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.     

Giant Magnetoresistance in the Half‐Metallic Double‐Perovskite Ferrimagnet Mn2FeReO6

The first transition‐metal‐only double perovskite compound, Mn²⁺₂Fe³⁺Re⁵⁺O₆, with 17 unpaired d electrons displays ferrimagnetic ordering up to 520 K and a giant positive magnetoresistance of up to 220 % at 5 K and 8 T. These properties result from the ferrimagnetically coupled Fe and Re sublattice and are affected by a two‐to‐one magnetic‐structure transition of the Mn sublattice when a magnetic field is applied. Theoretical calculations indicate that the half‐metallic state can be mainly attributed to the spin polarization of the Fe and Re sites.     

Fe K‐Edge X‐ray Absorption Fine Structure Determination of γ‐Al2O3‐Supported Iron‐Oxide Species

The structure of FeO_x species supported on γ‐Al₂O₃ was investigated by using Fe K‐edge X‐ray absorption fine structure (XAFS) and X‐ray diffraction (XRD) measurements. The samples were prepared through the impregnation of iron nitrate on Al₂O₃ and co‐gelation of aluminum and iron sulfates. The dependence of the XRD patterns on Fe loading revealed the formation of α‐Fe₂O₃ particles at an Fe loading of above 10 wt %, whereas the formation of iron‐oxide crystals was not observed at Fe loadings of less than 9.0 wt %. The Fe K‐edge XAFS was characterized by a clear pre‐edge peak, which indicated that the Fe?O coordination structure deviates from central symmetry and that the degree of Fe?O?Fe bond formation is significantly lower than that in bulk samples at low Fe loading (<9.0 wt %). Fe K‐edge extended XAFS oscillations of the samples with low Fe loadings were explained by assuming an isolated iron‐oxide monomer on the γ‐Al₂O₃ surface.     

Réduction électrochimique des oxydes de fer: Application à la mesure de non stoechiométrie

Electroreduction of iron oxides Fe₂O₃, Fe₃O₄, Fe_1?xO, inserted in carbon paste electrode is followed by spectrophotometry.Results confirm an overall process of dissolution. The stoechiometry of the iron oxide can be estimated on some 0.1 mg from the charge corresponding to the reduction of Fe(III) and from the quantity of Fe(II) measured by spectrophotometry.     

Transition‐Metal Carbodiimides as Molecular Negative Electrode Materials for Lithium‐ and Sodium‐Ion Batteries with Excellent Cycling Properties

We report evidence for the electrochemical activity of transition‐metal carbodiimides versus lithium and sodium. In particular, iron carbodiimide, FeNCN, can be efficiently used as negative electrode material for alkali‐metal‐ion batteries, similar to its oxide analogue FeO. Based on ⁵⁷Fe Mössbauer and infrared spectroscopy (IR) data, the electrochemical reaction mechanism can be explained by the reversible transformation of the Fe?NCN into Li/Na?NCN bonds during discharge and charge. These new electrode materials exhibit higher capacity compared to well‐established negative electrode references such as graphite or hard carbon. Contrary to its oxide analogue, iron carbodiimide does not require heavy treatments (such as nanoscale tailoring, sophisticated textures, or coating) to obtain long cycle life with current density as high as 9 A g^?1 for hundreds of charge–discharge cycles. Similar to the iron compound, several other transition‐metal carbodiimides M_x(NCN)_y with M=Mn, Cr, Zn can cycle successfully versus lithium and sodium. Their electrochemical activity and performance open the way to the design of a novel family of anode materials.     

Thermodynamic Stability of Transition‐Metal‐Substituted LiMn2−xMxO4 (M=Cr,Fe, Co,and Ni) Spinels

The formation enthalpies from binary oxides of LiMn₂O₄, LiMn_2?xCr_xO₄ (x=0.25, 0.5, 0.75 and 1), LiMn_2?xFe_xO₄ (x=0.25 and 0.5), LiMn_2?xCo_xO₄ (x=0.25, 0.5, and 0.75) and LiMn_1.75Ni_0.25O₄ at 25 °C were measured by high temperature oxide melt solution calorimetry and were found to be strongly exothermic. Increasing the Cr, Co, and Ni content leads to more thermodynamically stable spinels, but increasing the Fe content does not significantly affect the stability. The formation enthalpies from oxides of the fully substituted spinels, LiMnMO₄ (M=Cr, Fe and Co), become more exothermic (implying increasing stability) with decreasing ionic radius of the metal and lattice parameters of the spinel. The trend in enthalpy versus metal content is roughly linear, suggesting a close‐to‐zero heat of mixing in LiMn₂O₄—LiMnMO₄ solid solutions. These data confirm that transition‐metal doping is beneficial for stabilizing these potential cathode materials for lithium‐ion batteries.     

A Reliable Aerosol‐Spray‐Assisted Approach to Produce and Optimize Amorphous Metal Oxide Catalysts for Electrochemical Water Splitting

An aerosol‐spray‐assisted approach (ASAA) is proposed and confirmed as a precisely controllable and continuous method to fabricate amorphous mixed metal oxides for electrochemical water splitting. The proportion of metal elements can be accurately controlled to within (5±5) %. The products can be sustainably obtained, which is highly suitable for industrial applications. ASAA was used to show that Fe₆Ni₁₀O_x is the best catalyst among the investigated Fe‐Ni‐O_x series with an overpotential of as low as 0.286 V (10 mA cm^?2) and a Tafel slope of 48 mV/decade for the electrochemical oxygen evolution reaction. Therefore, this work contributes a versatile, continuous, and reliable way to produce and optimize amorphous metal oxide catalysts.     

Structural‐Distortion‐Driven Magnetic Transformation from Ferro‐ to Ferrimagnetic Iron Chains in B6‐based Nb6FeIr6B8

We report on a structural distortion of kinetically stable B₆‐based ferromagnetic Nb₆FeIr₆B₈ that induces an unprecedented transformation of a ferromagnetic Fe chain into two ferrimagnetic Fe chains through superstructure formation. Density functional theory calculations showed that the ferromagnetic Fe–Fe intrachain interactions found in the undistorted structure become ferrimagnetic in the distorted superstructure, mainly because the two independent iron atoms building each chain interact antiferromagnetically and carry different magnetic moments. High‐temperature SQUID magnetometry confirmed ferrimagnetic ordering at 525 K with a high and negative Weiss constant of ?972 K indicating the presence of strong antiferromagnetic interactions, as predicted. This finding paves the way for the development of low‐dimensional magnetic intermetallic systems based on Heisenberg ferrimagnetic chains, which have previously been studied only in molecular‐based compounds.     

Charge Fluctuations and Ethylene‐Group‐Ordering Transition in β′′‐(BEDT‐TTF)4[(H3O)Fe(C2O4)3]⋅Y Molecular Charge‐Transfer Salts

The polarized infrared reflectance and Raman spectra of the three quasi‐two‐dimensional β′′‐(BEDT‐TTF)₄[(H₃O)Fe(C₂O₄)₃]?Y bifunctional charge‐transfer salts, where BEDT‐TTF=bis(ethylenedithio)tetrathiafulvalene and Y=C₆H₅Br, (C₆H₅CN)_0.17(C₆H₅Br)_0.83, (C₆H₅CN)_0.4(C₆H₅F)_0.6, have been measured as a function of the temperature. Signatures of charge inhomogenity have been found in both Raman and infrared spectra of the β′′‐(BEDT‐TTF)₄[(H₃O)Fe(C₂O₄)₃]?Y superconductors. A 100 K transition to a mixed insulating/metallic state is clearly seen for the first time in the temperature dependence of the electronic spectra of superconducting β′′‐(BEDT‐TTF)₄[(H₃O)Fe(C₂O₄)₃]?C₆H₅Br. We suggest that this phase transition is due to subtle changes in the ethylene groups ordering, which are related to a structural phase transition in the anionic layer. The infrared and Raman spectra of quasi‐two‐dimensional metal α‐′pseudo‐κ′‐(BEDT‐TTF)₄[(H₃O)Fe(C₂O₄)₃]C₆H₄Br₂ are also investigated.     

Synthesis and Structural Analysis of ϵ‐Fe2O3

Powder material of ?‐Fe₂O₃ was obtained by thermal decomposition of the clay mineral nontronite and subsequent isolation of the ferric oxide by leaching the silicate phases. Additionally, crystals of ?‐Fe₂O₃ were grown as precipitates by internal oxidation of a Pd₉₆Fe₄ alloy. Analysis of the precipitate crystals by electron diffraction yields an orthorhombic crystal system and space group Pna2₁ ab initio. X‐ray diffraction data of the powder containing small amounts of Al substituting Fe were refined by the Rietveld method. The refinement yields lattice parameters a = 507.15 pm, b = 873.59 pm and c = 941.78 pm, and atom positions. ?‐Fe₂O₃ is isostructural with κ‐Al₂O₃, AlFeO₃, and GaFeO₃ having an anion stacking sequence /ABAC/, and 1/4 of the cations in tetrahedral co‐ordination. Some strongly distorted FeO₆ octahedrons with one large Fe‐O distance, which may be considered as a 5+1 co‐ordination, appear to be characteristic for ?‐Fe₂O₃. The structure shows elements known from silicates and oxyhydroxides of iron, respectively.     

Mechanism of Iron Oxide Formation from Iron Pentacarbonyl‐Doped Low‐Pressure Hydrogen/Oxygen Flames

A chemical reaction mechanism was developed for the formation of iron oxide (Fe₂O₃) from iron pentacarbonyl (Fe(CO)₅) in a low‐pressure hydrogen–oxygen flame reactor. In this paper, we describe an extensive approach for the flame‐precursor chemistry and the development of a novel model for the formation of Fe₂O₃ from the gas phase. The detailed reaction mechanism is reduced for the implementation in two‐dimensional, reacting flow simulations. The comprehensive simulation approach is completed by a model for the formation and growth of the iron oxide nanoparticles. The exhaustive and compact reaction mechanism is validated using experimental data from iron‐atom laser‐induced fluorescence imaging. The particle formation and growth model are verified with new measurements from particle mass spectrometry.     

An [FeIII34] Molecular Metal Oxide

The dissolution of anhydrous iron bromide in a mixture of pyridine and acetonitrile, in the presence of an organic amine, results in the formation of an [Fe₃₄] metal oxide molecule, structurally characterised by alternate layers of tetrahedral and octahedral Fe^III ions connected by oxide and hydroxide ions. The outer shell of the complex is capped by a combination of pyridine molecules and bromide ions. Magnetic data, measured at temperatures as low as 0.4 K and fields up to 35 T, reveal competing antiferromagnetic exchange interactions; DFT calculations showing that the magnitudes of the coupling constants are highly dependent on both the Fe‐O‐Fe angles and Fe?O distances. The simplicity of the synthetic methodology, and the structural similarity between [Fe₃₄], bulk iron oxides, previous Fe^III–oxo cages, and polyoxometalates (POMs), hints that much larger molecular Fe^III oxides can be made.     

Transition‐Metal‐Mediated Cleavage of a SiSi Double Bond

Reaction of carbene‐stabilized disilicon ( 1 ) with Fe(CO)₅ gives the 1:1 adduct L:Si?Si[Fe(CO)₄]:L (L:=C{N(2,6‐Prⁱ₂C₆H₃)CH}₂) ( 2 ) at room temperature. At raised temperature, however, 2 may react with another equivalent of Fe(CO)₅ to give L:Si[μ‐Fe₂(CO)₆](μ‐CO)Si:L ( 3 ) through insertion of both CO and Fe₂(CO)₆ into the Si₂ core, which represents the first experimental realization of transition metal‐carbonyl‐mediated cleavage of a Si?Si double bond. The structures and bonding of both 2 and 3 have been investigated by spectroscopic, crystallographic, and computational methods.     

Reversible Carrier‐Type Transitions in Gas‐Sensing Oxides and Nanostructures

Despite many important applications of α‐Fe₂O₃ and Fe doped SnO₂ in semiconductors, catalysis, sensors, clinical diagnosis and treatments, one fundamental issue that is crucial to these applications remains theoretically equivocal—the reversible carrier‐type transition between n‐ and p‐type conductivities during gas‐sensing operations. Herein, we present an unambiguous and rigorous theoretical analysis in order to explain why and how the oxygen vacancies affect the n‐type semiconductors α‐Fe₂O₃ and Fe‐doped SnO₂, in which they are both electronically and chemically transformed into a p‐type semiconductor. Furthermore, this reversible transition also occurs on the oxide surfaces during gas‐sensing operation due to physisorbed gas molecules (without any chemical reaction). We make use of the ionization energy theory and its renormalized ionic displacement polarizability functional to reclassify, generalize and explain the concept of carrier‐type transition in solids, and during gas‐sensing operation. The origin of such a transition is associated with the change in ionic polarizability and the valence states of cations in the presence of oxygen vacancies and physisorped gas molecules.     

General Self‐Template Synthesis of Transition‐Metal Oxide and Chalcogenide Mesoporous Nanotubes with Enhanced Electrochemical Performances

The development of a general strategy for synthesizing hierarchical porous transition‐metal oxide and chalcogenide mesoporous nanotubes, is still highly challenging. Herein we present a facile self‐template strategy to synthesize Co₃O₄ mesoporous nanotubes with outstanding performances in both the electrocatalytic oxygen‐evolution reaction (OER) and Li‐ion battery via the thermal‐oxidation‐induced transformation of cheap and easily‐prepared Co‐Asp(cobalt–aspartic acid) nanowires. The initially formed thin layers on the precursor surfaces, oxygen‐induced outward diffusion of interior precursors, the gas release of organic oxidation, and subsequent Kirkendall effect are important for the appearance of the mesoporous nanotubes. This self‐template strategy of low‐cost precursors is found to be a versatile method to prepare other functional mesoporous nanotubes of transition‐metal oxides and chalcogenides, such as NiO, NiCo₂O₄, Mn₅O₈, CoS₂ and CoSe₂.     

Hydrothermal Synthesis,Crystal Structure Determination,and Magnetic Properties of a New Calcium Iron Iridium Hydrogarnet Ca3(Ir2–xFex)(FeO4)2–x(O4H4)1+x with 0 ≤ x ≤ 1

The new calcium iron iridium hydrogarnet Ca₃(Ir_2–xFe_x)(FeO₄)_2–x(H₄O₄)_1+x (0 ≤ x ≤ 1) was obtained by hydrothermal synthesis under strongly oxidizing alkaline conditions. The compound adopts a garnet‐like crystal structure and crystallizes in the acentric cubic space group I4 3d (no. 220) with a = 12.5396(6) Å determined at T = 100 K for a crystal with a refined composition Ca₃(Ir_1.4Fe_0.6)(FeO₄)_1.4(O₄H₄)_1.6. Iridium and iron statistically occupy the octahedrally coordinated metal position, the two crystallographically independent tetrahedral sites are partially occupied by iron. Hydroxide groups are found to cluster as hydrogarnet defects, i.e. partially substituting oxide anions around the empty tetrahedral metal sites. The presence of hydroxide ions was confirmed by infrared spectroscopy and the hydrogen content was quantified by carrier gas hot extraction; the overall composition was verified by energy dispersive X‐ray spectroscopy. The structure model is supported by ⁵⁷Fe‐Mössbauer spectroscopic data evidencing different Fe sites and a magnetic ordering of the octahedral iron sublattice at room temperature. The thermal decomposition proceeds via three steps of water loss and results in Ca₂Fe₂O₅, Fe₂O₃ and Ir. Mössbauer and magnetization data suggest magnetic order at ambient temperature with complex magnetic interactions.     

Stabilization of Low‐Valent Iron(I) in a High‐Valent Vanadium(V) Oxide Cluster

Low‐valent iron centers are critical intermediates in chemical and bio‐chemical processes. Herein, we show the first example of a low‐valent Fe^I center stabilized in a high‐valent polyoxometalate framework. Electrochemical studies show that the Fe^III‐functionalized molecular vanadium(V) oxide (DMA)[Fe^IIIClV^V₁₂O₃₂Cl]³⁻ (DMA=dimethylammonium) features two well‐defined, reversible, iron‐based electrochemical reductions which cleanly yield the Fe^I species (DMA)[Fe^IClV^V₁₂O₃₂Cl]⁵⁻. Experimental and theoretical studies including electron paramagnetic resonance spectroscopy and density functional theory computations verify the formation of the Fe^I species. The study presents the first example for the seemingly paradoxical embedding of low‐valent metal species in high‐valent metal oxide anions and opens new avenues for reductive electron transfer catalysis by polyoxometalates.     

Structural analysis of Gd6FeBi2 from single‐crystal X‐ray diffraction methods and electronic structure calculations

The crystal structure of the gadolinium iron bismuthide Gd₆FeBi₂ has been characterized by single‐crystal X‐ray diffraction data and analyzed in detail using first‐principles calculations. The structure is isotypic with the Zr₆CoAl₂ structure, which is a variant of the ZrNiAl structure and its binary prototype Fe₂P (Pearson code hP9, Wyckoff sequence g f d a). As such, the structure is best viewed as an array of tricapped trigonal prisms of Gd atoms centered alternately by Fe and Bi. The magnetic‐ordering temperature of this compound (ca 350 K) is much higher than that of other rare‐earth metal‐rich phases with the same or related structures. It is also higher than the ordering temperature of many other Gd‐rich ternary phases, where the magnetic exchange is typically governed by Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions. First‐principles calculations reveal a larger than expected Gd magnetic moment, with the additional contribution arising from the Gd 5d electrons. The electronic structure analysis suggests strong Gd 5d–Fe 3d hybridization to be the cause of this effect, rather than weak interactions between Gd and Bi. These details are of importance for understanding the magnetic response and explaining the high ordering temperature in this material.     

Temperature‐Induced Intersite Charge Transfer Involving Cr ions in A‐Site‐Ordered Perovskites ACu3Cr4O12 (A=La and Y)

Changes in the valence state of transition‐metal ions in oxides drastically modify the chemical and physical properties of the compounds. Intersite charge transfer (ISCT), which involves simultaneous changes in the valence states of two valence‐variable transition‐metal cations at different crystallographic sites, further expands opportunities to show multifunctional properties. To explore new ISCT materials, we focus on A‐site‐ordered perovskite‐structure oxides with the chemical formula AA′₃B₄O₁₂, which contain different transition‐metal cations at the square‐planar A′ and octahedral B sites. We have obtained new A‐site‐ordered perovskites LaCu₃Cr₄O₁₂ and YCu₃Cr₄O₁₂ by synthesis under high‐pressure and high‐temperature conditions and found that they showed temperature‐induced ISCT between A′‐site Cu and B‐site Cr ions. The compounds are the first examples of those, in which Cr ions are involved in temperature‐induced ISCT. In contrast to the previously reported ISCT compounds, LaCu₃Cr₄O₁₂ and YCu₃Cr₄O₁₂ showed positive‐thermal‐expansion‐like volume changes at the ISCT transition.     

A Molecular Placeholder Strategy To Access a Family of Transition‐Metal‐Functionalized Vanadium Oxide Clusters

Systematic access to metal‐functionalized polyoxometalates has thus far been limited to lacunary tungsten oxide and molybdenum oxide clusters. The first controlled, stepwise bottom‐up assembly route to metal‐functionalized molecular vanadium oxides is now presented. A di‐vacant vanadate cluster with two metal binding sites, (DMA)₂[V₁₂O₃₂Cl]^3? (DMA=dimethylammonium) is formed spontaneously in solution and characterized by single‐crystal X‐ray diffraction, ESI mass spectrometry, ⁵¹V NMR spectroscopy, and elemental analyses. In the cluster, the metal binding sites are selectively blocked by hydrogen‐bonded DMA placeholder cations. Reaction of the cluster with transition metals TM (Fe³⁺, Co²⁺, Cu²⁺, Zn²⁺) gives access to mono‐functionalized vanadate clusters (DMA)[{TM(L)}V₁₂O₃₂Cl]^n? (L=ligand). Metal binding is accomplished by significant distortions of the vanadium oxide framework reminiscent of a pincer movement. Cluster stability under technologically relevant conditions in the solid‐state and solution is demonstrated.