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.     

Mössbauer spectroscopy study of mechanically alloyed Fe2O3–(Al, Co and WC) systems

Mössbauer spectroscopy revealed that a central hyperfine interaction doublet and an additional sextet characterized the appearance of new phases in the mechanically alloyed Fe₂O₃–Al and Fe₂O₃–Co systems. In the Fe₂O₃–Al system, the intensity of the central super paramagnetic doublet which represents small particles of iron, increased with increasing milling time from 5 to 30 h of mechanical alloying. The magnetic sextet characterizing hematite vanished in the room temperature Mössbauer spectra of samples produced after 25 h of mechanically alloying the 50% Fe₂O₃ and 50% Al system. In general XRD peak broadening was observed as a result of extensive material structural distortion and formation of small particles. Fe, Al₂O₃ and mixed iron–aluminium oxide phases were identified in the XRD patterns with a small persistence of the iron oxide up to 20 h of mechanically alloying the 1:1 system Al–Fe₂O₃. In the 50% Co–50% Fe₂O₃ system, a 55% abundant new phase CoFe₂O₄ was observed, from the Mössbauer spectra of the system. The presence of this new phase was confirmed by the XRD analysis. The high energy ball milling of WC–Fe₂O₃ revealed a more effective grinding compared to hematite alone. The hematite particles were reduced to nanosized particles.     

Mössbauer and optical investigation of Co3−x Fe x O4 thin films grown by sol–gel process

Structural transformation and the related variation in magnetic and optical properties of Co_3?x Fe _x O₄ thin films grown by a sol–gel method have been investigated as the Fe composition varies up to x?=?2. The normal spinel phase is dominant below x?=?0.55 and the inverse spinel phase grows as x increases further. Conversion electron Mössbauer spectroscopy (CEMS) measurements indicate that the normal spinel phase have octahedral Fe³⁺ ions mostly while the inverse spinel phase contain octahedral Fe²⁺ and tetrahedral Fe³⁺ ions. For higher Fe composition (x?>?1.22), Co²⁺ ions are found to substitute the octahedral Fe²⁺ sites. The measured optical absorption spectra for the Co_3?x Fe _x O₄ films by spectroscopic ellipsometry support the CEMS interpretation.     

The effect of the sputtering process ambient on the magnetic state and phase composition of the film nanocomposites (Fe0.45Co0.45Zr0.10) x (Al2O3)1−x

The effect of oxygen-containing ambient arising at sputtering of granular nanocomposites (Fe_0.45Co_0.45Zr_0.10) _x (Al₂O₃)_1−x (30 at.%≤x≤65 at.%) on their magnetic state and phase composition has been investigated. It was shown that the presence of oxygen resulted in the formation of oxide shells preventing the ferromagnetic interaction between Co_0.45Fe_0.45Zr_0.10 nanoparticles and also the formation of metallic percolative net beyond the percolation threshold (as opposed to the films prepared in pure argon atmosphere).     

Magnetic Relaxation in (Zn,Cd) x Fe1.7−x Co0.9Ti0.4O4 Spinel Oxides

Hyperfine Interactions - We report results of Mössbauer and X-ray diffraction measurements on powdered samples of (Zn, Cd) x Fe1.7?x Co0.9Ti0.4O4 spinel oxides (where x=0 to 1.0)...     

Characterisation of the reduction properties of La0.5Sr0.5Fe0.5Co0.5O3

We report here on the characterisation by temperature programmed reduction, ⁵⁷Fe Mössbauer spectroscopy and X-ray absorption spectroscopy of the phases resulting from treatment of the perovskite-related material La_0.5Sr_0.5Fe_0.5Co_0.5O₃ in a flowing 90% hydrogen/10% nitrogen atmosphere. The results show that treatment of La_0.5Sr_0.5Fe_0.5Co_0.5O₃ (which contains approximately 50% Fe⁴⁺ and 50% Fe³⁺) in the flowing 90% hydrogen/10% nitrogen atmosphere at 600°C does not result in the reduction of any of the constituent elements of the material and that the perovskite structure is still retained. The Mössbauer spectrum recorded following heating in the gaseous reducing environment at 1,000°C shows the presence of metallic iron, an Fe³⁺-containing phase with parameters compatible with the presence of SrLaFeO₄ which has a K₂NiF₄-type structure, and a paramagnetic Fe³⁺ phase. The X-ray absorption spectroscopy results show the presence of metallic cobalt. The Mössbauer spectrum recorded following heating at 1,200°C continues to show the Fe³⁺-containing components plus a larger contribution from metallic iron. The X-ray absorption spectroscopy results show the presence of metallic cobalt, SrLaFeO₄, La₂O₃ and SrO.     

Characterisation of soil-oxide analogs by applied-field 57Fe Mössbauer spectroscopy

In this contribution, the effect of high external magnetic fields upon the Mössbauer spectra of aluminous α- and γ-Fe₂O₃, of aluminous α- and γ-FeOOH and of aluminous Fe₃O₄ is reviewed. It is shown that the shapes of these spectra are characteristic of these materials and also to some extent of their crystallinities and Al-for-Fe substitutions. Based upon this evaluation, the potential for application of external-field Mössbauer spectroscopy to soil-related analytical purposes is demonstrated for two soil samples. Limitations of the technique are discussed. Finally, some suggestions for further research in this field are indicated.     

A Mössbauer study of the silica-like structures of FePO4 and (Al0.67Fe0.33)PO4

Summary Samples of FePO₄ and (Al_0.67Fe_0.33)PO₄ have been investigated by M?ssbauer spectroscopy. The data obtained on quadruple splittings are in agreement with neutron scattering and XRD data on (Al, Fe)O₄ tetrahedra structures.     

Emission Mössbauer studies of 57Co2+ and 119Sb5+ adsorbed on γ-Fe2O3 and NiFe2O4 surfaces

Summary Surface chemical states of⁵⁷Co²⁺ and¹¹⁹Sb⁵⁺ ions adsorbed on γ-Fe₂O₃ and NiFe₂O₄ were studied in the presence of the aqueous phase using the magnetic interaction between the substrates and⁵⁷Fe²⁺ or¹¹⁹Sn⁴⁺ arising from the adsorbed species. Two chemical forms were observed for the⁵⁷Co²⁺ adsorbed on γ-Fe₂O₃; one giving a peak in the high-field region and another giving a broad distribution in the lower fields, which were attributed to⁵⁷Co²⁺ at surfaceB sites of the spinel structure of γ-Fe₂O₃ and⁵⁷Co²⁺ species weakly bonded to the substrate, respectively. In case of the NiFe₂O₄ substrate, the latter species was dominant and was converted to the former by heating. Most of the¹¹⁹Sb⁵⁺ ions adsorbed on the γ-Fe₂O₃ and NiFe₂O₄ particles were found to have a certain magnetic interaction with the magnetically ordered ions of the substrates. Paper presented at the ICAME-95, Rimini, 10–16, September 1995     

Antiferromagnetism in double-layered perowskitic oxides SrLa2Fe2O7 and BaLa2Fe2O7

Il Nuovo Cimento D - Two double-layered perowskitic compounds have been investigated by XRD analysis and Mössbauer spectroscopy. The experimental data allow the conclusion that magnetization...     

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...     

Mössbauer and magnetic study of Mn2+- and Cr3+-substituted spinel magnesioferrites of the composition Mg1−xMnxFe2−2xCr2xO4

Chromium and manganese co-substituted spinel magnesioferrites of the composition Mg_1?x Mn _x Fe_2?2x Cr_2x O₄ (x?=?0.0, 0.1, 0.2, 0.3, and 0.5) were investigated with X-ray diffraction (XRD), Mössbauer spectroscopy and magnetic measurements. The cation distribution inferred suggests that Mn²⁺ and Cr³⁺ ions dominantly occupy the A- and B-sites respectively. The gradual decrease of the hyperfine fields and Curie temperatures with increasing x reflects a gradual weakening in the AB exchange interaction. Mössbauer data of the sample with x = 0.5 is suggestive of cation clustering and/or superparamagnetism. The magnetization data is suggestive of Yafet-Kittel-type canted magnetism.     

The study of superexchange interaction of ordered Li 0.5 Fe 1.0 Rh 1.5 O 4

Li_0.5Fe_1.0Rh_1.5O₄ has been studied by X-ray diffraction, Mössbauer spectroscopy. The crystal structure is characterized by the additional reflection (200) that is described by 1:1 ordered structure of Li, Fe at tetrahedral (A) site and can be assigned to the space group F $\bar{4}Li_0.5Fe_1.0Rh_1.5O₄ has been studied by X-ray diffraction, M?ssbauer spectroscopy. The crystal structure is characterized by the additional reflection (200) that is described by 1:1 ordered structure of Li, Fe at tetrahedral (A) site and can be assigned to the space group F3m. The lattice constant (a ₀) is 8.4348 ?. The temperature dependence of the magnetic hyperfine field is analyzed by the Néel theory of ferrimagnetism. The inter-sublattice superexchange interaction is found to be antiferromagnetic with a strength of J _A–B = –3.78 k _B while the intra-sublattice superexchange interactions are ferromagnetic with strengths of J _A–A = 5.40 k _B and J _B−B = 7.39 k _B . The Debye temperatures of the tetrahedral and octahedral sites are determined to be 388 and 464 ± 3 K, respectively, and the Néel temperature has been found to be 260 ± 3 K.     

57Fe Mössbauer spectroscopic study of (25−x)MnO–xZnO–15Fe2O3–60B2O3 glasses

The ⁵⁷Fe Mössbauer technique has been used to investigate the effect of zinc oxide substitution in (25???x)MnO–xZnO–15Fe₂O₃–60B₂O₃ glass system (x?=?0, 5, 10, 15 and 20 mol% of ZnO ). Mössbauer absorption spectra for all the samples recorded at room temperature suggest the existence of the two paramagnetic quadrupole doublets. The observed variations in hyperfine parameters have been explained on the basis of cations distribution and exchange interaction at the lattice sites and it is concluded that B–B interaction increases while the metal–metal interaction decreases due to replacement of manganese oxide by zinc oxide. These results suggest that the present glass system exhibits a paramagnetic behaviour that changes towards the weak paramagnetic when manganese oxide was replaced with zinc oxide.     

Morin transition suppression in Polycrystalline 57Hematite (α-Fe2O3) exposed to 56Fe(II)

We used the isotope selectivity of ⁵⁷Fe Mössbauer spectroscopy to investigate changes in the magnetic properties of polycrystalline hematite exposed to ferrous iron (Fe(II)). We found that sorption of ⁵⁶Fe(II), followed by interfacial electron exchange, alters the bulk magnetic properties of ⁵⁷hematite. After reaction with ⁵⁶Fe(II), we observed partial suppression of the Morin transition of ⁵⁷hematite to below 13 K. This is significantly lower than the Morin temperature (T _M) of ～230 K measured for isotopically enriched polycrystalline ⁵⁷hematite, as well as the T _M of 264?±?2 K reported for normal polycrystalline hematite.     

Anomalous isomeric shifts in Mössbauer spectra of substituted leucites (K,Rb, Cs)(Al,B, Fe)Si2O6

Summary A systematic investigation on the properties of synthetic iron-substituted leucite-like silicates has been performed by M?ssbauer spectroscopy. The obtained data on the isomeric shifts indicate that along the chains of tetrahedra present in the anionic framework of these silicates a readjustement of the bond lengths takes place, due to the large ionic radius of iron which exceeds those of silicon and aluminum.     

Investigation of near-source basaltic glasses using 57Fe Mössbauer spectroscopy

Two basaltic glass samples have been studied, one from the Masaya volcano in Nicaragua and one from the Villarrica volcano in Chile using ⁵⁷Fe Mössbauer spectroscopy. The iron(II) to iron(III) ratio has been determined for each sample. The 80 K data shows some separation of the sites and is fitted allowing for discrete doublets.     

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.     

The Mössbauer spectrum of synthetic hureaulite: Fe5 2+(H2O)4(PO4H)2(PO4)2

The crystal structure of synthetic ferrous hureaulite, Fe₅ ²⁺ (H₂O)₄(PO₄H)₂(PO₄)₂, was refined from single-crystal X-ray data. It is monoclinic, space group C2/c, with a=17.487(4), b=9.017(2), c=9.338(2) Å, β=96.27(3)°, V=1463.6(6) Å³, Z=4 and D _calc=3.327 g/cm³. This end member of the hureaulite series was crystallized under distinctly acidic conditions, by a method that gives perfect crystals, large enough for X-ray single crystal studies. The main feature of the hureaulite structure is that it has an equal number of normal (PO₄)³⁺ and acid (PO₄H)²⁺ tetradentate groups. These are centered on Fe²⁺ atoms and share corners with edge-linked octahedra, forming pentamer units. The five Fe²⁺ atoms are distributed on three distinct sites in these units. This can be directly observed in the Mössbauer spectrum at 295 K, which contains three doublets whose relative intensities correspond to the 1:2:2 distributions of crystallographic sites.     

Nd0.5Pb0.5-xSrx (Mn, Fe)O3的局域结构畸变和磁输运性质的研究

通过X光衍射，穆斯堡尔谱和输运性质的测量研究了掺杂Fe的Nd0.5Pb0.5-xSrxMnO3样品的结构和磁输运性质。x光衍射和穆斯堡尔谱都表明了样品随Sr的增多结构畸变减小。Sr的掺杂同时也降低了室温时的电阻率并使磁电阻增大。由于在样品中Mn3+ 与 Mn4+的比率是一样的，也就意味着具有相同的双交换作用。所以Sr对样品性质的改变可以认为是由于晶格畸变所产生的。由于Jahn-Teller 效应而产生的Mn(Fe)O6八面体的局域结构畸变可以局域电子。由我们的实验可以看到在居里温度Tc以上，除了双交换作用之外，Jahn-Teller 效应对于输运性质也起了重要作用。