High-spin Fe3 Research Articles

Hydrothermal synthesis of ordered corkite, [formula omitted], a S = 5/2 kagomé antiferromagnet

Corkite, PbFe3(PO4)(SO4)(OH)6, an understudied relative of the jarosite family of Heisenberg antiferromagnets, has been synthesized and its magnetic properties characterized for the first time. Relative to natural samples, synthetic corkite displays signatures in both infrared and Raman spectra of a more ordered arrangement of polyanion groups about the kagomé sublattice that retains inversion symmetry. Magnetic susceptibility measurements reveal that corkite undergoes a transition to a long-range, antiferromagnetically-ordered state below TN ​= ​48 ​K, lower than that observed in the majority of jarosite phases, and indicative of further spin frustration. Curie-Weiss fitting of the measured magnetic susceptibility yields an effective magnetic moment of peff ​= ​5.87(3) μB/Fe3+ and θCW ​= ​−526.6(4.4) K, analogous to that observed in similar high-spin Fe3+ systems, and indicative of strong antiferromagnetic coupling. Estimation of the change in magnetic entropy as a function of temperature from T ​= ​0 ​K to T ​= ​195 ​K, ΔSmag ​= ​14.86 J⋅molFe3+−1K−1, is also in good agreement with the ΔSmag ​= ​Rln(2S ​+ ​1) ​= ​14.9 ​J ⋅ mol−1K−1 expected for a S ​= ​5/2 system. Annealing of synthetic corkite at 200 ​°C results in changes to both the measured IR and Raman spectra, indicative of partial removal of bridging hydroxyl groups from the kagomé sublattice. In comparison to the pure jarosites, where both structure and magnetism remain largely invariant upon a variety of chemical substitutions, the replacement of one sulfate group per formula unit with a higher-valent phosphate group applies additional steric and electronic pressure on the kagomé lattice in corkite, further frustrating the magnetic ground state of the material. Corkite thus represents both an outlier in the known body of jarosite-type materials, and an illustration of how existing structures may be further strained in the development of highly frustrated magnetic systems.

Enabling Aqueous Processing of Prussian White Cathodes for Sodium Ion Cells By Understanding the Effect of Structural Water

Sodium-ion batteries (SIBs) have been identified as one of the most promising alternative technologies for future stationary large-scale storage and light electromobility applications. Indeed, the potential cost decrease with respect to the lithium-ion technology, and the sustainability provided by using Earth abundant elements lead to lower economic and geopolitical impact.[1,2] Great achievements in terms of materials development have been reported in the recent years. However, further work is still needed to fully understand the structure-function correlation in several SIB materials. Easy-synthesis Prussian blue analogues (PBAs) have attracted much attention as potential cathode material in SIBs. Their open framework structure with large interstitial spaces allows a range of different compositions and enables fast Na+ insertion/extraction. However, a key factor affecting the electrochemical performance of PBAs is the crystal water in the structure arising from aqueous synthesis environments.[3] Thus, many have investigated improved synthesis methods aiming at reducing crystal water and associated structural defects.[4] Goodenough et al[5] firstly reported the improved electrochemical stability of PBAs through dehydration converting Na-rich monoclinic Na2MnFe(CN)6 to a rhombohedral structure. More recent studies, including Zhou et al[6] and Chou et al[7], further highlighted the negative impact of structural water on electrochemical performance. So far, the water sensitivity of PBAs has hindered aqueous electrode processing as demonstrated by the several studies conducted on N-methyl pyrrolidone-based electrodes. A more comprehensive understanding of the effect of crystal water and the induced degradation processes is necessary for obtaining high-performance electrode materials produced through environmentally friendly aqueous processing.Herein, we investigate the electrochemical behaviour of an aqueous-processed Fe-based Prussian white (PW) cathode material (Na2-x Fe[Fe(CN)6]1-y·zH2O).[8] The use of such material and processing matches the requirements for low toxicity, cost, and resource abundance in large-scale applications. We uniquely show that, by carefully tuning the water content in the PW structure, the electrochemical performance can be significantly improved. As structural water is removed by tuning the temperature and vacuum conditions during the electrode preparation, the crystallinity of the structure can be controlled, and optimal electrochemical behaviour can be achieved. Moreover, with the combination of cyclic voltammetry, impedance spectroscopy, X-Ray diffraction, X-ray absorption spectroscopy and scanning electron microscopy, we elucidate the activity of the low spin/ high spin Fe2+/3+redox reactions, the Na+ transport properties and the electrode’s structural and morphological changes. This allows to understand holistically the effect of crystal water on the electrochemical performance of PW cathodes. We then demonstrate that our approach is scalable and can be applied to different cell formats representing a step forward in the upscale of sustainable and low-cost sodium-ion cells.

The impact of indium ion on structural, magnetic, and electrodynamic traits of Co-Ni nanospinel ferrites

Indium doped cobalt–nickel nano-spinel ferrites, Co0.5Ni0.5Fe2-xInxO4 (x ≤ 0.10) (NSFs), have been fabricated through solution-combustion (SC) approach and characterized for structural, magnetic morphological behavior. XRD (X-ray powder diffraction) affirmed that all NSFs have a single phase without any impurities. SEM (Scanning Electron Microscope) along with EDX (Energy dispersive X-ray), TEM (Transmission Electron Microscope), HR-TEM (High-Resolution Transmission Electron Microscope) analyses, and elemental mappings confirmed the morphology and chemical composition of all products, respectively. The crystallite size has decreased from ∼50 nm to ∼19 nm, while the lattice constant has been seen to vary because of the replacement of Fe3+ ions by In3+ ions. The fitting of room temperature Mössbauer spectra provided the hyperfine parameters, and it was found that the In3+ ions occupy the A site. The isomer shift values are associated with the characteristics of the high spin Fe3+ charge state. The magnetic behaviors of produced nanospinel ferrites (NSFs) were assessed through M vs. H loops registered at 10 and 300 K. It was found that the pristine as well as In3+ ions substituted Co-Ni NSFs exhibit ferrimagnetic nature at both temperatures. In addition, considerable variations in the magnetic parameters were noticed with respect to the level of the substituent element. The values of Ms at both 10 and 300 K increased up to x = 0.03 and then decreased with a further increase in the indium content. Unlike the variations in Ms and Mr, the Hc values at both 10 and 300 K are found to reduce almost linearly with the indium concentration (x). The variations in different magnetic parameters were explained based on the cation distribution, magnetic moments, ionic radii, magnetocrystalline anisotropy, structural and morphological changes, etc. It was predicted that In3+ ions tend to substitute Fe3+ ions at the Td (A) sites for x ≤ 0.03 and then start to partially substitute Fe3+ ions at the Oh (B) sites as x reaches above 0.03. Such a prediction in combination with the variations in the entire magnetic moment could explain the observed variations for Ms values. The reduction in Hc values was mainly ascribed to a drop in the anisotropy constant. Electrodynamic properties were investigated as S-parameters (S11-S21) in the 33–50 GHz frequency range. Main parameters such as real and imaginary parts of permittivity and permeability vs. frequency were calculated based on the standard model. A good correlation between the indium content and high-frequency properties was determined. It was observed the decrease of the electrical and magnetic losses with the increase of the indium concentration due to the diamagnetic origin and differences between Fe3+/In3+ ionic radii of the substituent.

Bi3+ and V3+ co-substituted Ni-Co spinel ferrites: Synthesis, optical, magnetic characterization and hyperfine interaction

Ni0.5Co0.5VxBixFe2-2xO4 spinel ferrite nanoparticles with composition (x = 0.00, 0.02, 0.04, 0.06 and 0.08) (CoNiVBiFeO (x = 0.00–0.08) SFNPs) were well fabricated hydrothermally. The consequence of co-substitution of both Bi3+ and V3+ ions on structure, morphology, and magnetic features of Ni0.5Co0.5Fe2O4 SFNPs were studied using XRD (X-ray diffractometry), scanning (SEM) and transmission (TEM) electron microscopies along with EDX, XPS (X-ray photoelectron spectroscopy), VSM (vibrating sample magnetometry) and Mossbauer spectroscopic techniques. The pure spinel ferrite (SFs) phase was approved via XRD. The morphology of SFNPs were revealed by SEM and TEM. Hyperfine parameters are determined from fitting room temperature Mössbauer spectra. The V3+ and Bi3+ ions occupy mainly at B site. The isomer shift values are coupled with the characteristics of the high spin Fe3+ ion. The influence of V3+ and Bi3+ ions co-doping on the magnetic features of Ni-Co SFNPs has been also investigated at 300 and 10 K. The saturation magnetization (Ms), remanence (Mr), coercive field (Hc), and Bohr magneton number (nB) are extracted from the recorded M-H hysteresis loops. Nanomagnetic materials with x content of 0.02, 0.08, and 0.10 exhibited superparamagnetic (SPM) behavior, whereas other products with x content of 0.00, 0.04, and 0.06 displayed soft ferrimagnetic behavior at ambient temperature. The different prepared nanoferrites displayed hard ferrimagnetic characteristic at 10 K. The Ms and Mr values are decreased as result of V3+ and Bi3+ ions co-doping. However, Hc does not show a monotonic trend. The fluctuations in magnetic moments of ions, super-exchange coupling, and distribution of cations are correlated with the changes in magnetic parameters to explain their variations.

In Situ Exploring of the Origin of the Enhanced Oxygen Evolution Reaction Efficiency of Metal(Co/Fe)–Organic Framework Catalysts Via Postprocessing

The oxygen evolution reaction (OER) is of vital importance for electrochemical energy conversion technologies. The use of metal–organic framework (MOF) catalysts after postprocessing is an important method to optimize catalytic activity; however, little attention has been paid to the transformation of the electronic structure and coordinated ions under operando conditions. Here, we focus on a Prussian blue analogue (PBA) with the formula Na2Co2+[Fe2+(CN)6]·nH2O after postprocessing at a temperature of 400 °C. The catalysts exhibit a significant improvement in the OER activity with an overpotential of 254 mV against 320 mV for the initial PBA at 10 mA cm–2 in 1 M KOH. Our ex situ soft X-ray absorption spectroscopy (XAS) results demonstrate that some of the high-spin Co2+ and low-spin Fe2+ ions of PBA exhibit exchange with N and C ligands. Our fast operando hard XAS study revealed a dynamic evolution process of the transformation of ligands from N(C) in the initial Na2Co[Fe(CN)6]·nH2O to oxygen and the formation of pure low-spin Co3+ oxide and high-spin Fe3+ oxide after OER. The amorphous (Co,Fe)OOH chemical state was identified as the catalytically active state based on our operando X-ray spectroscopy results. The results of this study provide insights into a dynamic evolution process of MOFs after postprocessing and the formation of real active sites during electrocatalysis.

High spin Fe3+-related bonding strength and electron transfer for sensitive and stable SERS detection.

The intrinsic electronic states of transition metal-containing SERS substrates, especially the effect of spin state on the detection sensitivity, still remain unknown. Herein, we propose a simple co-precipitation approach to form trimetallic MIL-101(FeNiTi) with high-spin (HS) Fe3+ as a result of geometric distortion of the octahedral symmetry. Using methylene blue as a demonstration, the trimetallic MIL-101(FeNiTi) shows a high enhancement factor (EF) of 6.1 × 106, a low detection limit of 10-9 M and excellent detection stability after long-term preservation. X-ray absorption fine structure and photoelectron spectra demonstrate that coupling between high-spin Fe3+ and aliovalent transition metals Ni2+ and Ti4+ with different filling degree of 3d eg-orbitals results in electron delocalization. The DFT calculation suggests that MIL-101(FeNiTi) with high-spin Fe3+ favors molecular adsorption and the charge transfer from the molecule to MIL-101(FeNiTi) is promoted, benefitting from the enhanced electron delocalization, which both contribute to the distinguished SERS performance of MIL-101(FeNiTi). This finding provides in-depth mechanistic understanding of the effect of the spin state of transition metals on mediating SERS activity, which is expected to efficiently promote the development of SERS platforms based on non-noble metals.

Structural Evolution, Redox Mechanism, and Ionic Diffusion in Rhombohedral Na2FeFe(CN)6 for Sodium-Ion Batteries: First-Principles Calculations

Prussian blue analogs (Na2FeFe(CN)6) have been regarded as potential cathode materials for sodium-ion batteries (SIBs) due to their low-cost iron resources and open framework. Herein, the detailed first-principles calculations have been performed to investigate the electrochemical properties of Na x FeFe(CN)6 during Na ion extraction. The material undergoes a phase transition from a dense rhombohedral to open cubic structure upon half-desodiation, which is resulted from competition of the Na−N Coulomb attraction and d−π covalent bonding of Fe−N. The analyses on the density of states, magnetic moments and Bader charges of Na x FeFe(CN)6 reveal that there involve in the successive redox reactions of high-spin Fe2+/Fe3+ and low-spin Fe2+/Fe3+ couples during desodiation. Moreover, the facile three-dimensional diffusion channels for Na+ ions exhibit low diffusion barriers of 0.4 eV ∼ 0.44 eV, which ensures a rapid Na+ transport in the Na x FeFe(CN)6 framework, contributing to high rate performance of the battery. This study gives a deeper understanding of the electrochemical mechanisms of Na x FeFe(CN)6 during Na+ extraction, which is beneficial for the rational design of superior PBA cathodes for SIBs.

Enzyme Control Over Ferric Iron Magnetostructural Properties.

Fe3+ complexes in aqueous solution can exist as discrete mononuclear species or multinuclear magnetically coupled species. Stimuli-driven change to Fe3+ speciation represents a powerful mechanistic basis for magnetic resonance sensor technology, but ligand design strategies to exert precision control of aqueous Fe3+ magnetostructural properties are entirely underexplored. In pursuit of this objective, we rationally designed a ligand to strongly favor a dinuclear μ-oxo-bridged and antiferromagnetically coupled complex, but which undergoes carboxylesterase mediated transformation to a mononuclear high-spin Fe3+ chelate resulting in substantial T1 -relaxivity increase. The data communicated demonstrate proof of concept for a novel and effective strategy to exert biochemical control over aqueous Fe3+ magnetic, structural, and relaxometric properties.

Stereoisomerism in Tetrametallic Propeller‐Like Complexes: A Solid‐State and Solution NMR Study on a Tetragallium(III) Derivative

AbstractTetragallium(III) complex in [Ga4(L4‐Py)2(dpm)6] ⋅ EtOH, with H3L4‐Py=2‐(hydroxymethyl)‐2‐(pyridin‐4‐yl)propane‐1,3‐diol and Hdpm=dipivaloylmethane, was investigated as a diamagnetic analogue of tetrametallic, propeller‐like single‐molecule magnets (SMMs). The chiral molecular structure partitions the six CH2 protons of each tripodal (L4‐Py)3− ligand into two diastereotopic sets. The two signals were clearly detected by 1H NMR spectroscopy in C6D6, proving that Λ and Δ enantiomers interconvert slowly over NMR timescale. Density functional theory calculations provided quantitative agreement with the observed values of chemical shifts and scalar coupling constants across both geminal and long‐range interaction pathways. The solid‐state structure suggests the occurrence of a lower symmetry stereoisomer (27 mol%), which was clearly identified in the NMR spectra. Since high spin Fe3+ forms distinctly more inert complexes than Ga3+, comparable or greater configurational stability is expected for the isostructural FeIII4, FeIII3CrIII, and FeIII3VIII SMMs, which are difficult to investigate by solution NMR because of the strong paramagnetism.

Dimensionality induced enhancement of ferromagnetic spin order and ferroelectric polarization in Ga doped α-Fe2O3 thin films

The thin films of Ga doped hematite (α-Fe2O3) were grown on p-Si 〈111〉 substrate using Pulsed Laser Deposition technique. The as-grown films indicated crystalline peaks of rhombohedral structure. The films exhibited enhancement of ferromagnetic properties. The reduction of dimensionality of the materials in thin films (2D) has shown unusual magnetic spin order and exchange bias effect at temperatures < 70 K. The ferromagnetic parameters (magnetization, coercivity) showed dependence on composition, deposition conditions and heat treatment of the films. The as-grown films showed a good signature of ferroelectric polarization. The as-grown films of Fe1.8Ga0.2O3, having relatively large surface roughness, exhibited smaller magnetic moment in comparison to as-grown films of Fe1.6Ga0.4O3 with small surface roughness. The X-ray photoelectron spectroscopy measurements confirmed a chemically homogeneous state of high spin Fe3+ ions in as-grown films of Fe1.6Ga0.4O3 composition, whereas chemical state of Fe ions is consisting of the mixture of high spin Fe3+ and Fe2+ states in as-grown films of Fe1.8Ga0.2O3.

Influence of Sn doping on the structural, magnetic, optical and photocatalytic properties of hematite (α-Fe2O3) nanoparticles

Sn-doped hematite (α-Fe2O3) nanoparticles of fairly uniform and Sn-dependent size and shape were synthesized via a simple combination of hydrothermal co-precipitation and calcination. The effects of Sn doping on the unit cell size, crystallinity, particle size and shape, as well as the magnetic, optical and photocatalytic properties of hematite nanoparticles were analyzed. The incorporation of Sn4+ ions into the crystal structure of hematite was confirmed by determination of the unit cell expansion due to the replacement of octahedrally coordinated Fe3+ ions by significantly larger Sn4+ ions, as well as a substantially reduced hyperfine magnetic field due to magnetic dilution upon the substitution of non-magnetic Sn4+ ions for magnetic high-spin Fe3+ ions. Sn doping caused a decrease in length and width and an increase in thickness of elongated hematite nanoparticles. Fairly uniform Sn-doped hematite nanoellipsoids or nanocuboids were formed, depending on the Sn content. Temperature dependence of magnetization measurements showed the disappearance of the magnetic phase transition (Morin transition) in hematite upon Sn doping. Magnetic coercivity decreased upon Sn doping due to a decrease in shape anisotropy induced by the change in particle shape from nanorods to nanoellipsoids and nanocuboids. The optical and electronic properties of hematite nanoparticles were significantly affected by Sn doping – the absorption edge was shifted to higher wavelengths, while direct and indirect optical band gaps narrowed with the increasing Sn4+-for-Fe3+ substitution. Sn-doped hematite nanoellipsoids containing 4.3 mol% Sn exhibited a substantial visible light photocatalytic activity in the heterogeneous photo-Fenton process, but this activity significantly decreased with higher Sn doping.

From Slater to Mott physics by epitaxially engineering electronic correlations in oxide interfaces

Using spin-assisted ab initio random structure searches, we explore an exhaustive quantum phase diagram of archetypal interfaced Mott insulators, i.e. lanthanum-iron and lanthanum-titanium oxides. In particular, we report that the charge transfer induced by the interfacial electronic reconstruction stabilises a high-spin ferrous Fe2+ state. We provide a pathway to control the strength of correlation in this electronic state by tuning the epitaxial strain, yielding a manifold of quantum electronic phases, i.e. Mott-Hubbard, charge transfer and Slater insulating states. Furthermore, we report that the electronic correlations are closely related to the structural oxygen octahedral rotations, whose control is able to stabilise the low-spin state of Fe2+ at low pressure previously observed only under the extreme high pressure conditions in the Earth’s lower mantle. Thus, we provide avenues for magnetic switching via THz radiations which have crucial implications for next generation of spintronics technologies.

A Terminal Imido Complex of an Iron-Sulfur Cluster.

We report the synthesis and characterization of the first terminal imido complex of an Fe-S cluster, (IMes)3 Fe4 S4 =NDipp (2; IMes=1,3-dimesitylimidazol-2-ylidene, Dipp=2,6-diisopropylphenyl), which is generated by oxidative group transfer from DippN3 to the all-ferrous cluster (IMes)3 Fe4 S4 (PPh3 ). This two-electron process is achieved by formal one-electron oxidation of the imido-bound Fe site and one-electron oxidation of two IMes-bound Fe sites. Structural, spectroscopic, and computational studies establish that the Fe-imido site is best described as a high-spin Fe3+ center, which is manifested in its long Fe-N(imido) distance of 1.763(2) Å. Cluster 2 abstracts hydrogen atoms from 1,4-cyclohexadiene to yield the corresponding anilido complex, demonstrating competency for C-H activation.

Structural, magnetic and Mössbauer studies of Nd 0.6Sr0.4FexMn1-xO3 perovskites

The effect of iron doping on the structural and magnetic properties of the manganite perovskites Nd0.6Sr0.4FexMn1-xO3 (0.1 ≤ x ≤ 1.0) was studied. Auto-combustion sol-gel method was used to prepare the compounds. X-ray diffraction refinement indicated that all the compounds crystallized in orthorhombic single-phase with Pnma space group. Scanning electron microscopy micrographs revealed a group of packed grains of about 0.2–1.0 μm whereas the average particle size was found to be in the range of about 40–80 nm via transmission electron microscopy. The zero-field cooled and field cooled magnetization curves, under an applied magnetic field of 0.02 T, revealed that the samples with 0.1 ≤ x ≤ 0.4 exhibit a paramagnetic to ferromagnetic transition with decreasing temperature while the samples with (0.5 ≤ x ≤ 1.0) do not demonstrate such phase transition. At room temperature the dependence of magnetization on the x iron concentration, as revealed by the corresponding M − H curves, can be interpreted as the cumulative result of changes in superparamagnetic, paramagnetic and antiferromagnetic contributions. 57Fe transmission Mӧssbauer spectra were recorded at room-temperature (295 K) and liquid nitrogen temperature (78 K) for all the samples. Iron ions were found to be present in these compounds exclusively in a paramagnetic high-spin Fe3+ state for 0.1 ≤ x ≤ 0.5 at 295 K and for 0.1 ≤ x ≤ 0.3 at 78 K. At higher iron concentrations an antiferromagnetic state of iron clusters is formed due to the Fe3+ – Fe3+ superexchange interaction. Room temperature 57Fe quadrupole splitting was found to increase near linearly with the x iron concentration up to x = 0.6. For x ≥ 0.6 the formation of paramagnetic Fe4+ is observed at 295 K, which at lower temperatures undergoes charge disproportionation leading to the appearance of Fe5+ species as observed at 78 K. Perturbations of the magnetic state of iron ions was found to lead to substantial broadening of the Mössbauer absorption peaks of magnetic components, which is interpreted as the effect of nearest neighbor Fe5+ and Mn ions at 78 K, and as that of electron-hopping related Fe3+ - Fe4+ valence fluctuations at 295 K.

2D ferrous nitroprussides stabilized through organic molecules as pillars: preparation, crystal structure and related properties

Transition metal (T) nitroprussides form a family of coordination complexes with a wide structural diversity including 3 D, 2 D, 1 D, 0 D and ionic phases. Recently, a reversible thermally induced spin transition has been reported for 2 D ferrous nitroprussides stabilized through pyridine and its derivatives as pillars between neighboring layers. The aim of this study is to explore the possibility of obtaining analog solids with spin-crossover behavior using other molecules as pillars. In this contribution we are reporting the crystal structure, solved and refined from powder XRD data, for the resulting hybrid solids containing 1,3-oxazole, 1H-pyrazole, pyridazine, 3-pyridine-3-carboxaldheyde, imidazo[1,2-a]pyridine, and 4-(2-pyridin-4-ylethyl)pyridine as pillar molecules in the interlayer region. The XRD data were complemented with TG curves, and IR, Raman, Mössbauer spectra, and magnetic measurements as a source of complementary structural information. According to the Mössbauer spectra and magnetic data recorded in the temperature range of 5 to 300 K, no evidence of thermally induced spin-crossover was observed for the 2 D ferrous nitroprussides with these six organic ligands as pillars. Such behavior is in contrast with the one observed for pyridine and its derivatives and it is explained in terms of the local symmetry around the high spin Fe2+ ion, as revealed in the recorded Mössbauer spectra. This suggests that the spin-crossover behavior could be induced by introducing guest molecules, as adsorbed species, in the interlayer region to force to a higher symmetry for the iron ion coordination environment.

Influence of low-spin Co3+ for high-spin Fe3+ substitution on the structural, magnetic, optical and catalytic properties of hematite (α-Fe2O3) nanorods

Pure hematite nanorods and very long and thin cobalt-doped hematite nanorods (nanoneedles) were prepared by a combined precipitation and annealing method. The influence of the level of cobalt doping on different properties of hematite nanorods was investigated. Incorporation of cobalt in the form of low-spin Co3+ ions into the structure of hematite by substitution for high-spin Fe3+ ions was proved by determination of a significant unit cell contraction by powder X-ray diffraction (PXRD) and characteristic positions of the absorption edge in X-ray absorption near edge structure (XANES) spectra and peaks in X-ray emission (XES) spectra. Cobalt doping caused a gradual elongation and thinning of hematite nanorods – very long and thin 1D nanoparticles (nanoneedles) were formed in the presence of 10 and 12 mol% Co. Magnetic measurements showed a strong increase in low-temperature remanent magnetization and coercivity upon cobalt doping, as well as a disappearance of the Morin transition, which was also confirmed by Mössbauer spectroscopy. Cobalt doping significantly affected optical properties of hematite nanorods – absorption in the visible and NIR ranges increased, which was accompanied by narrowing of the optical band gap. Compared with pure hematite nanorods cobalt-doped hematite nanoneedles showed a significantly better electrocatalytic activity for the oxygen evolution reaction (OER).

Rational Ligand Design Enables pH Control over Aqueous Iron Magnetostructural Dynamics and Relaxometric Properties.

Complexes of Fe3+ engage in rich aqueous solution speciation chemistry in which discrete molecules can react with solvent water to form multinuclear μ-oxo and μ-hydroxide bridged species. Here we demonstrate how pH- and concentration-dependent equilibration between monomeric and μ-oxo-bridged dimeric Fe3+ complexes can be controlled through judicious ligand design. We purposed this chemistry to develop a first-in-class Fe3+-based MR imaging probe, Fe-PyCy2AI, that undergoes relaxivity change via pH-mediated control of monomer vs dimer speciation. The monomeric complex exists in a S = 5/2 configuration capable of inducing efficient T1-relaxation, whereas the antiferromagnetically coupled dimeric complex is a much weaker relaxation agent. The mechanisms underpinning the pH dependence on relaxivity were interrogated by using a combination of pH potentiometry, 1H and 17O relaxometry, electronic absorption spectroscopy, bulk magnetic susceptibility, electron paramagnetic resonance spectroscopy, and X-ray crystallography measurements. Taken together, the data demonstrate that PyCy2AI forms a ternary complex with high-spin Fe3+ and a rapidly exchanging water coligand, [Fe(PyCy2AI)(H2O)]+ (ML), which can deprotonate to form the high-spin complex [Fe(PyCy2AI)(OH)] (ML(OH)). Under titration conditions of 7 mM Fe complex, water coligand deprotonation occurs with an apparent pKa 6.46. Complex ML(OH) dimerizes to form the antiferromagnetically coupled dimeric complex [(Fe(PyCy2AI))2O] ((ML)2O) with an association constant (Ka) of 5.3 ± 2.2 mM-1. The relaxivity of the monomeric complexes are between 7- and 18-fold greater than the antiferromagnetically coupled dimer at applied field strengths ranging between 1.4 and 11.7 T. ML(OH) and (ML)2O interconvert rapidly within the pH 6.0-7.4 range that is relevant to human pathophysiology, resulting in substantial observed relaxivity change. Controlling Fe3+ μ-oxo bridging interactions through rational ligand design and in response to local chemical environment offers a robust mechanism for biochemically responsive MR signal modulation.

57Fe Mössbauer spectroscopic study on the Fe doped supercapacitor birnessite, $$ ({\text{Na}},{\text{K}})_{x} {\text{Mn}}^{4 + }_{2 - x} {\text{Mn}}^{3 + }_{x} {\text{O}}_{4} \cdot 1.5{\text{H}}_{2} {\text{O}} $$

In the present work, the oxidation and spin state of Fe and the local structure around Fe in the supercapacitor birnessite with different contents of the Fe dopant were investigated using Mossbauer spectroscopy. It was found that Fe ions were exclusively present as high spin Fe3+ in octahedral coordination with about 70% iron occupying the Mn3+ positions and about 30% iron occupying the Mn4+ positions in the [MnO6] octahedra for all Fe-doped birnessite samples. Based on these new findings, the trend of typical cell parameters, selected bond lengths of the Fe-doped birnessites and their corresponding quadrupole splittings in the Mossbauer spectra were well explained by considering both the weakened Jahn–Teller effect during the replacement of Mn3+ by Fe3+ and the expansion of octahedra during the replacement of Mn4+ by Fe3+. The present work offers some new insights into the understanding of the mechanism of the heterogeneous atomic doping on the crystal structure of birnessite, with importance for both mineralogy and material science.

Study on Magnetic Properties and the Phase Structure of MnO2 and LiFePO4/C Composite Materials

MnO2 and LiFePO4/C composite materials were prepared by two-step sol-gel. The main peak patterns of lithium iron phosphate and manganese dioxide appeared in the analysis of the phase structure of composite materials. The SEM analysis shows that the particle quality of composite materials is obvious and the dispersion is better, and the particle size is mainly 168 nm. Magnetic hysteresis loops at room temperature show that the introduction of manganese dioxide causes the saturated magnetization strength of composite materials Ms (emu/g), residual magnetization strength Mr (emu/g) and the area of hysteresis loops (kOe emu/g) to increase with the increase of manganese dioxide. The Mossbauer spectrum of MnO2 and LiFePO4/C indicates that the main component of composite materials is high spin Fe2+ compounds.

AnisoDipFit: Simulation and Fitting of Pulsed EPR Dipolar Spectroscopy Data for Anisotropic Spin Centers

Pulsed electron paramagnetic resonance dipolar spectroscopy (PDS) allows to measure the distances between electron spin centers and, in favorable cases, their relative orientation. This data is frequently used in structural biology for studying biomolecular structures, following their conformational changes and localizing paramagnetic centers within them. In order to extract the inter-spin distances and the relative orientation of spin centers from the primary, time-domain PDS signals, a specialized data analysis is required. So far, the software to do such analysis was available only for isotropic S = 1/2 spin centers, such as nitroxide and trityl radicals, as well as for high-spin Gd3+ and Mn2+ ions. Here, a new data analysis program, called AnisoDipFit, was introduced for spin systems consisting of one isotropic and one anisotropic S = 1/2 spin centers. The program was successfully tested on the PDS data corresponding to the spin systems Cu2+/organic radical, low-spin Fe3+/organic radical, and high-spin Fe3+/organic radical. For all tested spin systems, AnisoDipFit allowed determining the inter-spin distance distribution with a sub-angstrom precision. In addition, the spatial orientation of the inter-spin vector with respect to the g-frame of the metal center was determined for the last two spin systems. Thus, this study expands the arsenal of the PDS data analysis programs and facilitates the PDS-based distance and angle measurements on the highly relevant class of metolloproteins.