Defects induced by electron irradiation in hollandite ceramics, specific radioactive cesium-host wasteforms: a 57Fe Mössbauer study

Several single phase hollandite ceramics having Ba²⁺ _x Cs⁺ _y (C = Al,Fe)³⁺ _2x+y Ti⁴⁺ _8?2x?y O₁₆ composition have been synthesized and irradiated under external electron beams simulating the β-irradiation of radioactive cesium. These samples have been characterized by ⁵⁷Fe transmission Mössbauer spectroscopy. Mössbauer results show that the irradiation modifies the local arrangements of Ba cations around Fe³⁺ ions by atomic displacement of Ba ions.     

Mössbauer studies of ɛ-Fe3−x Ni x N and γ′-Fe4−y Ni y N nanoparticles

Nanocrystalline ?-Fe_3?x Ni _x N (0.0?≤?×?≤?0.8) particles are synthesized by precursor technique and nitridation of decomposed products in NH₃ (g) in the temperature range 673 K-823 K. For x?=?0.1–0.4 compositions, single phase ?-Fe_3?x Ni _x N hexagonal structure with space group P6₃/mmc is formed, while for x?=?0.5–0.8, fcc γ′-Fe_4?y Ni _y N phase is also precipitated. The room temperature Mössbauer spectrum for all the compositions shows the presence of superparamagnetic doublet, which is attributed to ?-Fe_3?x Ni _x N phase. For x?=?0.5–0.8 compositions, two additional sextets are observed corresponding to two different iron sites, the corner position (Fe^c) and the fcc position (Fe^f), in γ′-Fe_4?y Ni _y N. The added Ni atoms preferentially substitute the corner Fe^c positions. The isomer shift, quadrupole splitting and hyperfine field values are found to change with the Ni content.     

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.     

Debye temperature and magnetic ordering in K x Ba1−x Fe2S3

⁵⁷Fe Mössbauer measurements are reported for the series K _x Ba_1?x Fe₂S₃, x ≤ 0.3, at temperatures between 4.2 K ≤ T ≤ 294 K. A decrease of the Debye temperature from 435 to 405 K with x, indicates a weakening of the stiffness of the Fe sublattice. The ordering temperatures, taken from the appearance of magnetic hyperfine splitting in the spectra, are approximately 40 K lower for x ≥ 0.1. The values of the centre shift and the small temperature dependence of the quadrupole splitting strongly supports that similar to the border compound BaFe₂S₃ also the K containing samples should be characterised as mixed valence compounds.     

Creation of ferromagnetic properties of V–Fe and Zr–Fe alloys by hydrogen absorption

The results of investigations of V_1?y Fe _y H _x and Zr_1?y Fe _y H _x alloys by ⁵⁷Fe Mössbauer spectroscopy are presented and discussed in view of hydrogen ability to create ferromagnetic properties of the alloy. The results indicate two different possibilities of hydrogen influence on the hyperfine magnetic field. Hydrogen absorption causes the ferromagnetic behaviour of the alloys at significant lower iron concentration compared to the concentration of magnetic transition in binary alloys. The main reason for such behaviour is the anisotropic lattice expansion in hydrogenated V–Fe and Zr–Fe alloys as well as the decomposition of paramagnetic Zr-rich intermetallic compounds in the aftermath of the strong electron affinity of hydrogen for zirconium. These trends give rise to growth of magnetic clusters of Fe atoms so strong that they can participate in the overall magnetic properties of the system under investigation.     

Mössbauer study of the (Ru1-xFex)Sr2GdCu2O8-δ system and two of its possible impurities: SrRuO3 and Gd2CuO4

Mössbauer spectra of a series of samples of the weak ferromagnetic $ {\left( {Ru_{{1 - x}} Fe_{x} } \right)}Sr_{2} GdCu_{2} O_{{8 - \delta }} M?ssbauer spectra of a series of samples of the weak ferromagnetic system reveal the existence of three dissimilar sites where the Fe atoms can go into the structure. The M?ssbauer parameters of the three observed quadrupole doublets, together with the relative population on each site, allow the following site assignment for the iron atoms: Fe³⁺ in four-fold planar coordination at Ru sites; Fe³⁺ in five-fold pyramidal coordination also at Ru sites and Fe²⁺ or Fe³⁺ in five-fold coordination at Cu sites. This assignment implies the formation of oxygen-vacancies at the charge reservoir (the RuO₂ planes) that affect the structure and the superconducting and magnetic properties of the undoped system. Moreover, a close correlation between the oxygen content, calculated through the M?ssbauer data, and the measured cell volume is established. We also report the M?ssbauer spectra of two compounds (SrRu_0.95Fe_0.05O₃ and Gd₂Cu_0.95Fe_0.05O₄) that could be formed as impurities during the synthesis of our samples.     

La–Zn Substituted Hexaferrites Prepared by Chemical Method

La–Zn substituted M-type Ba hexaferrite powders were prepared by sol-gel (Mx) and organometallic precursor (Sk) methods with Fe/Ba ratio of 11.6 and 10.8, respectively. The compositions (LaZn) _x Ba_{1 ? x} Fe_{12 ? x} O₁₉ with 0.0 ≤ x ≤ 0.6 were annealed at 975°C/2 h. The cationic site preferences of nonmagnetic La³⁺ instead of Ba²⁺ ions and Zn²⁺ instead of Fe³⁺ ions were determined by Mössbauer spectroscopy. The La³⁺ ions substitute the large Ba²⁺ ions at 2a site and for x ≥ 0.4 also at 4f₂ site. The nearly all Zn²⁺ ions are placed at the 4f₁ sites. The thermomagnetic analysis of χ(?) confirms that only the small substitutions for x ≤ 0.4 can be taken as a single-phase hexaferrites. The coercivity H _c almost does not change at x = 0.2 for (Mx) samples and further decrease up to x = 0.6. For (Sk) samples at substitution x = 0.2 the values of H _c are decreasing and at higher x the values nearly do not change. The Curie points, T _c, slowly decrease with x for both (Mx) and (Sk) samples.     

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.     

A Mössbauer study of octahedral site symmetries in systems Co x Mn3−x−y Fe y O4 and Ni x Mn3−x−y Fe y O4

A Mössbauer study of cation distribution in systems Co _x Mn_3?x?y Fe _y O₄ and Ni _x Mn_3?x?y Fe _y O₄ has been made. It has been found that in both systems all specimens withy<0.6 value exhibit quadrupole doublets corresponding to site symmetries Fe³⁺(I) and Fe³⁺(II) of octahedral site. As more and more cobalt or nickel is introduced into the matrix the intensity of the inner quadrupole doublet increases while on introducing iron that of the outer quadrupole doublet increases. After a certain concentration of iron the inner doublet starts becoming more intense. It is suggested that this arises possibly from the substitution of cations in the second co-ordination sphere of Fe³⁺(I) and Fe³⁺(II) sites. Fory>0.6 the Mössbauer spectra show relaxation effects.     

Magnetic behavior of the oxide spinels: Li0.5Fe2.5−2x Al x Cr x O4

In order to study the effect of substitution of Fe³⁺ by Al³⁺ and Cr³⁺ in Li_0.5Fe_2.5O₄ on its structural and magnetic properties, the spinel system Li_0.5Al _x Cr _x Fe_2.5?2x O₄ (x=0.0, 0.2, 0.4, 0.5, 0.6 and 0.8) has been characterized by X-ray diffraction, high field magnetization, low field ac susceptibility and ⁵⁷Fe Mossbauer spectroscopy. Contrary to the earlier reports, about 50% of Al³⁺ is found to occupy the tetrahedral sites. The system exhibits canted spin structure and a central paramagnetic doublet was found superimposed on magnetic sextet in the Mössbauer spectra (x>0.5).     

Local magnetic ordering of Fe impurities in Pd2MnSn

Mössbauer effect of Fe⁵⁷ embedded as very dilute substitutional impurities in Pd₂MnSn was studied. The impurities are seen to replace the three elements in the alloy. Although the Curie temperature of the alloy is 189K, well below the room temperature, the Mössbauer spectrum recorded at room temperature consisted of two distinct 6-finger magnetic hyperfine spectra and a single unsplit line. One of the 6-finger patterns which corresponds to an internal magnetic field ofH _int=?375 kOe is inferred to arise due to local magnetic coupling of the localized magnetic moments of Fe impurities at the Pd sites with those of the 4 Mn first nearest neighbours of the Fe impurities. The other 6-finger pattern which corresponds to an internal magnetic field ofH _int=?335 kOe is inferred to arise due to the local magnetic coupling of the localized magnetic moments of the Fe impurities at the Sn sites with those of the 6 Mn second nearest neighboours of the Fe impurities. The difference in the internal magnetic fields observed at the Pd and Sn sites in the alloy could be understood qualitatively, on the basis of RKKY theory, as arising due to the different conduction electron polarization contributions to the net internal magnetic field at the Fe impurity sites. The results of the measurements suggest that the localized magnetic moments of Fe⁵⁷ impurities at Pd and Sn sites are antiferromagnetically coupled with the moments of their neighbouring Mn atoms.     

Magnetotransport and magnetic properties of sulfospinels Zn x Fe1−x Cr2S4

Cr-based chalcogenide spinels, which do not have heterovalency and distortion-induced ions such as manganese oxides with perovskite structure, have demonstrated the existence of colossal magnetoresistance. In order to investigate the magnetotransport phenomena and magnetic properties of sulfospinels Zn _x Fe_1?x Cr₂S₄, polycrystalline Zn _x Fe_1?x Cr₂S₄ samples were synthesized in the 0?≤?x?≤?0.2 range by a solid reaction method. The crystal structure for x?=?0.05, 0.1, and 0.2 turned out to be cubic at room temperature by X-ray diffraction measurement. In magnetoresistance measurement, Zn _x Fe_1?x Cr₂S₄ samples indicate that this system is semiconducting below about 150 K. The temperature of maximum magnetoresistance is almost consistent with Curie temperature. The isomer shift and the electric quadrupole shift of Zn _x Fe_1?x Cr₂S₄ samples by Mössbauer experiment show that Fe²⁺ ions occupy the tetrahedral site in the spinel structure. As the Zn ions are substituted for Fe ions, the Jahn–Teller relaxation slows down and the electric quadrupole shift increases. The magnetotransport phenomena of Zn _x Fe_1?x Cr₂S₄ is related to Jahn–Teller effect and half-metallic electronic structure, which are different from the double exchange interactions of the manganite La–Ca–Mn–O system or the triple exchange interactions of sulfospinel Cu _x Fe_1?x Cr₂S₄.     

Mössbauer studies of phosphate glasses for the immobilisation of toxic and nuclear wastes

Various iron-containing phosphate glasses were investigated by Mössbauer spectroscopy. Iron was found to occur predominantly as Fe³⁺ in all glasses, and largely occupied sites with distorted octahedral coordination for both redox states. Using a base glass of nominal composition 60 P₂O₅–40 Fe₂O₃ (mol%), stepwise molar replacement of Fe₂O₃ by (0.67 Na₂O?×?0.33 Al₂O₃) increased the redox ratio, Fe²⁺/ΣFe, from 0.13 at 40% Fe₂O₃ to 0.25 at 10% Fe₂O₃. The centre shift increased and quadrupole splitting decreased by up to ～20% over this range, interpreted as a decrease in the average distortion of Fe sites from cubic symmetry, and an increase in average iron coordination. Literature revealed that recoil-free fraction ratio f (Fe³⁺)?/?f (Fe²⁺)?≈?1.3 in iron phosphate glasses, and this was considered when assessing redox. Mössbauer parameters of these and other glasses demonstrated a combination of structural stability and compositional flexibility which makes them so suitable for waste immobilisation.     

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.     

Electric quadrupole interaction at 111Cd and 119Sn in isostructural NiIn and CoSn compounds

The perturbed angular correlation technique and Mössbauer spectroscopy were applied to study the electric field gradient on ¹¹¹Cd and ¹¹⁹Sn probe atoms in isostructural NiIn and CoSn compounds. The ¹¹¹Cd PAC measurements performed in the temperature range 80--1100 K demonstrated the existence of two axially symmetric EFG's in each of the investigated compounds, related to the 2(d) and 1(a) probe sites in the B35 structure. A 1(a)-site preference for Cd probes in CoSn compound was observed. The temperature dependence of the quadrupole frequencies for ¹¹¹Cd in both compounds, interpreted in terms of the empirical model proposed by Christiansen et al.[1], follows a T^3/2 relation with different slope parameters for each of the observed frequencies. These results are combined with the data from the Mössbauer experiment. The ¹¹⁹Sn Mössbauer spectra taken at liquid nitrogen and at room temperatures showed two quadrupole split doublets with the intensity ratio 2:1 for CoSn and 8:1 for NiIn_0.99Sn_0.01sample, giving an evidence of 2(d)-site preference for tin atoms in NiIn. The EFG values measured on ¹¹⁹Sn are 2.5 to 4 times larger than those on ¹¹¹Cd nuclei, while the ratio of the respective Sternheimer antishielding factors is equal to 0.77. Within the limits of errors no differences were observed in the magnitude and temperature dependence of Debye--Waller factors for 2(d) and 1(a) ¹¹⁹Sn positions in CoSn and NiIn lattices.     

Characterization of the kinetics of Fe (II) binding by the R2 protein subunit of E. coli ribonucleotide reductase

¹¹⁹Sn Mössbauer spectroscopy was used to study Sn_1???x ⁵⁷Fe _x O_2???δ and Fe _x Sb _y Sn_1???x???y O_2???δ sol–gel synthesized ferromagnetic powders. ¹¹⁹Sn Mössbauer spectra of powder samples showed envelopes characteristic of a relatively broad Sn(IV) line. Small but significant differences can be found among the spectra belonging to different compositions and heat treatments. The spectra were decomposed into two doublets, one of these with unusually high quadrupole splitting was assigned to the defects playing very important role in the realisation of ferromagnetism in these materials.     

Magneto-crystalline properties of BaFe12−2x M x Sn x O19 (M = Sn, Ni, Zn) ferrite powders

BaFe_12?2x M _x Sn _x O₁₉ compounds, where M?=?Sn²⁺, Ni²⁺ or Zn²⁺ ions, were synthesized by mechanical milling and partially by citrate precursor methods. Analysis of magneto-crystalline structure has been carried out by Mössbauer spectroscopy. The Sn⁴⁺ ions replace Fe³⁺ ions on 2b and slightly on 2a?+?4f₁ sites, Zn²⁺ ions strongly prefer 4f₁ sites, Sn²⁺ ions prefer 4f₁ sites too and Ni²⁺ ions occupy 4f₂ and 12k or 2a sites. The magnetic properties were evaluated by the vibrating sample magnetometry and the thermomagnetic analysis. A large variation of the intrinsic coercivity H _c (330 to 78 kA/m) and of temperature coefficient of coercivity of ΔH _c /Δ? (0.39 to 0.22 kA/m°C) were achieved as a function of the (Zn–Sn) and (Sn–Sn) substitutions, respectively. The Curie temperature T _c decreased with the (Ni–Sn) substitution from 447 to 399°C.     

Cationic Order in Double Perovskite Oxide, Sr2Fe1−x Sc x ReO6 (x = 0.05, 0.1)

We have synthesized by sol–gel method the following polycrystalline double perovskite samples: Sr₂Fe_1?x Sc _x ReO₆ (x = 0, 0.05, 0.1). The results of the Rietveld refinements presented single double perovskite phases with orthorhombic symmetry for the system Sr₂Fe_1?x Sc _x ReO₆, the differences in atomic radii between Fe³⁺ and Sc³⁺ cause a lowering in symmetry with respect to the parent Sr₂FeReO₆ tetragonal compound. The Curie temperatures are found at about 426 and 436 (±5) K for Sr₂Fe_0.9Sc_0.1ReO₆ and Sr₂Fe_0.9Sc_0.05ReO₆, respectively. The Mössbauer spectra measured at 77 K show complex hyperfine structures resulting from different magnetic contributions at Fe³⁺ sites; the average hyperfine field is estimated 50 T and the isomer shift at 0.5 mm/s. At room temperature an intermediate valence state for Fe is also observed.     

Neutron diffraction and Mössbauer study on FeGa x Cr2−x S4

Ga doped sulphur spinel FeGa _x Cr_2?x S₄ (x = 0.1 and 0.3) have been studied with X-ray, neutron diffraction, and Mössbauer spectroscopy. Rietveld refinement of X-ray, neutron diffraction, and Mössbauer spectroscopy lead to the conclusion that the samples are in inverse spinel type, where most Ga ions are present at octahedral site (B). The neutron diffractions on FeGa _x Cr_2?x S₄ (x = 0.1) above 10 K show long range interaction behaviors and reveal a ferrimagnetic ordering, with the magnetic moment of Fe²⁺(?3.45 μ_B) aligned antiparallel to Cr³⁺ (+2.89 μ_B) at 10 K. Fe ions migrate from the tetrahedral (A) site to the octahedral (B) site with an increase in Ga substitutions. The electric quadrupole splittings of the A and B sites in Mössbauer spectra give direct evidence that Ga ions stimulate an asymmetric charge distribution of Fe ions in the A site.     

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