Mössbauer study of Fe57 and Co57 in Co1 + xV2−xO4

Mössbauer sources and absorbers, prepared by doping Fe⁵⁷ and radioactive Co⁵⁷ into samples of Co_1?xV_2?xO₄ (0 ? x ? 1), were studied in the temperature range of 80–500 K. The source and absorber spectra are very similar. The absence of any Fe²⁺ at the A site can be understood by partial covalent bond formation with an anion. However, the predominance of Fe³⁺ at the B site (with some Fe²⁺ for x = 1) cannot be explained by simple crystal-field or molecular-orbital theories. The x dependence of the isomer shift and of the Fe³⁺B-site quadrupole interaction can be related to changes in the lattice constant and the oxygen parameter. The temperature dependence of the Fe²⁺B-site quadrupole interaction can be fitted in the motional-averaging model. In the range of 0 ? x ? 0.5 the temperature dependence of the isomer shift shows effects of chemical bonding beyond the second-order Doppler shift.     

Etude De La Conductivite Electrique Des Ferrites Oxyfluores A Structure Spinelle

An investigation of the electrical conductivity of some oxyfluoride spinels of formula Zn_x²⁺Fe_1?x³⁺[M²⁺ Fe³⁺]O_4?xF_x (M = Fe, Co, Ni) and Fe³⁺[N_x²⁺Fe²⁺Fe_1?x³⁺]O_4?xF_x (N = Fe, Ni) shows that the conduction depends on the composition of the B sites: the activation energy increases, the conductivity and the Fe₃O₄ transition temperature decrease as the substitution rate of Fe³⁺ by N²⁺ in the B sites increases. The authors conclude to a hopping mechanism between the B cations; the anionic sublattice and the cationic A sublattice do not participate in the conduction.     

Mossbauer study of ferrite systems Co x Mn1−x Fe2O4 and Ni x Mn1−x Fe2O4

Mössbauer spectra of Co _x Mn_1?x Fe₂O₄ and Ni _x Mn_1?x Fe₂O₄ ferrites withx values ranging from 0·1 to 0·8 in steps of 0·1 have been recorded at room temperature. All spectra exhibit well-defined Zeeman hyperfine patterns. It has been observed that hyperfine field at Fe³⁺ nucleus increases more rapidly by nickel substitution than by cobalt substitution. This has been explained in terms of exchange interactions and cation distribution in the spinels. Hyperfine fields, isomer shifts and quadrupole splittings have been determined.     

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.     

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 Fe x Mn1 − x S single crystals

Single crystals of iron manganese sulfides Fe _x Mn_{1 ? x} S (0.25 ≤ x ≤ 0.29) are experimentally investigated using Mössbauer spectroscopy and x-ray diffraction. The Mössbauer spectra measured at 300 K exhibit a single broadened line characteristic of paramagnets. The isomer shift of this line is equal to 0.92–0.94 mm/s, which is typical of Fe²⁺ ions in the octahedral position. The quadrupole splitting (0.18–0.21 mm/s) suggests a distortion of the coordination polyhedron of iron ions in the Fe _x Mn_{1 ? x} S compounds.     

Conductivite electrique des magnetites faiblement substituees a l'aluminium ou au chrome et faiblement oxydees au voisinage de la temperature de transition de Verwey

The electrical conductivity of aluminum or chromium slightly substituted and slightly oxidized magnetites (0 < x < 0.27; 0 < δ 0.040) whose formula is Fe³⁺[□_δFe_(1?3δ)²⁺Fe_(1?x) + 28³⁺M_x³⁺]O₄^2? over the temperature range 300-77°K is affected by the composition of octahedral sites. In particular the Verwey transition temperature and its magnitude decrease with x and δ while the electrical conductivity and activation energy evolve differently on either side of this discontinuity.     

Magnetocrystalline anisotropy and inversion degree of manganese-ferrous-ferrites

Torque measurements have been performed at 4 and 77 K on single crystals of Mn_1?xFe_2+xO₄ (0<x<0.05) and MnFe_2?xTi_xO₄ (0<x<0.1). The crystals were either quenched or slowly cooled causing a change of inversion by 0.1. It is found that the magnetic anisotropy due to the ferrous ions in the Ti-doped samples is 80 per cent larger than in the Mn_1?xFe_2+xO₄ crystals. In both crystal series the ferrous ion anisotropy in the slowly cooled crystals is 70 per cent higher than in the quenched crystals. The K₁ of MnFe₂O₄ at 4 K is found to decrease from ?3.6 × 10⁵ erg/cm³ after show cooling, down to ?2.6 × 10⁵ erg/cm³ after quenching.     

A XPS study of Fe1-χCuχCr2S4

The X-ray photoelectron spectroscopy is applied for a chalcogen chromite system Fe_{1 ? x}Cu_xCr₂S₄ to obtain microscopic information about valence states of the constituting atoms. The features of the Cu 2p spectra show that copper is always in a monovalent state in this system. This supports a model Fe³⁺_{1 ? x}Cu¹⁺_xCr³⁺₂S^1-_{2x ? 1}S^2-_{5 ? 2x} for a compositional range 0.5 < x < 1.     

Investigation of La1−x Ca x FeO3−y (0≦x≦0.50) by means of X—ray diffraction and Mössbauer spectroscopy

Compounds La_1?x Ca _x FeO_3?y (0≤x≤0.50) were prepared and characterized by X-ray diffraction and Mössbauer measurements. The diffraction patterns were defined to be orthorhombic. The lattice constants of orthorhombic perovskite La_1?x Ca _x FeO_3?y decrease linearly with increasingx. The Mössbauer spectra at room temperature indicate that the Néel temperature drops and the isomer shift decreases with increasingx. The spectrum of La_0.50 Ca_0.50 FeO_3?y at 80 K shows two hyperfine splitting patterns which may be related to the Fe³⁺ and Fe⁵⁺ ions.     

Ordering in FeII–III Hydroxysalt green rusts from XRD and Mössbauer analysis (chloride, carbonate, sulphate, oxalate...); about the structure of hydrotalcite-like compounds

The domain of existence of Fe^II–III hydroxysalt green rusts (GR) is characterised by the ratio x = [Fe^III]/[Fe_total] lying in [1/4, 1/3] as it is for any layered double hydroxide (LDH). Since Fe^III cations balance the charges of intercalated anions, cation and anion distributions are correlated. However, the distribution of anions depends on their charge, shape and size. For Cl^? and CO₃ ^2?, x belongs to the complete range whereas for C₂O₄ ^2? and SO₄ ^2? it is limited to x = 1/4 and 1/3, respectively. Mössbauer spectra present in all cases two ferrous and one ferric doublets, which characterise each GR by their relative abundance. Intermediate values of x are in fact obtained by intrincate domains of two types, one where the cation periodicity is 2×a ₀ and the other one √3×a ₀ if a ₀ is the lattice parameter.     

Non affinity of Fe2+ for B sites in iron thiochromites and compared band structures of oxides and sulfides spinels

Departure from stoichiometry in vapor grown FeCr₂S₄ was studied using Mössbauer spectroscopy. The paramagnetic Mössbauer spectra give evidence of two singlets and two doublets which correspond respectively to A site Fe²⁺ ? Fe³⁺, Fe^II in T_d symmetry and in symmetry lower than T_d. The following ionic distribution has been deduced:

$\text{(Fe}{}^{\mathrm{2+}}{}_{\mathrm{1?y}}\text{Fe}{}^{\mathrm{3+}}{}_{\mathrm{y}}\text{)|Cr}{}^{\mathrm{3+}}{}_{\mathrm{2?x}}\text{□}{}_{\mathrm{x}}\text{|S}{}_{\mathrm{4?z}}\text{□}{}_{\mathrm{z}}$

Compounds in the system Fe_1+xCr_2?xS₄ have been studied for 0 ? x ? 0.1. The spectra are solved assuming Fe^II in A site with T_d symmetry, A site Fe^II with lower symmetry and B site Fe³⁺. No Fe²⁺ appears in B site. These features are discussed in terms of schematic band structures implying single electron narrow bands. The non-affinity of Fe²⁺ for B sites of iron thiochromites is discussed in relation with B site Cr²⁺ level.     

Transition from the Kondo regime to long-range magnetic order in the FexV1−x S system

A study is reported on the electrical and magnetic characteristics of the Fe_xV_1?x S solid-solution system with x≤0.5. A maximum in the temperature dependence of resistivity ρ(T) characteristic of the Kondo effect has been observed for small x(x<0.01). For x>0.1, long-range magnetic order sets in in the system with T _K ≈ 100 K. Near x=0.05, the Fe²⁺ impurity behavior crosses over to a magnetically ordered phase. The electronic properties of Fe_xV_1?x S are typical of those of strongly correlated electronic systems. Both the electrical and magnetic data imply that carrier delocalization is the strongest at x=0.4.     

57Fe,119Sn and151Eu Mössbauer study of EuBa2(Cu1−x−y 119Sn x 57Fe y )3O7−δ superconductor

¹⁵¹Eu,¹¹⁹Sn and⁵⁷Fe Mössbauer spectroscopy was used to study a EuBa₂(Cu_1?x?y Sn _x Fe _v )₃O_7?δ superconductor in which sites were replaced by all three Mössbauer isotopes in the same sample in order to get information about the site preferences and structural changes. We have found changes in the Mössbauer parameters of the⁵⁷Fe,¹¹⁹Sn and¹⁵¹Eu spectra compared to those recorded in the separately substituted EuBa₂Cu₃O_7?δ, EuBa₂(Cu_1?x Sn _x )₃O_7?δ, YBaCu_3?x Fe _x O_7?δ YBaCu_3?x Sn _x O_7?δ materials. These results can be interpreted as a consequence of the site preferences and the changes in the structure due to the presence of other additional substitutional elements.     

Studies of oxides related to high temperature superconductors

Samples of the series of compounds YBa₂(Cu_1?x Fe _x )₃ O_6.5+p with 0<x≤0.2 have been characterised by X-ray diffraction, oxidation titration and Mössbauer spectroscopy yielding information on the oxygen content parameterp and the charge state ratios Cu³⁺/Cu²⁺ and Fe⁴⁺/Fe³⁺.     

Mössbauer studies on cation distribution and lattice distortion in magnesium manganites

Mössbauer studies are reported on a magnesium manganite series Mg_xMn_2.9?xFe_0.1O₄ for 0.5 ? x ? 2.4. In the region x ? 1.4 two quadrupole doublets are observed while for x ? 1.8 only one doublet could be observed. Quadrupole splitting (QS) has been studied as a function of x and an unusually large QS for the outer doublet has been attributed to Jahn-Teller distortion whose onset has been located in the region 1.8 > x > 1.4; this is corroborated by X-ray diffraction studies. Probable cation distributions are proposed. Oxygen ion parameters are estimated for the compositions corresponding to x = 1 (tetragonal phase) and x = 2 (cubic phase) under point charge approximation for electric field gradient at Fe³⁺ sites.     

Mössbauer studies in the spinel system coxfe3−xo4

Mössbauer spectra have been obtained for the cobalt-iron spinels having the general formula CO_xFe_3?xO₄ for the whole range of composition, materials from x = 0 to x = 2.76 having been studied. The spectra fall into three groups: (A) Those for spinels for which 0 < χ < 1 which show two six-peak sets of lines corresponding to exchanging Fe²⁺/Fe³⁺ and to Fe³⁺ iron; (B) those for spinels containing Fe³⁺ iron only which show magnetic splitting 1 < χ < 2; (C) those for spinels containing Fe³⁺ iron only which show quadrupole splitting but no magnetic splitting (x = 2.3, 2.5 and 2.8). The significance of the various spectra in relation to the structure of these mixed oxides is discussed.     

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.     

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.     

Mössbauer studies of the atomic distributions and hyperfine interactions in Mn-Fe alloys with β-Mn structure

Mn_20?x Fe_x alloys (x=1–6.4) with β-Mn structure are studied by Mössbauer spectroscopy. The electric field gradient tensor for the crystallographically nonequivalent sites 8(c) and 12(d) are calculated. Partial Mössbauer spectra are identified, and the ⁵⁷Fe hyperfine-interaction parameters determined. Iron atoms substituting for manganese in the Mn_20?x Fe_x system exhibit strong preference for occupying 8(c) sites. With increasing iron concentration, the longrange order parameter in the Fe atom arrangement increases, and the degree of order, decreases.