Interplay between Cu and Fe Valences in BaR(Cu0.5Fe0.5)2O5+δ Double Perovskites with R=Lu, Yb, Y, Eu, Sm, Nd, and Pr

We have conducted a systematic ⁵⁷Fe Mössbauer study on BaR(Cu_0.5Fe_0.5)₂O_5+δ double perovskites with various oxygen contents and rare-earth elements (R=Lu, Yb, Y, Eu, Sm, Nd, and Pr). In samples based on R=Lu, Yb, Y, Eu, Sm the oxygen content remained at δ≈0, upon reductive or oxidative heat treatments under normal pressure. The larger rare-earth elements, i.e. Nd or Pr, readily allowed for continuous oxygen content tuning up to δ≈0.3. By employing high-pressure heat treatments higher oxygen contents were achieved for all samples. The Néel temperature of the samples was found to decrease with increasing amounts of oxygen entering the lattice. In high-pressure oxygenated samples the decrease was less severe indicating that despite the incorporation of oxygen a large amount of Fe still remains in the high-spin trivalent state. By using charge-neutrality arguments together with the relative intensities of the Mössbauer spectral components the average valences of Fe and Cu were obtained. Oxygenation under normal pressure led to a corresponding increase of the valence of Fe, while Cu remained divalent. Upon high-pressure heat treatment equal amounts of Fe³⁺ and Cu²⁺ were found to be oxidized to Fe⁵⁺ and Cu³⁺, respectively.     

Properties of the perovskites, SrMn1−xFexO3−δ (x=1/3, 1/2, 2/3)

The SrMn_1−xFe_xO_3−δ (x=1/3, 1/2, 2/3) phases have been prepared and are shown by powder X-ray and neutron (for x=1/2) diffraction to adopt an ideal cubic perovskite structure with a disordered distribution of transition-metal cations over the six-coordinate B-site. Due to synthesis in air, the phases are oxygen deficient and formally contain both Fe³⁺ and Fe⁴⁺. Magnetic susceptibility data show an antiferromagnetic transition at 180 and 140 K for x=1/3 and 1/2, respectively and a spin-glass transition at 5, 25, 45 K for x=1/3, 1/2 and 2/3, respectively. The magnetic properties are explained in terms of super-exchange interactions between Mn⁴⁺, Fe^(4+δ)+ and Fe⁽³⁺⁾⁺. The XAS results for the Mn-sites in these compounds indicate small Mn-valence changes, however, the Mn-pre-edge spectra indicate increased localization of the Mn-e_g orbitals with Fe substitution. The Mössbauer results show the distinct two-site Fe⁽³⁺⁾+/Fe^(4+δ)+ disproportionation in the Mn- substituted materials with strong covalency effects at both sites. This disproportionation is a very concrete reflection of a localization of the Fe-d states due to the Mn-substitution.     

Syntheses and characterization of zero-dimensional molybdoantimonites, A2(Mo4Sb2O18) (A=Y, La, Nd, Sm, Gd and Dy)

Six new isostructural A₂(Mo₄Sb₂O₁₈) (A=Y, La, Nd, Sm, Gd and Dy) compounds have been synthesized by solid-state reactions and characterized by single crystal X-ray diffraction and spectroscopic techniques. They crystallize in C2/c space group with 4 formula units and contain A³⁺ cations and discrete centrosymmetric anionic (Mo₄Sb₂O₁₈)⁶⁻ aggregates, made of tetrahedral MoO₄ and disphenoidal SbO₄ moieties. They exhibit characteristic Sb³⁺ photoluminescence.     

Syntheses, structure and rare earth metal photoluminescence of new and known isostructural A2Mo4Sb2O18 (A=Ce, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) compounds

Nine new A₂Mo₄Sb₂O₁₈ (A=Ce, Pr, Eu, Tb, Ho, Er, Tm, Yb, Lu) compounds have been synthesized by solid-state reactions. They are isostructural with six reported analogues of yttrium and other lanthanides and the monoclinic unit cell parameters of all fifteen of them vary linearly with the size of A³⁺ ion. Single crystal X-ray structures of eight A₂Mo₄Sb₂O₁₈ (A=Ce, Pr, Eu, Gd, Tb, Ho, Er, Tm) compounds have been determined. Neat A₂Mo₄Sb₂O₁₈ (A=Pr, Sm, Eu, Tb, Dy, Ho, Er, Tm) compounds exhibit characteristic rare earth metal photoluminescence.     

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.     

Manganite charge and orbitally ordered and disordered states probed by Fe substitution into Mn site in , and (, Gd, Sm, Nd, Pr, La)

The layered manganese oxides LnBaMn_1.96Fe_0.04O_y (Ln=Y, Gd, Sm, Nd, Pr, La) have been synthesized for y=5,5.5 and 6. In the oxygen-saturated state (y=6) they exhibit the charge and orbital order at ambient temperature for Ln=Y, Gd, Sm, but unordered e_g-electronic system for Ln=La, Pr, Nd. Fourfold increase of quadrupole splitting was observed owing to the charge and orbital ordering. This is in agreement with the jumplike increase in distortion of the reduced perovskite-like cell for the charge and orbitally ordered manganites compared to the unordered ones. Substitution of 2% of Mn by Fe suppresses the temperatures of structural and magnetic transitions by 20–50 K. Parameters of the crystal lattices and the room-temperature Mössbauer spectra were studied on 40 samples whose structures were refined within five symmetry groups: P4/mmm, P4/nmm, Pm-3m, Icma and P2/m. Overwhelming majority of the Fe species are undifferentiated in the Mössbauer spectra for most of the samples. Such the single-component spectra in the two-site structures are explained by the preference of Fe towards the site of Mn(III) and by the segmentation of the charge and orbitally ordered domains.     

Iron valence in double-perovskite (Ba,Sr,Ca)2FeMoO6: isovalent substitution effect

In the Fe-Mo based B-site ordered double-perovskite, A₂FeMoO_6.0, with iron in the mixed-valence II/III state, the valence value of Fe is not precisely fixed at 2.5 but may be fine-tuned by means of applying chemical pressure at the A-cation site. This is shown through a systematic ⁵⁷Fe Mössbauer spectroscopy study using a series of A₂FeMoO_6.0 [A=(Ba,Sr) or (Sr,Ca)] samples with high degree of Fe/Mo order, the same stoichiometric oxygen content and also almost the same grain size. The isomer shift values and other hyperfine parameters obtained from the Mössbauer spectra confirm that Fe remains in the mixed-valence state within the whole range of A constituents. However, upon increasing the average cation size at the A site the precise valence of Fe is found to decrease such that within the A=(Ba,Sr) regime the valence of Fe is closer to II, while within the A=(Sr,Ca) regime it is closer to the actual mixed-valence II/III state. As the valence of Fe approaches II, the difference in charges between Fe and Mo increases, and parallel with this the degree of Fe/Mo order increases. Additionally, for the less-ordered samples an increased tendency of clustering of the antisite Fe atoms is deduced from the Mössbauer data.     

Investigations of vanadium oxide bronzes θ-(Fe1−yAly)xV2O5

The vanadium oxide bronzes θ-(Fe_1?yAl_y)_xV₂O₅ are Curie-Weiss paramagnets and hopping semiconductors. The samples studied were synthesized by direct solid-state reaction and investigated by the X-ray diffraction, differential thermal analysis, electrical resistivity, magnetic susceptibility, and Mössbauer techniques. The crystal lattice parameters, effective magnetic moments of Fe³⁺ cations, Curie-Weiss temperatures, and the values of ⁵⁷Fe hyperfine interaction parameters were determined. Endothermic effects were observed for some of the samples.     

Electronic state of Fe used as Mössbauer probe in the perovskites LaMO3 (M=Ni and Cu)

For the first time a comparative study of rhombohedral LaNiO₃ and LaCuO₃ oxides, using ⁵⁷Fe Mössbauer probe spectroscopy (1% atomic rate), has been carried out. In spite of the fact that both oxides are characterized by similar crystal structure and metallic properties, the behavior of ⁵⁷Fe probe atoms in such lattices appears essentially different. In the case of LaNi_0.99Fe_0.01O₃, the observed isomer shift (δ) value corresponds to Fe³⁺ (3d⁵) cations in high-spin state located in an oxygen octahedral surrounding. In contrast, for the LaCu_0.99Fe_0.01O₃, the obtained δ value is comparable to that characterizing the formally tetravalent high-spin Fe⁴⁺(3d⁴) cations in octahedral coordination within Fe(IV) perovskite-like ferrates. To explain such a difference, an approach based on the qualitative energy diagrams analysis and the calculations within the cluster configuration interaction method have been developed. It was shown that in the case of LaNi_0.99Fe_0.01O₃, electronic state of nickel is dominated by the d⁷ configuration corresponding to the formal ionic “Ni³⁺-O²⁻” state. On the other hand, in the case of LaCu_0.99Fe_0.01O₃ a large amount of charge is transferred via Cu-O bonds from the O:2p bands to the Cu:3d orbitals and the ground state is dominated by the d⁹L configuration (“Cu²⁺−O” state). The dominant d⁹L ground state for the (CuO₆) sublattice induces in the environment of the ⁵⁷Fe probe cations a charge transfer Fe³⁺+O⁻(L)→Fe⁴⁺+O²⁻, which transforms “Fe³⁺” into “Fe⁴⁺” state. The analysis of the isomer shift value for the formally “Fe⁴⁺” ions in perovskite-like oxides clearly proved a drastic influence of the 4s iron orbitals population on the Fe−O bonds character.     

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%.     

Structure and magnetic properties of the cubic oxide fluoride BaFeO2F

Fluorination of the parent oxide, BaFeO_3−δ, with polyvinylidine fluoride gives rise to a cubic compound with a=4.0603(4) Å at 298 K. ⁵⁷Fe Mössbauer spectra confirmed that all the iron is present as Fe³⁺. Neutron diffraction data showed complete occupancy of the anion sites, indicating a composition BaFeO₂F, with a large displacement of the iron off-site. The magnetic ordering temperature was determined as T_N=645±5 K. Neutron diffraction data at 4.2 K established G-type antiferromagnetism with a magnetic moment per Fe³⁺ ion of 3.95 μ_B. However, magnetisation measurements indicated the presence of a weak ferromagnetic moment that is assigned to the canting of the antiferromagnetic structure. ⁵⁷Fe Mössbauer spectra in the temperature range 10-300 K were fitted with a model of fluoride ion distribution that retains charge neutrality of the perovskite unit cell.     

Anomalous spin state of iron(II) in 57Fe Mössbauer emission spectra of 57Co-Labeled tris(2-methylphenanthroline) cobalt(II) perchlorate

⁵⁷Fe Mössbauer emission spectra of the ⁵⁷Co labeled complex compound [⁵⁷Co(2-CH₃-phen)₃] (ClO₄)₂·2H₂O have been measured as a function of temperature between 293 and 4.6 K. The spectra exclusively show high-spin iron(II) resonances beside a small fraction of an high-spin iron(II) species, whereas the corresponding iron(II) compound is known to exhibit thermally induced high-spin ⁵T_2g(O_h) ? low spin ¹A_1g(O_h) transition. The electronic nature of the anomalous spin state has been found to be ⁵A₁(D₃) by a theoretical treatment of the temperature dependence of the quadrupole splitting. The results are in good agreement with those obtained from Mössbauer absorption measurements on [⁵⁷Fe_0.01Co_0.99(2-CH₃-(phen)₃] (ClO₄)₂·2H₂O.     

Metal valences in electron-doped (Sr,La)2FeTaO6 double perovskite: A Fe Mössbauer spectroscopy study

Substitution of divalent Sr by trivalent La is found to affect the valence states of both of the two B-site cations, Fe and Ta, in the double perovskite oxide (Sr_1−xLa_x)₂FeTaO₆. Moreover, it improves the degree of order of these cations. From ⁵⁷Fe Mössbauer spectra the average Fe valence was found to decrease with increasing La substitution level, x. However, the valence of Fe decreased less than expected if the valence of Ta was assumed to remain constant. Hence, we conclude that also the valence of Ta decreases.     

Local structure and hyperfine interactions of Fe and Sn atoms in brownmillerite-like ferrite Sr2Fe1.98Sn0.02O5+x

Mössbauer spectroscopy has been applied for studying local environment of ⁵⁷Fe and ¹¹⁹Sn probe atoms within tin-doped Sr₂Fe_1.98Sn_0.02O_5+x (x?0.02) ferrite with the brownmillerite-type structure. ⁵⁷Fe Mössbauer spectra indicate no appreciable local distortions induced by the tin dopant atoms. The ¹¹⁹Sn spectra recorded below the magnetic ordering temperature (T_N) can be described as a superposition of two Zeeman sextets, which indicate that Sn⁴⁺ dopant ions are located in two non-equivalent crystallographic and magnetic sites. The observed hyperfine parameters were discussed supposing Sn⁴⁺ cations to replace iron cations in the octahedral (Sn_O) and tetrahedral (Sn_T) sublattices. It has been supposed that Sn⁴⁺ cations being stabilized in the tetrahedral sublattice complete their nearest anion surrounding up to the octahedral oxygen coordination “Sn_T⁴⁺”. Annealing of the Sr₂Fe_1.98Sn_0.02O_5+x in helium flux conditions at 950°C leads to formation of divalent Sn²⁺ cations with a simultaneous decrease of the contribution for the Sn_T⁴⁺ sub-spectrum. The parameters of combined electric and magnetic hyperfine interactions of the ¹¹⁹Sn²⁺ sub-spectrum underline that impurity atoms are stabilized in the sp³d-hybrid state in the oxygen distorted tetragonal pyramid. The analysis of the ¹¹⁹Sn spectra indicates a chemical reversibility of the processes Sn_T²⁺?Sn_T⁴⁺ within the tetrahedral sublattice of the brownmillerite-type ferrite.     

Origin of the quadrupole splitting in the Mössbauer 57Fe spectrum of cubic (disordered) LiFeO2

The room-temperature Mössbauer ⁵⁷Fe spectrum of polycrystalline disordered cubic α-LiFeO₂ contains a quadrupole splitting |Δ_obs| of 0.65(2) mm/sec. This value is relatively large for an Fe atom in an essentially Fe³⁺_HS state. To account for its magnitude, the distribution of the electric-field gradient (EFG) values associated with the Fe atoms was investigated by means of exact geometric analysis involving the 12 nearest cation neighbors (model A) as well as large-scale computer simulation involving more distant cations (models B to E). It is found that (1) the major contribution to |Δ_obs| comes from the distribution of +1 and +3 charges among the 12 nearest cation neighbors of a reference Fe atom; (2) this contribution by itself largely accounts for |Δ_obs|; (3) the contribution from cations beyond the seventh-nearest neighbors is marginal; (4) displacing the oxygen atoms from their lattice sites toward adjacent Fe atoms produces a significant effect on the distribution of EFG values at a reference Fe atom, while incipient cation ordering appears to have a relatively small effect; and (5) the contribution of the EFG = 0 component to model |EFG| distributions will be overemphasized unless cations beyond the first-nearest neighbors are included in the EFG summation. The 144 distinct (up to rotation and reflection) Li¹⁺_12?kFe³⁺_k configurations on the coordination cuboctahedron of nearest cation neighbors (required for the examination of model A) are listed, together with their symmetries and multiplicities, and it is shown that the 144 configurations engender only 17 distinct |EFG| values. Observations are also made on various geometric aspects of calculating EFG at ⁵⁷Fe³⁺_HS on cubic lattices.     

Ln(Fe3+M2+)O4 compounds with layer structure [Ln : Y,Er, Tm,Yb, and Lu, M : Mg,Mn, Co,Cu, and Zn]

A series of new compounds Ln(Fe³⁺M²⁺)O₄ [Ln : Y, Er, Tm, Yb, and Lu, M : Mg, Mn, Co, Cu, and Zn] were successfully synthesized and their lattice constants were determined. These compounds have the same crystal structure as YbFe₂O₄ and Fe³⁺ and M²⁺ are both surrounded by five oxygen ions forming a trigonal bipyramid. The synthetic conditions are presented. They are strongly dependent upon the constituent cations of the compound.     

Study on the valence fluctuation and the magnetism of an iron mixed-valence complex, (n-C4H9)4N[FeIIFeIII(mto)3](mto = C2O3S)

In the case of iron mixed-valence complexes whose spin states are situated in the spin-crossover region, conjugated phenomena coupled with spin and charge are expected. In general, the Fe site coordinated by six S atoms is in the low-spin state, while the Fe site coordinated by six O atoms is in the high-spin state. From this viewpoint, we have synthesized and investigated physical properties for an monothiooxalato-bridged (mto = C₂O₃S) iron mixed-valence complex, (n-C₄H₉)₄N[Fe^IIFe^III(mto)₃], consisting of Fe^IIIO₃S₃ and Fe^IIO₆ octahedra, which behaves as a ferrimagnet with its magnetic transition temperature of T_N = 38 K and Weiss temperature of θ = ?93 K. From the analysis of ⁵⁷Fe Mössbauer spectra of ⁵⁷Fe enriched complexes, (n-C₄H₉)₄N[⁵⁷Fe^IIFe^III(mto)₃] and (n-C₄H₉)₄N[Fe^II57Fe^III(mto)₃], the charge transfer between Fe^II and Fe^III exists in the paramagnetic phase. Considering the time window of ⁵⁷Fe Mössbauer spectroscopy, the time scale of the valence fluctuation is at least slower than 10^?7 s. In order to confirm the valence fluctuation between Fe^II and Fe^III, we investigated the dielectric constant and found an anomalous enhancement attributed to the Fe valence fluctuation between 170 and 250 K.     

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.     

Les oxydes A2BaCuO5 (A = Y,Sm, Eu,Gd, Dy,Ho, Er,Yb)

A series of new phases, A₂BaCuO₅ (A = Y, Sm, Eu, Gd, Dy, Ho, Er, Yb), has been isolated. These compounds are orthorhombic, with $\text{a ? 7.1, b ? 12.2}$, and $\text{c ? 5.6}$Å. The probable space groups deduced from the electron diffraction patterns are Pbnm and Pbn2₁. The structure has been resolved from X-ray powder patterns. The framework can be considered as built up from distorted monocapped trigonal prisms AO₇ which share one triangular face forming A₂O₁₁ blocks. The edge-sharing A₂O₁₁ blocks form a three-dimensional network which delimits cavities where Ba²⁺ and Cu²⁺ are located. Barium is coordinated to 11 oxygen atoms, while the coordination polyhedron of copper is a distorted tetragonal pyramid CuO₅.     

Mixed valence states of Cr and Fe in La1−xSrxFe0.8Cr0.2O3−y

The La_1?xSr_xFe_0.8Cr_0.2O_3?y (x = 0.2, 0.4, 0.6 and 0.8) phases were studied by X-ray photoelectron spectroscopy at room temperature and ⁵⁷Fe Mössbauer spectroscopy at different temperatures. Mixed valence states were observed both for chromium and iron ions, justifying the complex magnetic behaviour exhibited by these compounds. The Mössbauer results indicate the simultaneous presence of Fe³⁺, Fe⁴⁺ and Fe⁵⁺ at 4.2 K and the co-existence of Fe³⁺ and Fe⁽³⁺ⁿ⁾⁺ at T = 293 K, with the latter fraction increasing with increasing strontium content. The presence of Cr^3+/4+ is interpreted as being mainly responsible for the incomplete charge disproportionation reaction of iron at low temperature, as deduced from the Mössbauer results.