A Mössbauer study of oxygen vacancy and cation distribution in 6H-BaTi1−xFexO3−x/2

We have synthesized samples in the system BaTi_1−xFe_xO_3−x/2 with x=0.1−0.6 at temperatures of 1200-1300°C under reducing conditions of oxygen fugacity. After drop quenching, samples were characterized using the electron microprobe, X-ray diffraction and Mössbauer spectroscopy. All samples were hexagonal with a 6H-BaTiO₃ type structure. Mössbauer spectroscopy showed all iron to be present as Fe³⁺, occurring in octahedral and pentahedral sites. Analysis of area ratios indicates that oxygen vacancies are distributed randomly over O1 sites, and that a random distribution of Fe and Ti cations over M1 and M2 sites is consistent with the data. No evidence for ordering of oxygen vacancies was found. Results are consistent with conductivity results, which show generally increasing ionic conductivity with increasing oxygen vacancy concentration.     

Mo¨ssbauer studies of thiospinels. IV. The system FeCr2S4-Fe3S4

Spinel compounds of the composition Fe_1+xCr_2?xS₄, with 0 ≦ x ≦ 0.5, have been prepared in polycrystalline form. The ionic distribution Fe²⁺[Cr³⁺_2?xFe³⁺_x]S^2?₄ is derived from both X-ray and ⁵⁷Fe Mo¨ssbauer data. Room temperature Mo¨ssbauer spectra show the typical behavior of tetrahedral-site Fe²⁺ surrounded by different octahedral-site neighbors. Octahedral-site Fe³⁺ absorbs as a doublet with Δ ≈ 0.5 mm/s. Samples of overall composition FeCr₂S₄ consist mainly of a spinel Fe²⁺[Cr³⁺_2?yFe³⁺_y]S^2?₄, y ≈ 0.02.     

An X-ray and Mössbauer study of the FeTi2O5Ti3O5 system

Mössbauer spectra and X-ray diffraction data have been recorded for compounds in the solid solution series Fe_xTi_3?xO₅, 0 ? x ? 1. Compounds with x > 0.35 have the orthorhombic pseudobrookite structure, and those with x < 0.35 a monoclinic distortion of it. Ferrous ions in the fourfold and eightfold sites give rise to separate quadrupole doublets in the Mössbauer spectra, and thus it has been possible to determine the site occupancy as a function of composition by a least-square analysis of the spectra. The Mössbauer method has also proved to be a sensitive tool for the investigation of the orthorhombic-monoclinic distortion.     

Oxygen-ion and electron conductivity in Sr2(Fe1−xGax)2O5

High-temperature electrical conductivity measurements, structural data from powder X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy were combined to study the interrelationship of oxygen ion transport and p- and n-type transport in Sr₂(Fe_1−xGa_x)₂O₅, where x=0, 0.1 and 0.2. Although gallium substitution generally decreases the total ion-electron transport, the transition of the orthorhombic brownmillerite structure to a cubic phase on heating results in the recurrence of the conductivity to the same high level as in the parent ferrite (x=0). The changes of the partial contributions to the total conductivity as a function of x are shown to reflect a complicated interplay of the disordering processes that develop in the oxygen sublattice on heating in response to replacement of iron with gallium.     

Substitution of Co in YBa2Fe3O8

The accommodation of Co in the oxygen-saturated solid-solution phase YBa₂(Fe_1−zCo_z)₃O_8+w has been investigated by powder X-ray and neutron diffraction techniques, as well as by Mössbauer spectroscopy. Of the nominal composition range 0.00?z?1.00 tested, the solid-solution limit under syntheses at 950°C in is z=0.47(5). No symmetry change in the nuclear and magnetic structures is seen as a consequence of the Co substitution, and the Co atoms are distributed evenly over the two sites that are square-pyramidally and octahedrally coordinated for w=0. The oxygen-saturated samples maintain their oxygen content roughly constant throughout the homogeneity range, showing that Co³⁺ replaces Fe³⁺. Despite the nearly constant value of w, Mössbauer spectroscopy shows that the amount of tetravalent Fe slightly increases with increasing z, and this allows Co to adopt valence close to 3.00 to a good approximation. The magnitude of the antiferromagnetic moment (located in the a,b plane) decreases with z in accordance with the high-spin states of the majority Fe³⁺ and Co³⁺ ions. Bond-valence analyses are performed to illustrate how the structural network becomes increasingly frustrated as a result of the substitution of Fe³⁺ by the smaller Co³⁺ ion. A contrast is pointed out with the substitution of cobalt in YBa₂Cu₃O₇ where it is a larger Co²⁺ ion that replaces smaller Cu²⁺.     

Structure alterations in microporous (Mg,Fe)2Al4Si5O18 crystals induced by energetic heavy-ion irradiation

The microporous framework structure of (Mg_1−xFe_x)₂Al₄Si₅O₁₈ (=cordierite) has been subject to a comparative study on the effect of structural alterations originating from exposure to high-energy heavy ions. Oriented samples (with x=0.061, 0.122, and 0.170) were irradiated with swift ¹²⁴Xe, ¹⁹⁷Au and ⁹⁶Ru ions with 11.1 MeV per nucleon energy and fluences of 1×10¹² and 1×10¹³ ions/cm². Irradiated and non-irradiated samples were investigated by means of X-ray diffraction, Mössbauer spectroscopy and optical absorption spectroscopy. Structural investigations reveal an essentially unchanged Al,Si ordering, which appears to be unaffected by irradiation. The most remarkable macroscopic change is the ion-beam induced colouration, which could be assigned to electronic charge transfer transitions involving the Fe cations. Mössbauer spectra indicate an increased amount of ^[4]Fe³⁺ for the irradiated sample. The most noticeable structural alteration concerns irradiation-induced dehydration of extra-framework H₂O, which is accompanied by a reduction in the molar volume by ∼0.2 vol%.     

Mössbauer study of the FeV2O4---Fe3O4 system

Mössbauer spectra of the Fe_1+xV_2−xO₄ spinel solid solutions are taken to investigate the cation distribution. Room temperature spectra can be interpreted by assuming that the cation distribution is represented approximately as Fe²⁺[Fe³⁺_xV³⁺_2−x]O₄ for 0 x 0.35 and Fe³⁺[Fe²⁺Fe³⁺_x−1V³⁺_2−x]O₄ for 1 x 2 and the ionic valence arrangement changes from the 2-3-3 type (Fe²⁺[Fe³⁺_xV³⁺_2−x]O₄) to the 3-2-3 one (Fe³⁺[Fe²⁺V³⁺]O₄) in the range 0.35 x 1. Fe₂VO₄ is found to be 3-2-3 spinel, Fe³⁺[Fe²⁺V³⁺]O₄. Its paramagnetic spectrum at 473°K is, however, composed of a broad single line with isomer shift value of 0.61 mm/sec relative to stainless steel, in which the line splitting due to the ferric and ferrous ions is rendered indistinguishable.     

Reduction properties of phases in the system La0.5Sr0.5MO3 (M=Fe, Co)

Phases formed by the reduction of compounds of the type La_0.5Sr_0.5MO₃ (M=Fe, Co) have been characterized by means of temperature programmed reduction, X-ray powder diffraction, ⁵⁷Fe Mössbauer spectroscopy and Fe K-, Co K-, Sr K-, and La L_III-edge X-ray absorption spectroscopy. The results show that treatment of the material of composition La_0.5Sr_0.5FeO₃ (which contains 50% Fe⁴⁺ and 50% Fe³⁺) at 650 °C in a flowing 90% hydrogen/10% nitrogen atmosphere results in the formation of an oxygen-deficient perovskite-related phase containing only trivalent iron. Further heating in the gaseous reducing environment at 1150 °C results in the formation of the Fe³⁺-containing phase SrLaFeO₄, which has a K₂NiF₄-type structure, and metallic iron. The material of composition La_0.5Sr_0.5CoO₃ is more susceptible to reduction than the compound La_0.5Sr_0.5FeO₃ since, after heating at 520 °C in the hydrogen/nitrogen mixture, all the Co⁴⁺ and Co³⁺ are reduced to metallic cobalt with the concomitant formation of strontium- and lanthanum-oxides.     

Electron-doping through La-for-Sr substitution in (Sr1−xLax)2FeTaO6: Effects on the valences and ordering of the B-site cations, Fe and Ta

We have employed aliovalent A-site cation substitution, La^III-for-Sr^II, to dope the Sr(Fe_0.5Ta_0.5)O₃ perovskite oxide with electrons. Essentially single-phase samples of (Sr_1−xLa_x)(Fe_0.5Ta_0.5)O₃ were successfully synthesized up to x≈0.3 in a vacuum furnace at 1400 °C. The samples were found to crystallize (rather than with orthorhombic symmetry) in monoclinic space group P2₁/n that accounts for the partial ordering of the B-site cations, Fe and Ta. With increasing La-substitution level, x, the degree of Fe/Ta order was found to increase such that the La-richest compositions are best described by the B-site ordered double-perovskite formula, (Sr,La)₂FeTaO₆. From Fe L₃ and Ta L₃ XANES spectra it was revealed that upon electron doping the two B-site cations, Fe^III and Ta^V, are both prone to reduction. Magnetic susceptibility measurements showed spin-glass type behaviour for all the samples with a transition temperature slightly increasing with increasing x.     

Cation Distribution of the Transparent Conductor and Spinel Oxide Solution Cd1+xIn2−xSnxO4

The cation distribution in the transparent conducting oxide Cd_1+xIn_2−xSn_xO₄ was investigated to determine if there is a correlation between structure and electronic properties. Combined Rietveld refinements of neutron and X-ray diffraction data and ¹¹⁹Sn Mössbauer spectroscopic analysis were used to show that the cation distribution changed with x(0≤x≤0.7) from a primarily normal spinel (x=0) to an increasingly random spinel. CdIn₂O₄ quenched from 1175°C has an inversion parameter of 0.31 (i.e., (Cd_0.69In_0.31)^tet(In_1.69Cd_0.31)^octO₄). The inversion parameter decreases to 0.27 as the quench temperature is lowered from 1175°C to 1000°C. The decrease in inversion parameter with temperature correlates with an increase in optical gap from 3.0 eV to 3.3 eV for specimens prepared at 1175°C and 800°C, respectively. We show that this is a consequence of an increase in the fundamental band gap.     

Influence of the Mg-content on the cation distribution in cubic MgxFe3−xO4 nanoparticles

The influence of the Mg-content on the structural and magnetic properties of cubic Mg_xFe_3−xO₄ nanoparticles prepared by combustion reaction was investigated using X-ray diffraction, transmission electron microscopy (TEM), Mössbauer spectroscopy, and Raman spectroscopy. Lattice parameter, nanoparticle size, and cation (Mg²⁺, Fe³⁺) distribution were quantified as a function of the Mg-content in the range 0.5≤x≤1.5. We found a mixed-like spinel structure at the smaller x-value end whereas the inverse-like spinel structure dominates samples with larger x-values. Moreover, in the x-value range investigated (0.5≤x≤1.5) we found no change in the quadrupole splitting and isomer shift values, though the hyperfine field decreases as the x-value increases. The splitting of the A_1g Raman mode was used to both quantify the Mg²⁺/Fe³⁺ contents in the tetrahedral site and obtain the cation distribution in the Mg_xFe_3−xO₄ structure. The cation distribution obtained from the Raman data is in very good agreement with the cation distribution obtained from the Mössbauer data.     

Structural aspects of Pr1−xSrxFeO3−w

Structural aspects of the distorted perovskite ABO₃ phase Pr_1−xSr_xFeO_3−w,x=0.00-0.80,w=0.000-0.332, were studied by powder X-ray diffraction, powder neutron diffraction, Mössbauer spectroscopy, and Fe K-, Sr K-, and Pr L_III-edge EXAFS techniques. The diffraction data revealed no indications for ordering of Pr and Sr at the A site, nor for oxygen vacancy ordering at O sites for heavily reduced samples. Mössbauer spectroscopy showed octahedral, square pyramidal, and tetrahedral Fe coordinations with relative amounts closely following the predictions for a binomial distribution of oxygen vacancies. In addition to Fe³⁺ and Fe⁴⁺, also Fe⁵⁺ appears at 77 K for (G-type) antiferromagnetic samples with high average Fe valence. This suggests dynamic 2 Fe⁴⁺↔Fe³⁺+Fe⁵⁺ fluctuations. At 296 K, a mixed valence Fe⁽³⁺ⁿ⁾⁺ component significantly improved the fit of Mössbauer spectra for the most oxidized paramagnetic samples. The qualitative EXAFS study shows that the local environments for Fe, Pr, and Sr strongly depend on x and w. The local Pr- and Sr-site geometries differ significantly from the cubic average structure for Pr_0.50Sr_0.50FeO_2.746.     

Magnetic structures of the α-Li3Fe2(PO4)3−x(AsO4)x (x=1, 1.5, 2, 3) solid solution

Mössbauer spectroscopy and neutron diffraction studies have been carried out for the α-Li₃Fe₂(PO₄)_3−x(AsO₄)_x (x=1, 1.5, 2, 3) solid solution, potential candidate for the cathode material of the lithium secondary batteries. The crystal and magnetic structures of all these phases are based on the structural and magnetic model corresponding to the α-Li₃Fe₂(PO₄)₃ phosphate parent, but with some differences promoted by the arsenate substitution. The PO₄ and AsO₄ groups have a random distribution in the structure. In all compounds the coupling of the magnetic moments takes place in the (001) plane, but the value of the angle between the moments and the x direction decreases from 38.3° (α-Li₃Fe₂(AsO₄)₃) to 4.7° (α-Li₃Fe₂(PO₄)₂(AsO₄)₁). This rotation arises from the change in the tilt angle between the Fe(1)O₆ and Fe(2)O₆ crystallographically and magnetically independent octahedra in the structures, and affects the effectiveness of the magnetic exchange pathways. The ordering temperature T_N decreases with the increase of phosphate amount in the compounds. The existence of a phenomenon of canting and the evolution of the ferrimagnetic behavior in this solid solution is also discussed.     

Correlation of crystal structures with electric field gradients in the fluorite- and pyrochlore-type compounds in the Gd2O3-ZrO2 system

Correlation of crystal structure with electric field gradient (EFG) in the fluorite- and pyrochlore-type compounds in the Gd₂O₃-ZrO₂ system Gd_xZr_1−xO_2−x/2 with 0.18?x?0.62 were investigated by ¹⁵⁵Gd Mössbauer spectroscopy, powder X-ray diffraction and point-charge model (PCM) calculation. An intermediate ordered pyrochlore phase forms for 0.45?x?0.55, sandwiched with a disordered fluorite phase for 0.18?x<0.45 and 0.55<x?0.62. Some ¹⁵⁵Gd Mössbauer parameters, especially the quadrupole coupling constant (e²qQ), were found to exhibit a characteristic maximum around the ideal-pyrochlore Gd₂Zr₂O₇ (x=0.50) composition. The validity of the proposed pyrochlore-based structural model was examined by comparing the experimental values of EFG at the Gd sites with those calculated by the PCM calculations.     

Nanocrystalline ferrites NixZn1−xFe2O4: Influence of cation distribution on acidic and gas sensing properties

This work is devoted to a detailed analysis of the interconnection between composition, cation distribution and acidic properties of the surface of nanocrystalline ferrites Ni_xZn_1−xFe₂O₄ obtained by aerosol pyrolysis. The detailed analysis of the Mössbauer spectra allows us to determine the distribution of cations between tetrahedral and octahedral positions in spinel structure. Depending on samples composition, the tetrahedral positions can be occupied by only Fe³⁺ cations (inverse spinel, x≥0.4) or by Fe³⁺ and Zn²⁺ cations (mixed spinel, x=0, 0.2). Increasing the nickel concentration in the ferrite leads to decrease in the number of strong acid centers on the surface. It was found that the decrease in the contribution of strong surface acid sites leads to an increase in sensory sensitivity of the ferrite towards ammonia. For ethanol detection an inverse relationship between sensor signal and surface acidity was observed.     

Mössbauer studies of thiospinels. V. The systems Cu1−xFexMe2S4 (Me = Cr, Rh) and Cu1−xFexCr2(S0.7Se0.3)4

With the exception of FeRh₂S₄, powder samples of all systems studied have been obtained as spinel phase without essential impurities. The lattice constants follow Vegard's law. From the Seebeck coefficients and the Mössbauer spectra the valence distribution Cu¹⁺_1−xFe²⁺_2x−1Fe³⁺_1−x[Me³⁺₂]X²⁻₄ is derived for 0.5 x 1, while there is only Fe³⁺ present for 0 < x 0.5. Samples with the overall composition FeRh₂S₄ contain mostly Rh₂S₃ and iron sulfide phases, but less than 20% of a spinel phase.     

Structural characterization of the FeTiO3-MnTiO3 solid solution

We have synthesized the (Mn_1−xFe_x)TiO₃ (0.0≤x≤1.0) solid solution compounds by high-temperature sintered methods, and characterized their crystal structures by combining X-ray diffraction, Mössbauer spectroscopy and Raman spectroscopy. Lattice constants and volumes show a linear decrease with increase in FeTiO₃ content. All experimental results illustrate a decreasing distortion of TiO₆ (or FeO₆/MnO₆) octahedra with increase in FeTiO₃ content. The vibrational frequency of OTiO bending motions presents a direct dependence on the corresponding bond angle (the ∠OTiO).     

Magnetic properties and Eu Mössbauer effects of mixed valence europium copper sulfide, Eu2CuS3

Ternary europium copper sulfide Eu₂CuS₃ have been investigated by X-ray diffraction, ¹⁵¹Eu Mössbauer spectroscopy, magnetic susceptibility, magnetization, and specific heat measurements. In this compound, Eu²⁺ and Eu³⁺ ions occupy two crystallographically independent sites. The ¹⁵¹Eu Mössbauer spectra indicate that the Eu²⁺ and Eu³⁺ ions exist in the molar ratio of 1:1, and the Debye temperatures of Eu²⁺ and Eu³⁺ are 180 and 220 K, respectively. In its magnetic susceptibility, the divergence between the zero-field cooled and field cooled susceptibilities appears below 3.4 K. The specific heat has a λ-type anomaly at the same temperature. From the field dependence of magnetization at 1.8 K, the Eu²⁺ ion was found to be in the ferromagnetic state with the saturation magnetization M_S=6.7 μ_B.     

Preparation and magnetic properties of Fe-Nb co-doped SnO2

Fe³⁺-Nb⁵⁺ co-doped SnO₂ was prepared at 1200 °C by a simple chemical co-precipitation method. The Sn_1−2xFe_xNb_xO₂ solid solutions kept cassiterite structure in the range of 0<x?0.33, and their cell parameters decrease with increasing x. While x=0.40, a second phase with orthorhombic FeNbO₄ structure co-exists with the cassiterite phase, and the second phase becomes dominant while x?0.45. The magnetic measurements indicated that low doping ratio sample (x=0.03) exhibits paramagnetic behavior. A paramagnetic-to-antiferromagnetic phase transition was observed for the samples with higher doping ratio (x?0.15).     

Stabilization of over-stoichiometric Mn in layered Na2/3MnO2

Sodium manganates with nominal composition Na_2/3MnO₂ were prepared by solid state reaction between Na₂CO₃ and MnCO₃ at 1000 °C. The composition and structure of Na_xMnO₂ were controlled by the rate of cooling from the temperature of preparation. This is a consequence of the capability of Na_2/3MnO₂ to accommodate over‐stoichiometric Mn⁴⁺ ions up to 12.5%. Structural characterization was carried out by XRD powder diffractions, TEM analysis and Raman spectroscopy. The composition and oxidation state of manganese were determined by chemical analysis and magnetic susceptibility measurements. The manganese distribution in the layers was analysed using electron paramagnetic resonance (EPR) spectroscopy. By quenching from 1000 °C, the orthorhombic distorted modification is stabilized. A phase separation into orthorhombic and hexagonal modifications takes place when Na_2/3MnO₂ is slow cooled. The structure changes are concomitant with an increase in the oxidation state of Mn. The over‐stoichiometric Mn⁴⁺ ions are accommodated in the hexagonal modification by creation of vacancies in the MnO₂‐layers.