbauer spectroscopy. Hyperfine magnetic fields for α″-Fe₁₆N₂ are 30, 31 and 39T at 298K, but the averaged hyperfine field is 33T and nearly equal to the value of pure α-Fe. σ-Fe₂ N is an antiferromagnet below 9K having a small magnetic moment less than 0.1 μ_B, although γ′-Fe₄N and ε-Fe_3–2N are ferromagnets. ZnS-type FeN is non-magnetic at 4.2K. M?ssbauer spectra obtained from NaCl-type FeN are complex and some Fe atoms in this nitride show a surprisingly large hyperfine magnetic field of 49T. Paper presented at the ICAME-95, Rimini, 10–16 September 1995.     

Synthesis and characterization of indium oxide-hematite magnetic ceramic solid solution

Indium oxide-doped hematite xIn₂O_3*(1-x)??-Fe₂O₃ (molar concentration x = 0.1?C0.7) solid solutions were synthesized using mechanochemical activation by ball milling. XRD patterns yield the dependence of lattice parameters and grain size as function of milling time. After 12 h of milling, the completion of In^3?+? substitution of Fe^3?+? in hematite lattice occurs for x = 0.1. For x = 0.3, 0.5 and 0.7, the substitutions between In^3?+? and Fe^3?+? into hematite and respectively, In₂O₃ lattices occur simultaneously. The lattice parameters of ??-Fe₂O₃ (a and c) and In₂O₃ (a) vary with milling time. For x = 0.1, Mössbauer spectra were fitted with one, two, or three sextets versus milling time, corresponding to gradual substitution of In^3?+? for Fe^3?+? in hematite lattice. For x = 0.3, Mössbauer spectra after milling were fitted with three sextets and two quadrupole-split doublets, representing In^3?+? substitution of Fe^3?+? in hematite lattice and Fe^3?+? substitution of In^3?+? in two different sites of In₂O₃ lattice. For x = 0.5 and 0.7, Mössbauer spectra fitting required two sextets and one quadrupole-split doublet, representing coexistence of In^3?+? substitution of Fe^3?+? in hematite lattice and Fe^3?+? substitution of In^3?+? in indium oxide lattice. The recoilless fraction studied versus milling time for each molar concentration exhibited low values, consistent with the occurrence of nanoparticles in the system. SEM/EDS measurements revealed that the mechanochemical activation by ball milling produced xIn₂O_3*(1-x)??-Fe₂O₃ solid solution system with a wide range of particle size distribution, from nanometer to micrometer, but with a uniform distribution of Fe, In, and O elements.     

Mössbauer spectroscopic properties of tin-doped iron oxides

The ⁵⁷Fe Mössbauer spectra recorded in situ from tin-doped Fe₃O₄ at elevated temperature in vacuo shows the Curie temperature to decrease with increasing concentrations of the dopant. Thermal treatment under oxidising conditions results in the initial formation of tin-doped γ-Fe₂O₃ which subsequently undergoes a phase transformation to tin-doped α-Fe₂O₃. ⁵⁷Fe Mössbauer spectroscopy at elevated temperatures shows the Néel temperature for tin-doped γ-Fe₂O₃ to be lower than that of pure γ-Fe₂O₃. The ¹¹⁹Sn Mössbauer spectra recorded from all the tin-doped iron oxides show the presence of a hyperfine magnetic field at the Sn⁴⁺ site which is more complex in the spectra recorded from tin-doped γ-Fe₂O₃ and α-Fe₂O₃.     

Formation of iron oxides in soils developed under natural fires and slash-and-burn based agriculture in a monsoonal climate (Chittagong Hill Tracts, Bangladesh)

Fire-induced mineral transformations have been investigated in composite mineral grains separated from the coarse sand fractions (400–2,000 μm) from Ultisols developed in the Chittagong Hill Tracts (Bangladesh). Magnetic and colour based separation (into light brown, dark red, and magnetic, dark red classes) were used to select the grains that were studied by Mössbauer spectroscopy. Aluminium substituted goethite (α-FeOOH) dominates the light brown particles. Fire transform the goethite into a poorly crystalline hematite (α-Fe₂O₃) dominating in the dark red particles. In the dark red, magnetic grains a recrystallized hematite dominates, but small amounts of maghemite (γ-Fe₂O₃) are also present. The latter is indicated by comparing the line intensities in spectra measurement with and without an external magnetic field.     

Monitoring by Mössbauer spectroscopy the thermal reduction of hematite into magnetite: the surface effect and charge disproportionality in iron oxide nanoparticles

Nanoparticles of magnetite Fe₃O₄ were synthesized by thermal reduction of hematite α-Fe₂O₃ powder in the presence of high boiling point solvent. The structural transformations and magnetic properties of the obtained nanoparticles were investigated by the ⁵⁷Fe Mössbauer spectroscopy, X-ray diffraction, and magnetic measurements. The content of hematite and magnetite phases was evaluated at each step of the chemical and thermal treatment of the product. An increase of saturation magnetization with the reaction time correlates with an increase of concentration of magnetite in the samples. The electron hoping between Fe^2?+? and Fe^3?+? ions in the octahedral sites of the magnetite nanoparticles and Verwey phase transition were investigated. It was established that not all iron ions in the octahedral sites participated in electron hoping Fe^2?+????Fe^3?+? above the Verwey temperature T _V, and the charge distribution could be expressed as $\big( {{\rm Fe}^{3+}}\big)_{{\rm tet}} \big[ {{\rm Fe}_{1.85}^{2.5+} {\rm Fe}_{0.15}^{3+} }\big]_{{\rm oct}} {\rm O}_4$ .     

On the transferred hyperfine interaction in substituted yig

NMR of⁵⁷Fe is studied in a number of (M_xY_3–x) Fe₅O₁₂ garnets for small concentrations of M (M is either trivalent RE ion –Ho 3+, Gd 3+, Nd3+, Pr 3+, La 3+ or Bi 3+ ion). Beside the main resonance lines, the satellites were observed, which correspond to those Fe, in vicinity of which the impurity M is located. After correcting for the dipolar field, the field corresponding to the change of the transferred hyperfine interaction in M³⁺–O^2–-Fe³⁺ vs. Y³⁺–O^2–-Fe³⁺ triad was deduced from the satellites splitting. The analysis of the results indicates that the observed change in the transferred hyperfine field is mainly connected with the transfer of electrons between M³⁺ and Fe³⁺ ions and not with the local deformation around the impurity.     

Thermally Driven and Ball-Milled Hematite to Magnetite Transformation

In this work, a study on the dynamics of transformation from hematite (α-Fe₂O₃) to magnetite (Fe₃O₄) by following two solid-state reaction methods is carried out. One of the procedures consists of a thermal treatment under a 20% H₂ and 80% N₂ atmosphere at 375°C, whereas the second method involves a planetary ball mill to induce the transformation. The phases evolution as a function of the thermal treatment time ranging from 0 up to 25 min every 2.5 min, and from 0 up to 6 hours every hour in the case of the milling method, was followed by using room-temperature Mössbauer spectroscopy and X-ray diffraction analysis. Results evidence a well-behaved structural transformation for which highly stoichiometric Fe₃O₄ as a single phase was obtained for treatment times above 12.5 min in the case of the thermally treated samples. Differently from this a less stoichiometric magnetite characterized by a distribution of hyperfine fields for milling times above 3 hours in the case of the ball milled samples was obtained. For reaction times below 12.5 min, two interpretation models based on the presence of an anion-deficient magnetite Fe₃O_4?δ and the presence of maghemite accounting for the intermediate states during the thermal transformation are also presented and discussed.

     

A study of Fe--N alloy systems

Magnetic properties of Fe nitrides have been reexamined by ⁵⁷Fe Mössbauer spectroscopy. Hyperfine magnetic fields for α″-Fe₁₆N₂, γ′-Fe₄N, ε-Fe₃N, ζ-Fe₂N, NaCl-type FeN and ZnS-type FeN have been determined at various temperatures. Although α′-, γ″-, and ε-nitride are all ferromagnets, ζ-Fe₂N is found to be an antiferromagnet below 9 K and ZnS-type FeN is non-magnetic at 4.2 K. Contrary to the ZnS-type FeN, the NaCl-type FeN is an antiferromagnet and shows a component with a surprisingly large hyperfine magnetic field of 49 T.     

Visible light activated photo-catalytic effect and local structure of iron silicate glass prepared by sol-gel method

A relationship between methylene blue (MB) decomposition ability under visible light and local structure of xFe₂O₃·(100-x)SiO₂ glass abbreviated as xFS prepared by sol-gel method was investigated by ⁵⁷Fe-Mössbauer spectroscopy, X-ray diffractometry (XRD) and ultraviolet-visible light absorption spectroscopy (UV-Vis). Mössbauer spectra of xFS glass with x of 10, 30 and 50 annealed at 1000 °C for 3 h were mainly composed of a paramagnetic doublet due to fayalite (Fe₂SiO₄), and magnetic sextets due to magnetite (Fe₃O₄) or hematite (α-Fe₂O₃). The absorption area (A) of α-Fe₂O₃ gradually increased from 0.0 to 10.3 and 100 % with the increasing Fe₂O₃ content (x) of annealed xFS glass. A leaching test performed by 20 mL of MB aqueous solution and 40 mg of annealed 50FS glass showed that MB concentration decreased from 16.2 to 4.7 μmol L^?1 after 2 h with the first order rate constant of 1.8 × 10^?4 s^?1. These results prove that annealed iron silicate glass containing α-Fe₂O₃ can decompose MB effectively under visible light irradiation.     

Zero-field and in-field Mössbauer spectroscopy as a tool for structural and magnetic characterization of maghemite (γ-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>) nanoparticles

Nowadays, nanoparticles of maghemite (γ-Fe₂O₃) represent one of the most useful materials in modern advanced nanotechnological applications due to their superior magnetic properties. For their characterization,⁵⁷Fe zero-field and in-field Mössbauer spectroscopy have proved themselves to be very powerful and effective tools which are crucial for an investigation of the local surrounding of iron atoms and observation of dynamic effects. The structural and magnetic characteristics of maghemite and its nanoparticles are thus discussed with regard to their zero-field and in-field Mössbauer spectra recorded at various temperatures and applied external magnetic fields. In addition, a special attention is also devoted to remarkable physical phenomena (superparamagnetism, spin canting) occurring largely in maghemite nanosized particles.     

57Fe NMR spectra in domain walls of hexagonal ferrites with magnetoplumbite structure

The domain wall NMR spectra of⁵⁷Fe were measured on polycrystalline samples of BaFe₁₂O₁₉ and SrFe₁₂O₁₉ at 4·2 K. We have calculated the anisotropy of the hyperfine field dipolar component. The measured NMR spectra were interpreted supposing that the hyperfine field anisotropy is caused only by the dipolar field anisotropy.     

Preparation and characterization of surface modified γ-Fe2O3 (maghemite)-silica nanocomposites used for the purification of benzaldehyde lyase

γ-Fe₂O₃ (maghemite)-silica nanocomposite particles were synthesized using a sol-gel method. The condensation products of 3-glycidoxy propyltrimethoxy silane (GPTMS) and nitrilotriacetic acid (NTA) were introduced onto the surfaces of the γ-Fe₂O₃-silica nanocomposite particles and subsequently, these modified surfaces were complexed with cobalt (Co⁺²) metal ions. A possibility of using these surface modified γ-Fe₂O₃-silica particles for the purification of 6×histidine tagged recombinant benzaldehyde lyase (BAL, EC 4.1.2.38) based on magnetic separation was investigated. X-ray diffraction (XRD), thermal analysis, and vibrating sample magnetometry (VSM) methods were used to characterize the surface modified superparamagnetic γ-Fe₂O₃ (maghemite)-silica nanoparticles. XRD (Scherer's equation) results indicate that the primary particle size of maghemite was around 11 nm. Magnetic characterization results confirmed that the γ-Fe₂O₃ (maghemite)-silica nanoparticles were superparamagnetic. According to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) results, these superparamagnetic nanoparticles specifically capture 6×His-tagged BAL from crude extract of Escherichia coli (E. coli) BL21(DE3)pLysS/BAL_HIS. This study shows that the surface modified γ-Fe₂O₃ (maghemite)-silica nanoparticles are eligible for immobilized metal-ion affinity adsorption for histidine tagged recombinant proteins with its high capacity (3.16±0.4 mg/g) and selectivity.