NdBaFe2O5+w and steric effect of Nd on valence mixing and ordering of Fe

NdBaFe₂O₅ above and below Verwey transition is studied by synchrotron X-ray powder diffraction and Mössbauer spectroscopy and compared with GdBaFe₂O₅ that adopts a higher-symmetry charge-ordered structure typical of the Sm-Ho variants of the title phase. Differences are investigated by Mössbauer spectroscopy accounting for iron valence states at their local magnetic and ionic environments. In the charge-ordered state, the orientation of the electric-field gradient (EFG) versus the internal magnetic field (B) agrees with experiment only when contribution from charges of the ordered d_xz orbitals of Fe²⁺ is included, proving thus the orbital ordering. The EFG magnitude indicates that only some 60% of the orbital order occurring in the Sm-Ho variants is achieved in NdBaFe₂O₅. The consequent diminishing of the orbit contribution (of opposite sign) to the field B at the Fe²⁺ nucleus explains why B is larger than for the Sm-Ho variants. The decreased orbital ordering in NdBaFe₂O₅ causes a corresponding decrease in charge ordering, which is achieved by decreasing both the amount of the charge-ordered iron states in the sample and their fractional valence separation as seen by the Mössbauer isomer shift. The charge ordering in NdBaFe₂O_5+w is more easily suppressed by the oxygen nonstoichiometry (w) than in the Sm-Ho variants. Also the valence mixing into Fe^2.5+ is destabilized by the large size of Nd. The orientation of the EFG around this valence-mixed iron can only be accounted for when the valence-mixing electron is included in the electrostatic ligand field. This proves that the valence mixing occurs between the two iron atoms facing each other across the structural plane of the rare-earth atoms.     

Verwey Transition under Oxygen Loading in RBaFe2O5+w (R=Nd and Sm)

Double-cell perovskites RBaFe₂O_5+w (R= Nd and Sm) are synthesized in the −0.03<w<0.47 portion of the nonstoichiometry range. The ideal composition with w=0 has di- and tri-valent iron in equal proportions and exhibits a charge-ordering Verwey transition upon cooling, preceded by a weaker premonitory transition. Both transitions are detectable by differential scanning calorimetry. The changes in entropy, volume, orthorhombic distortion and electrical conductivity at the main transition are larger for the variant with the smaller Sm atom than for R=Nd. The discontinuity of the SmBaFe₂O_5+w transition also persists into much higher levels of the oxygen nonstoichiometry w than for the R=Nd variant. Whereas 3D-charge-ordered SmBaFe₂O_5.00 is isostructural with TbBaFe₂O_5.00 adopting space group Pmma, NdBaFe₂O_5.00 undergoes a more profound structural change upon entering the charge-ordered state, the structure of which is refined in the P2₁ma symmetry from high-resolution X-ray synchrotron diffraction data.     

Corrigendum to “EuBaFe2O5+w: Valence mixing and charge ordering are two separate cooperative phenomena” [J. Solid State Chem. 180 (2007) 148-157]

An error in the expression for the Fe²⁺, Fe³⁺ valence-mixing probability in RBaFe₂O_5+w is acknowledged and correct formula is derived. The new formula slightly improves the least-squares fit to the experimental concentrations of the Mössbauer component Fe^2.5+ as a function of the oxygen-nonstoichiometry parameter w for R=Eu.     

XPES and UVPES studies of iron oxides

X-Ray and uv photoelectron spectra of FeO, Fe₂O₃, and Fe₃O₄ have been studied along with those of a few model compounds. It has been possible to assign distinct bands due to Fe²⁺ and Fe³⁺ in the 3d, 3p, 3s, and 2p bands of Fe₃O₄. The spectra of Fe₃O₄ do not show major changes through the Verwey transition.     

Relation of the verwey transition in magnetite to an order-disorder transition of strongly correlated electrons

The principal features of the Verwey transition in magnetite have been simulated by adopting elements of order-disorder theory. In certain limiting cases we obtain the model of Strässler and Kittel which had previously been used to rationalize the electronic and thermodynamic properties of magnetite. According to the present microscopic model the discontinuous Verwey transition in magnetite is driven by a change in a highly correlated electron system with temperature from a charge-ordered small-polaron state associated with local lattice deformations to a disordered state in which electrons resonate between Fe²⁺ and Fe³⁺ ions located on the octahedral cationic sites.     

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.     

Fe-assisted formation of α-Al2O3, starting from sol-gel precursors

The role of Fe³⁺ ions in the transformations from boehmites and pseudoboehmite xerogels via transition aluminas to corundum was studied here. Especially, the active iron species responsible for the decrease of the temperature of transformation to corundum were looked for. To enable the formation of various Fe³⁺ and Fe²⁺ species, samples were subjected to thermal treatments in different atmospheres as well as mechanically activated. Thermal analysis and ESR spectroscopy served to follow the processes and to characterise the resulting products. It was found that (i) isolated Fe³⁺ ions can indicate local structural changes but have (almost) no influence on the temperature of corundum formation, (ii) the temperature of corundum formation decreases in the result of action of small α-Fe₂O₃ particles and (iii) during thermal treatments Fe³⁺ ions are distributed between different phases or precursors thereof: transition aluminas, corundum, Fe₂O₃, and a Fe³⁺ pool.     

Observation of the verwey transition in Fe3O4 by high-resolution electron microscopy

High resolution electron microscopy (HREM) of Fe₃O₄ has been carried out at temperatures near the Verwey transition (∼120 K) with a point resolution of 3 Å. Lattice fringes of both the high- and the low-temperature phases have been observed at these temperatures. The crystal symmetry of the low-temperature phase indicated by the lattice images is consistent with the result obtained by earlier diffraction studies. A series of lattice images showing the transition from the low-temperature phase (to the high-temperature phase) has been obtained. The transformation to the high-temperature phase occurs through the penetration of the high-temperature phase into areas of the low-temperature phase. A quick motion of domain boundaries in the low-temperature phase, which is consistent with almost instantaneous rearrangements of charge ordering, has been observed. The possibility of determining the ordered arrangement of Fe²⁺ and Fe³⁺ ions directly by HREM is discussed.     

Structural and magnetic properties of the quaternary oxides Ba6Ln2Fe4O15 (Ln=Pr and Nd)

The crystal structures and magnetic properties of the quaternary lanthanide oxides Ba₆Ln₂Fe₄O₁₅ (Ln=Pr and Nd) are reported. They crystallize in a hexagonal structure with space group P6₃mc and have the “Fe₄O₁₅ cluster” consisting of one FeO₆ octahedron and three FeO₄ tetrahedra. Measurements of the magnetic susceptibility, specific heat, and powder neutron diffraction reveal that this cluster behaves as a spin tetramer with a ferrimagnetic ground state of S_T=5 even at room temperature. The cluster moments show a long-range antiferromagnetic ordering at 23.2 K (Ln=Pr) and 17.8 K (Nd), and the magnetic moments of the Ln³⁺ ions also order cooperatively. By applying the magnetic field (∼2 T), this antiferromagnetic ordering of the clusters changes to a ferromagnetic one. This result indicates that there exists a competition in the magnetic interaction between the clusters.     

Structure and magnetic order in the series BixRE1−xFe0.5Mn0.5O3 (RE=La,Nd)

The influence of Bi³⁺ on the structural and magnetic properties of the rare-earth-containing perovskites REFe_0.5Mn_0.5O₃ (RE=La,Nd) was studied, and the limit of bismuth substitution was determined to be x≤0.5 in Bi_xRE_1−xFe_0.5Mn_0.5O_3+δ (RE=La,Nd) at ambient pressure. Crystal structures in both La and Nd series were determined to be GdFeO₃-type Pnma with the exception of the Bi_0.3La_0.7Fe_0.5Mn_0.5O₃ sample, which is monoclinic I2/a in the a⁻b⁻b⁻ tilt scheme. The samples undergo a transition to G-type antiferromagnetic order along with a weak ferromagnetic component, mixed with cluster-glass type behavior. The substitution of bismuth into the lattice results in a drop in T_N relative to the lanthanide end-members. Long range ordering temperatures T_N in the range 240-255 K were observed, with a significantly lower ordered magnetic moment in the case of lanthanum (M∼1.7-1.9 μ_B) than in the case of neodymium (M∼2.1 μ_B).     

Infrared studies on the behavior in oxygen of cobalt-substituted magnetites: Comparison with zinc-substituted magnetites

Infrared spectroscopic measurements were carried out on the cobalt-substituted magnetites (Fe³⁺)_A(Co²⁺_xFe²⁺_1?xFe³⁺)_BO^2?₄, pretreated in oxygen, to investigate as a function of temperature the defect phases γ and their transformation to hematite. It has been found that the defect spinels for which x < 0.30 show a partial vacancy ordering on octahedral sites. Referring to the disappearance of the 720-cm^?1 absorption band of the defect phases γ or the appearance of the 470-cm^?1 absorption band of αFe₂O₃, we show that the transition temperature γ → α increases with cobalt substitution. By comparison with zinc-substituted magnetites, the divalent cation distribution is shown to be important to vacancy ordering and to setting the temperature of hematite precipitation.     

Structure and magnetism of rare-earth-substituted Ca3Co2O6

Yttrium- and rare-earth-substituted derivatives of Ca_3−vR_vCo₂O₆ (RY, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Yb, and Lu) have been synthesized and structurally characterized by powder X-ray and neutron diffraction. All phases adopt the K₄CdCl₆-type structure with space group R3¯c), in which the trivalent R³⁺ substituents randomly occupy the Ca²⁺ site. The homogeneity range of Ca_3−vR_vCo₂O₆ extends to v≈0.90 for the substituents concerned. A significant increase in the Co2-O distances within the trigonal-prismatic Co2O₆ co-ordination polyhedra upon introduction of R³⁺ confirms that extra electrons from the R³⁺-for-Ca²⁺ substitution exclusively enter the Co2 site of the quasi-one-dimensional Ca_3−vR_vCo₂O₆ structure, thereby formally reducing its oxidation state. This is furthermore supported by magnetic susceptibility and low-temperature neutron diffraction measurements. The long-range ferrimagnetic ordering temperature is reduced upon R substitution and appears to vanish for v>∼0.30.     

Thermodynamic properties and cation distribution of the ZnFe2O4Fe3O4 spinel solid solutions at 900°C

The thermodynamic properties of the Fe₃O₄ZnFe₂O₄ spinel solid solution were determined at 900°C by the use of the solid electrolyte galvanic cell Fe₂O₃ + Fe₃O₄|O^2?|Fe₂O₃ + Zn_xFe_3?xO₄The activity values obtained exhibit slight negative deviation from the ideal solution model. An analysis of the free energy of mixing of the spinel solid solution provided information on the distribution of cations between the tetrahedral and octahedral sites of the spinel lattice. This is the basis for the estimation of the free energy of formation of pure zinc ferrite from oxides. ΔG⁰_ZnFe2O4 = ?2740 ? 1.6 T cal mole^?1     

A Room‐Temperature Verwey‐type Transition in Iron Oxide,Fe5O6

Functional oxides whose physicochemical properties may be reversibly changed at standard conditions are potential candidates for the use in next‐generation nanoelectronic devices. To date, vanadium dioxide (VO₂) is the only known simple transition‐metal oxide that demonstrates a near‐room‐temperature metal–insulator transition that may be used in such appliances. In this work, we synthesized and investigated the crystals of a novel mixed‐valent iron oxide with an unconventional Fe₅O₆ stoichiometry. Near 275 K, Fe₅O₆ undergoes a Verwey‐type charge‐ordering transition that is concurrent with a dimerization in the iron chains and a following formation of new Fe?Fe chemical bonds. This unique feature highlights Fe₅O₆ as a promising candidate for the use in innovative applications. We established that the minimal Fe?Fe distance in the octahedral chains is a key parameter that determines the type and temperature of charge ordering. This model provides new insights into charge‐ordering phenomena in transition‐metal oxides in general.     

Structural and magnetic study of RFe0.5V0.5O3 (R=Y, Eu, Nd, La) perovskite compounds

B-site disordered RFe_0.5V_0.5O₃ compounds, with R=La, Nd, Eu and Y, have been prepared by solid-state reaction technique and their structures and magnetic properties have been investigated through X-ray powder diffraction, time-of-flight neutron powder diffraction and magnetization measurements at temperatures ranging from 5 to 700 K. The four compounds can be described as distorted perovskites with space group symmetry Pbnm and a⁺b⁻b⁻ tilt system. The studied compounds also show antiferromagnetic ordering with Neel temperatures of 299, 304, 304, and 335 K respectively. The magnetic structures of R=La, Nd and Y compounds were determined from the neutron powder diffraction as G_z with observed magnetic moments of 2.55, 2.54 and 2.69μ_B at 30, 40 and 40 K, respectively.     

Studies on magnetic susceptibility and specific heat for 6H-perovskite-type oxides Ba3LnIr2O9 (Ln=La, Nd, Sm-Yb)

Magnetic properties of the 6H-perovskite-type oxides Ba₃LnIr₂O₉ (Ln=La and Nd: monoclinic; Ln=Sm-Yb: hexagonal symmetry) were investigated. For all the title compounds, a specific heat anomaly was found at 5.3-17.4 K. At the corresponding temperatures, the magnetic susceptibilities show a slight variation in its gradient. These magnetic anomalies suggest the magnetic ordering of the magnetic moments (S=1/2) remaining in the Ir^4.5+₂O₉ face-shared bioctahedra. In addition, the Ln³⁺ ions show the onset of the antiferromagnetic ordering around these temperatures. The Ba₃NdIr₂O₉ only shows a ferromagnetic behavior below 17.4 K with a remnant magnetization of 1.25 μ_B. This behavior may be due to the ferromagnetic ordering of the Nd³⁺ moments.     

Crystal growth and structure of a new manganese vanado-antimonate MnVSbO6

Single crystals of a new compound of formula MnVSbO₆ were grown by slow cooling from a V₂O₅-B₂O3₃ flux at 900°C. The compound crystallizes in the orthorhombic space group Pbcn (No. 60), with cell parameters (in the Pcnb setting) a=4.6604(3) Å, b=4.9603(3) Å, c=17.1433(9)Å, Z=4. The crystal structure was solved from 1188 independent reflections to R_w=3.20% and goodness-of-fit 1.5 for 44 refined parameters. The structure can be described as a superstructure of the α-PbO₂ type with a cation ordering similar to that found in Fe₂WO₆. Cations occupy octahedral sites in the PbO₂-like layers. Zigzag chains of edge-sharing MnO₆ octahedra alternate with mixed Sb/V chains following a -Mn-Sb/V-Sb/V- sequence. The magnetic susceptibility of MnVSbO₆ follows the Curie-Weiss law down to ca. 15 K, where it orders antiferromagnetically. The bond lengths and Curie constant are consistent with the expected charge distribution Mn²⁺V⁵⁺Sb⁵⁺O₆.     

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.     

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⁺.     

Elucidation of methyl tert-butyl ether degradation with Fe/H2O2 by purge-and-trap gas chromatography-mass spectrometry

Degradation of methyl tert-butyl ether (MTBE) with Fe²⁺/H₂O₂ was studied by purge-and-trap gas chromatography-mass spectrometry. MTBE was degraded 99% within 120 min under optimum conditions. MTBE was firstly degraded rapidly based on a Fe²⁺/H₂O₂ reaction and then relatively slower based on a Fe³⁺/H₂O₂ reaction. The dissolved oxygen decreased rapidly in the Fe²⁺/H₂O₂ reaction stage, but showed a slow increase in the Fe³⁺/H₂O₂ reaction stage. tert-Butyl formate, tert-butyl alcohol, methyl acetate and acetone were identified as primary degradation products by mass spectrometry. A preliminary reaction mechanism involving two different pathways for the degradation of MTBE with Fe²⁺/H₂O₂ was proposed. This study suggests that degradation of MTBE can be achieved using the Fe²⁺/H₂O₂ process.