High-spin Fe3 Research Articles

Activation of Strong π-Acids at [Fe4S4]+ Clusters Enabled by a Noncanonical Electronic Structure.

Although Fe-S clusters are privileged metallocofactors for the reduction of N2, CO, and other π-acidic substrates, their constituent metal ions─high-spin Fe2+ and Fe3+─are typically not amenable to binding and activating strong π-acids. Here, we demonstrate that [Fe4S4]+ clusters can overcome this limitation by adopting a noncanonical electronic structure. Specifically, we report the synthesis and characterization of a series of 3:1 site-differentiated [Fe4S4]+ clusters in which the unique Fe site is bound by one of 10 electronically variable arylisocyanide ligands. Rather than being continuously tuned as a function of the arylisocyanides' electronic properties (e.g., as quantified by linear free energy relationships), the structures of the clusters are divided into two groups: (i) those with moderately π-acidic isocyanides, which adopt a "typical" structure characterized by standard bond metrics and geometric distortions from tetrahedral symmetry, and (ii) those with more strongly π-acidic isocyanides, which adopt a "contracted" structure with an unusually symmetric geometry and a compressed cluster core. Computational studies revealed that although the "typical" structure has a canonical electronic structure, the "contracted" structure has a noncanonical arrangement of spin density, with a full complement of π-backbonding electrons and more substantial Fe-Fe delocalization. These features of the "contracted" structure enable substantial C≡N bond weakening of the strongest π-acceptors in the series. More generally, the experimental characterization of the "contracted" electronic isomer suggests that other noncanonical electronic structures of Fe-S clusters remain to be discovered.

Low-Spin Fe3+ Evoked by Multiple Defects with Optimal Intermediate Adsorption Attaining Unparalleled Performance in Water Oxidation.

Electrocatalytic water splitting is long constrained by the sluggish kinetics of anodic oxygen evolution reaction (OER), and rational spin-state manipulation holds great promise to break through this bottleneck. Low-spin Fe3+ (LS, t2g 5eg 0) species are identified as highly active sites for OER in theory, whereas it is still a formidable challenge to construct experimentally. Herein, a new strategy is demonstrated for the effective construction of LS Fe3+ in NiFe-layered double hydroxide (NiFe-LDH) by introducing multiple defects, which induce coordination unsaturation over Fe sites and thus enlarge their d orbital splitting energy. The as-obtained catalyst exhibits extraordinary OER performance with an ultra-low overpotential of 244 mV at the industrially required current density of 500 mA cm-2, which is 110 mV lower than that of the conventional NiFe-LDH with high-spin Fe3+ (HS, t2g 3eg 2) and superior to most previously reported NiFe-based catalysts. Comprehensive experimental and theoretical studies reveal that LS Fe3+ configuration effectively reduces the adsorption strength of the O* intermediate compared with that of the HS case, thereby altering the rate-determining step from (O* → OOH*) to (OH* → O*) of OER and lowering its reaction energy barrier. This work paves a new avenue for developing efficient spin-dependent electrocatalysts for OER and beyond.

Resolving Structures of Paramagnetic Systems in Chemistry and Materials Science by Solid-State NMR: The Revolving Power of Ultra-Fast MAS.

Ultra-fast magic-angle spinning (100+kHz) has revolutionized solid-state NMR of biomolecular systems but has so far failed to gain ground for the analysis of paramagnetic organic and inorganic powders, despite the potential rewards from substantially improved spectral resolution. The principal blockages are that the smaller fast-spinning rotors present significant barriers for sample preparation, particularly for air/moisture-sensitive systems, and are associated with low sensitivity from the reduced sample volumes. Here, we demonstrate that the sensitivity penalty is less severe than expected for highly paramagnetic solids and is more than offset by the associated improved resolution. While previous approaches employing slower MAS are often unsuccessful in providing sufficient resolution, we show that ultra-fast 100+kHz MAS allows site-specific assignments of all resonances from complex paramagnetic solids. Combined with more reliable rotor materials and handling methods, this opens the way to the routine characterization of geometry and electronic structures of functional paramagnetic systems in chemistry, including catalysts and battery materials. We benchmark this approach on a hygroscopic luminescent Tb3+ complex, an air-sensitive homogeneous high-spin Fe2+ catalyst, and a series of mixed Fe2+/Mn2+/Mg2+ olivine-type cathode materials.

Impurity Ions Mn<sup>2+</sup> and Fe<sup>3+</sup> as Paired Spin Labels for the Study of Structural Transformations in Phyllosilicates by the ESR Method

Impurity paramagnetic ions Mn2+ and high spin Fe3+ (S = 5/2) are shown to be very informative “paired spin labels” to investigate structural transformations in natural aluminosilicate clay minerals by ESR spectroscopy. Second derivative ESR (SD ESR) enables to detect minor narrow lines of the ions against the background of intense broad lines of other paramagnetic impurities. Complex SD ESR spectra of the ions are explained by the Jahn-Teller effect and hyperfine interactions with OH-groups. SD ESR spectra before and after heating (620°C and 900°C) proved transformations of octahedral crystal cells accompanied by the loss of the OH-groups, displacement of the ions to equivalent positions.

Low-Coordinate Iron Hydride Chemistry at an N,N,C-Heteroscorpionate Platform.

Locally high-spin iron hydrides are proposed to play a critical role as intermediates in iron-molybdenum cofactor (FeMoco)-catalyzed N2 fixation. Inspired by these biological systems, we report herein our initial investigations into low-coordinate iron hydride chemistry supported by our N,N,C-heteroscorpionate ligands. Those ligands with smaller steric profiles are unable to completely suppress the formation of a binuclear [Fe(μ2-H)]2 complex; however, the incorporation of more substantial steric bulk allows for the isolation of a rare example of a terminal, high-spin (S = 2) Fe2+ hydride. Fourier transform infrared spectroscopy suggests an unusually weak Fe-H bond in the latter molecule. Mössbauer spectroscopies, coupled with density functional theory calculations, highlights the substantial influence of the terminal hydride ligand on 57Fe isomer shift.

Terminal Hydride Complex of High-Spin Mn.

The iron-molybdenum cofactor of nitrogenase (FeMoco) catalyzes fixation of N2 via Fe hydride intermediates. Our understanding of these species has relied heavily on the characterization of well-defined 3d metal hydride complexes, which serve as putative spectroscopic models. Although the Fe ions in FeMoco, a weak-field cluster, are expected to adopt locally high-spin Fe2+/3+ configurations, synthetically accessible hydride complexes featuring d5 or d6 electron counts are almost exclusively low-spin. We report herein the isolation of a terminal hydride complex of four-coordinate, high-spin (d5; S = 5/2) Mn2+. Electron paramagnetic resonance and electron-nuclear double resonance studies reveal an unusually large degree of spin density on the hydrido ligand. In light of the isoelectronic relationship between Mn2+ and Fe3+, our results are expected to inform our understanding of the valence electronic structures of reactive hydride intermediates derived from FeMoco.

High-field/high-frequency electron spin resonances of Fe-doped β−Ga2O3 by terahertz generalized ellipsometry: Monoclinic symmetry effects

We demonstrate detection and measurement of electron paramagnetic spin resonances (EPR) of iron defects in β−Ga2O3 utilizing generalized ellipsometry at frequencies between 110 and 170 GHz. The experiments are performed on an Fe-doped single crystal in a free-beam configuration in reflection at 45∘ and magnetic fields between 3 and 7 T. In contrast with low-field, low-frequency EPR measurements, we observe all five transitions of the s=5/2 high-spin state Fe3+ simultaneously. We confirm that ferric Fe3+ is predominantly found at octahedrally coordinated Ga sites. We obtain the full set of fourth-order monoclinic zero-field splitting parameters for both octahedrally and tetrahedrally coordinated sites by employing measurements at multiple sample azimuth rotations. The capability of high-field EPR allows us to demonstrate that simplified second-order orthorhombic spin Hamiltonians are insufficient, and fourth-order terms as well as consideration of the monoclinic symmetry are needed. These findings are supported by computational approaches based on density-functional theory for second-order and on ligand-field theory for fourth-order parameters of the spin Hamiltonian. Terahertz ellipsometry is a way to measure spin resonances in a cavity-free setup. Its possibility of varying the probe frequency arbitrarily without otherwise changing the experimental setup offers unique means of truly disentangling different components of highly anisotropic spin Hamiltonians. Published by the American Physical Society 2024

Magnetic suppression for a possible Fe-poor organic–inorganic hybrid superconductor Fe14Se16(tepa)0.8 (tepa = tetraethylenepentamine) with a superconducting transition at ∼42 K

Composition/structure-dependent superconductivity for FeSe-based superconductors attracted great attention not only due to their high superconducting transition temperatures (TC), but also for understanding the origin of iron-based superconductivity. Here, we report a new Fe-poor organic-inorganic hybrid material Fe14Se16(tepa)0.8 with a paramagnetic-diamagnetic transition at ∼42 K grown by a high-temperature organic-solution-phase method with soluble iron/selenium sources in a tepa solution, alternative to previous intercalation strategies. The Fe14Se16(tepa)0.8 phase is in a tetragonal layered hybrid structure with a nanoplate shape. Composition analyses reveal a Fe-poor characteristic of the hybrid in contrast to previous FeSe-intercalated superconductor, and selected area electron diffraction pattern is featured by Fe3Se4 superstructures with a √2 × √2 of Fe vacancy order. Ab initio density functional calculations show that minus Fe3Se4 ions are stable in the hybrid and ∼0.25e−/Fe0.75Se is obviously larger than the reported values of approximately 0.2e−/FeSe in other FeSe-intercalated superconductors. Typical hysteresis loops and temperature dependence of dc/ac susceptibilities of the Fe14Se16(tepa)0.8 measured below ∼42 K suggest a presence of the Meissner effect in this material. Effects of synthesis conditions on structures and magnetic properties of the hybrids show a magnetic evolution from a long-range ferrimagnetic (FIM) order of Fe14Se16(tepa) to a coexistence of FIM and superconducting (SC) orders of Fe14Se16(tepa)0.9 and an SC order of Fe14Se16(tepa)0.8. X-ray absorption spectrum (XAS) confirms the presence of ferric/ferrous irons. Mössbauer studies reveal that the high-TC superconductivity originates from a suppression of the FIM order through tuning the spin states of irons from high-spin Fe3+ (S = 5/2) and Fe2+ (S = 2) in the Fe14Se16(tepa) to low-spin Fe3+ (S = 1/2) and Fe2+ (S = 0) in the Fe14Se16(tepa)0.8. Although no zero resistance is detected even at a temperature of 2 K, the resistivity at 2 K decreases by more than 1600 times compared to that in a normal state calculated by a variable range hopping (VRH) model, suggesting that the high-TC superconductivity of Fe14Se16(tepa)0.8 is possible.

Constructing FeNiPt@C Trifunctional Catalyst by High Spin-Induced Water Oxidation Activity for Zn-Air Battery and Anion Exchange Membrane Water Electrolyzer.

Developing cost-efficient trifunctional catalysts capable of facilitating hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) activity is essential for the progression of energy devices. Engineering these catalysts to optimize their active sites and integrate them into a cohesive system presents a significant challenge. This study introduces a nanoflower (NFs)-like carbon-encapsulated FeNiPt nanoalloy catalyst (FeNiPt@C NFs), synthesized by substituting Co2+ ions with high-spin Fe2+ ions in Hofmann-type metal-organic framework, followed by carbonization and pickling processes. The FeNiPt@C NFs catalyst, characterized by its nitrogen-doped carbon-encapsulated metal alloy structure and phase-segregated FeNiPt alloy with slight surface oxidization, exhibits excellent trifunctional catalytic performance. This is evidenced by its activities in HER (-25mV at 10mA cm-2), ORR (half-wave potential of 0.93V), and OER (294mV at 10mA cm-2), with the enhanced water oxidation activity attributed to the high-spin state of the Fe element. Consequently, the Zn-air battery and anion exchange membrane water electrolyzer assembled by FeNiPt@C NFs catalyst demonstrate remarkable power density (168mW cm-2) and industrial-scale current density (698mA cm-2 at 1.85V), respectively. This innovative integration of multifunctional catalytic sites paves the way for the advancement of sustainable energy systems.

Chemical synthesis of Mn–Zn magnetic ferrite nanoparticles: Effect of secondary phase on extrinsic magnetic properties of Mn–Zn ferrite nanoparticles

This paper presents a comprehensive investigation of the magnetic properties of co-precipitated MnxZn1–xFe2O4 nanoferrites, where x takes the values of 0.5, 0.6, and 0.7. X-ray diffraction patterns indicated the presence of spinel phase. However, the ferrite composition x = 0.6 contains little trace of secondary phase α-Fe2O3. The occurrence of cation redistribution in the current ferrite samples can be concluded from the deviation of oxygen position parameter (U) to that of optimum value (0.375). The bond angles facilitate the reduction of the A–B interaction and the increase of the B–B interaction. With the substitution of Mn2+ ions, the saturation magnetization (Ms) exhibits both drops and increases whereas the coercivity (Hc) consistently declines. The ferrite composition x = 0.7 exhibits the maximum value of Ms and is equal to 27.9 emu/g. The blocking temperature (TB) rises in proportion to the degree of Mn2+ ion doping. The observation of quadruple doublets in the Mӧssbauer spectra indicates that the ferrite nanoparticles exist in single domain state exhibiting the quadruple interactions. The range of isomer shift (δ) 0.199–0.267 mm/s indicates the existence of exclusively high spin Fe3+ ions. Two paramagnetic doublets are anticipated to exist as a result of the core and shell of present ferrite nanoparticles. In the core-shell morphology of nanoparticles, the core is characterized by larger quadruple splitting (Δ) while the shell is characterized by smaller Δ. The results are integrated in terms of magnetocrystalline anisotropy and secondary phase presuming the core-shell structure of nanoparticles.

Slow magnetization relaxation of a high-spin iron(iii) in mayenite Ca12Al14O33

High-spin Fe3+ ion imbedded in the extended solid displays field-induced slow relaxation of magnetization with enhanced relaxation time values.

Hydrothermal synthesis and structure of ferric molybdates from sodium carbonate solutions

Several new iron molybdates have been synthesized under hydrothermal conditions at relatively low temperatures (200 °C) to stabilize the Fe3+ oxidation state. The mineralizers are all sodium ion based and three new phases were isolated as high quality single crystals from various reaction stoichiometries, Fe(MoO4)(OH) (I), Na0·15Fe2Mo3O12(H2O)·0.25H2O (IIa), and NaFe(MoO4)2(H2O) (III). The structures contain iron oxide octahedra arranged in chains (I), dimers (IIa), and isolated octahedra (III), that are linked by various molybdate oxyanions to form 3-D framework (I), 3-D framework with channels (IIa), and 2-D sheets in their iron molybdate motifs. Both I and III are lightly colored materials containing valence precise Fe3+, whereas IIa contains partially occupied Na + ions in columns within the lattice leading to concomitant valence mixing of Fe2+/Fe3+ with a corresponding darker color to the crystal. If sodium ions are not included in the synthesis and instead replaced by smaller Li + ions in the reaction, there is no alkali metal incorporation in the lattice and the resultant compound Fe2Mo3O12(H2O)·0.25H2O (IIb) is formed. This is isostructural to IIa but without the partially occupied Na + ions in the channels which makes it another valence precise Fe3+ compound that is nearly colorless as expected since all transtions are spin forbidden in the high spin Fe3+ case. The role of carbonate as a mineralizer was examined since the reaction is highly acidic (pH ∼ 1) in the absence of carbonate, and this does not lead to satisfactory product formation. The carbonate mineralizer increases the pH to 6–6.5, and this pH is maintained throughout the reaction, which does lead to high quality crystal products. Despite its absence in any of the products, the carbonate mineralizer plays an additional important (albeit unknown) role in the synthesis, because if simple hydroxide is used to increase the pH to 6–6.5, the resultant products are much lower in quality and yield.

Crystal structure, magnetic and Mössbauer studies of DyFexMn1-xO3 multiferroic manganites

The influence of iron replacement on the structural and magnetic properties of Dysprosium manganites was studied using the series of DyFexMn1-xO3 (x = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0) compounds. All samples were fabricated by means of the modified auto-combustion sol-gel method, followed by the ball milling technique. Orthorhombic structure (Pnma space group) with high phase purity and crystallinity featured all samples, according to X-ray diffraction analysis. The temperature dependence of magnetization curves indicated antiferromagnetic (AFM) to paramagnetic (PM) transition with the increase in temperature. The Néel temperature (TN) was observed to shift to higher temperatures, whereas the spin reorientation transition temperature (TSR) reduced as Fe3+ content increased. Cusps in the zero-field-cooled (ZFC) curves were detected at low temperatures (around 4 K) as an indication of the ordering of the Dy spin. 57Fe Mӧssbauer spectra in samples with low iron content were dominated by a high spin (Fe3+) paramagnetic doublet at both 295 K and 78 K, while the distribution in the hyperfine magnetic field accounted for the appearance of the broadening magnetic peaks. High iron samples revealed a magnetic sextet with fit-resolved patterns that indicated a strong AFM coupling of Fe3+ ions. The hyperfine magnetic field (Bhf) magnitude demonstrated a linear increase with the increase in iron doping.

Evaluation of Maturity (Late Diagenesis) in Source Rocks of Jaisalmer Basin in Rajasthan, India using 57Fe Mössbauer Spectroscopy

Abstract The potential of a source rock for hydrocarbon generation depends on several factors such as the amount, type and maturity level of the organic matter present in it as well as the redox conditions in which the sediments containing the source rocks were deposited. Organic-rich sediments deposited in reducing environment have better prospects for a good quality source rock as compared to those deposited in oxidizing environments. The information about the oxidizing/reducing environment at the time of deposition can be obtained from the presence or absence of certain iron-bearing minerals in sediments. The presence of oxide minerals goethite (α-FeOOH) and/or hematite (α-Fe2O3) indicates that sediments were deposited in an oxidizing environment. Therefore the study of iron-bearing minerals in sediments can be used to understand the qualities of the source rocks. Sedimentary samples were collected from different depths of three wells BT-3, DND-1 and LNR-1 located in different structures of the Jaisalmer basin were investigated to understand the chemical state of iron by using Mössbauer spectroscopy. In most of the samples, iron was found mainly in the form of pyrite (FeS2), siderite (Fe, Mg) CO3, 2:1 layer silicates in high spin Fe3+ and high spin Fe2+ state and (Ca, Fe, Mg) carbonates. In the samples from Lathi and Shumarwali formations that belong to the lower Jurassic and Triassic sequences, α-Fe2O3 (hematite) was indeed observed. It was present in a narrow sedimentary depth interval in the wells BT-3 and DND-1 and in a much broader thickness in well LNR-1. The other sedimentary sequences younger than the Lower Jurassic sediments have not shown the presence of any oxide mineral. The present study indicates that oxidizing environment was present in the entire basin in the Lower Jurassic era at least for some duration. However, the degree of oxidation was not uniform as evident from the distribution pattern of iron oxide mineral (α-Fe2O3). Mössbauer studies of source rocks can also be used as a fingerprint to get information about the thermal history (Palaeo-temperature) of sedimentary sequence which is crucial in hydrocarbon prospecting.

Magnetic properties of an oxo-bridged dinuclear iron(III) complex resulting from the oxidation of mononuclear iron metallacycles with a flexible ethylenediphenylenebis(oxamate) ligand

Transition metal ions such as copper(II), nickel(II), cobalt(II), and manganese(II) are widely used in the formation of mono- or oligonuclear metallacyclic complexes with the flexible N,N’-2,2′-ethylenediphenylenebis(oxamate) ligand (H4edpba); however similar supramolecular complexes with iron(II) and iron(III) are rarely obtained. In this work, we synthesized for the first time a new air-stable oxo-bridged diiron(III) complex of formula {[Fe(H2edpba)(dmso)]2(μ-O)}·dmso·H2O (2) from dioxygen oxidation of the putative mononuclear iron(II) complex precursor of formula [Fe(H2edpba)(H2O)2]Cl (1) in dimethylsulfoxide (dmso). Compound 2 crystallizes in the monoclinic system and contains one oxo group bridging two cationic mononuclear metallacyclic moieties, [Fe(H2edpba)(dmso)]+. The iron(III) ions are also coordinated to four oxygen atoms from the dianionic H2edpba2− ligands in a tetradentate fashion, while an oxygen atom from the coordinated dmso molecules. Mössbauer spectroscopy confirms the presence of high-spin (HS) Fe3+ ions (2). The magnetic properties of 2 reveal a strong antiferromagnetic interaction between the two HS Fe3+ ions (J = −226.46 cm−1), in agreement with the DFT calculations (J = −207.99 cm−1).

Magnetic studies of Mn2+substituted Zn-ferrite nanoparticles: Role of secondary phases, bond angles and magnetocrystalline anisotropy

Magnetic studies of MnxZn1–xFe2O4 (x = 0.5, 0.6, 0.7) nanoferrite particles prepared by sol-gel auto combustion method are presented. The deviation of the oxygen positional parameter (U) from the ideal value of 0.375 is an indicator for the cation redistribution in the present ferrite systems. The variation of bond angles estimated from the cation distribution seemed to be strengthening of A–B superexhange interaction. The nature of M − H loops revealed that the present ferrite nanoparticles are in the single domain state showing superparamagnetism. The saturation magnetization (Ms) increases while coercivity (Hc) decreases with the substitution of Mn2+ ion concentration. The highest value of Ms = 25 emu/g was reported for the composition x = 0.7. It was observed that the blocking temperature (TB) is decreasing and again increasing with the doping level of Mn2+. From FC-ZFC curves, the uneven variation of magnetization (M) at 10 K can be found. It attributes to the cation distribution in the ferrite samples at 10 K is different from those of at 300 K. The Mӧssbauer spectra revealed that the ferrite nanoparticles exhibited both quadruple and hyperfine interactions can be found in the composition x = 0.7 while the ferrite nanoparticles exhibited that only quadruple interaction can be in the compositions x = 0.5 and 0.6. The isomer shift (δ) values showed the presence of high spin Fe3+ ions only. The paramagnetic doublets having higher value of quadruple splitting (Δ) are assigned to the core structure while doublets lower value of quadruple splitting (Δ) is assigned to shell structure. The present results are incorporated in terms of cation distribution, magnetocrystalline anisotropy considering the core-shell morphology of ferrite nanoparticles.

Facile synthesis and selected characteristics of two-dimensional material composed of iron sulfide and magnesium-based hydroxide layers (tochilinite)

We report the reliable synthesis of 2D iron sulfide-magnesium hydroxide nanoflakes. The sulfide and hydroxide sheets assemble via opposite electric charges. Comparable amounts of high-spin Fe3+ and Fe2+ centers occur in the sulfide layers.

Синтез, структурные и магнитные свойства NaZnFe-=SUB=-2-=/SUB=-(VO-=SUB=-4-=/SUB=-)-=SUB=-3-=/SUB=-

A new magnetic compound NaZnFe2(VO4)3 obtained by solid-phase synthesis has been studied using X-ray diffraction, Mössbauer spectroscopy, electron paramagnetic resonance, measurement of the temperature dependence of the dielectric permeability, and magnetometry. The crystalline structure of NaZnFe2(VO4)3 is described by a triclinic spatial group of symmetry P1 with the parameters of an elementary crystalline chain: a = 6.74318 (7) Å, b = 8.1729 (1) Å, c = 9.8421 (1) Å, α = 106.2611 (9)º, β = 104.55 (1)º, γ = 102.337 (1)º, V = 479.88 (1) Å3, Z = 2. Magnetic Fe3+ cations in the cell occupy six positions populated together with diamagnetic Zn2+ cations, which leads to states of magnetic inhomogeneity and local violation of charge neutrality. Data from resonance and magnetic studies of NaZnFe2(VO4)3 confirm the main role of high-spin Fe3+ iron cations in the formation of magnetism with competing exchange magnetic interactions and a high value of the frustration index. It is shown that the magnetic subsystem of a sample with a negative asymptotic Neel temperature undergoes a magnetic transition from the paramagnetic state to the magnetic state of the spin glass when the temperature decreases. T.V. Drokina 1, M.S. Molokeev M.C. 1, 2, D.A. Velikanov 1, O.A. Bayukov 1, A.M. Vorotynov 1, A.L. Freidman 1, G.A. Petrakovskii 1 1 L.V. Kirensky Institute of Physics, Federal Research Center KSC Siberian Branch of Russian Academy of Science, Krasnoyarsk, 660036, Russia 2 Siberian Federal University, Krasnoyarsk, 660074, Russia

Pechini citrate-gel synthesised In3+ substituted X-type barium zinc hexaferrites: An investigation on structural, optical, magnetic and dielectric properties

Indium substituted X-type Ba2Zn2InxFe28-xO46 (0.0≤x≤2.0) ferrites were synthesised by a citrate-gel aqueous combustion technique, and heated at 1300 °C. XRD, FTIR, SEM with EDX, magnetic hysteresis, Mössbauer spectroscopy, UV–visible spectroscopy, and dielectric measurements up to 20 GHz have been carried out to study the modification of the structural, morphological, magnetic, optical and frequency dependant dielectric properties when indium is introduced, replacing iron in the lattice of X-type hexaferrites. XRD analysis reveals formation of a major X-type phase in all compositions. The systematic rise in lattice constants (a, c) along with cell volume (V) confirm the replacement of iron by indium. Magnetic hysteresis loops reveal a soft ferrite nature with low Mr/MS values, suggesting a multidomain structure in all samples. The HC values decrease from 22.22 kA m-1 to 12.18 kA m−1 (279 Oe to 153 Oe) with increasing indium, while magnetisation remained high between 56 A m2 /kg and 62 A m2/kg for all samples. Room temperature Mӧssbauer spectra of all samples were fitted with six Zeeman sextets of five different magnetic sublattices. It was ascertained that In-substitution reduces the Fe population in the k (spin up) sublattice, resulting in a reduction in magnetisation. The average hyperfine magnetic field <Hf> is also reported to decrease with Indium substitution for all samples. The value of the isomer shift (δ) confirmed that the iron-ions are in high spin Fe3+ state. The complex impedance and electric modulus plots reveal non-Debye type relaxation in all samples at 100 Hz to 2 MHz.

The Elusive Mononitrosylated [Fe4 S4 ] Cluster in Three Redox States.

Iron-sulfur clusters are well-established targets in biological nitric oxide (NO) chemistry, but the key intermediate in these processes-a mononitrosylated [Fe4 S4 ] cluster-has not been fully characterized in a protein or a synthetic model thereof. Here, we report the synthesis of a three-member redox series of isostructural mononitrosylated [Fe4 S4 ] clusters. Mononitrosylation was achieved by binding NO to a 3 : 1 site-differentiated [Fe4 S4 ]+ cluster; subsequent oxidation and reduction afforded the other members of the series. All three clusters feature a local high-spin Fe3+ center antiferromagnetically coupled to 3 [NO]- . The observation of an anionic NO ligand suggests that NO binding is accompanied by formal electron transfer from the cluster to NO. Preliminary reactivity studies with the monocationic cluster demonstrate that exposure to excess NO degrades the cluster, supporting the intermediacy of mononitrosylated intermediates in NO sensing/signaling.