Fe-Fe Interactions Research Articles

Machine learning-based prediction of FeNi nanoparticle magnetization

This work proposes a computationally efficient approach for estimating the magnetization of Fe0.7Ni0.3 body-centered cubic (bcc) nanoparticles (NPs) at room temperature using machine-learning algorithms, in terms of the average magnetic moment per atom, ⟨μ⟩. The magnetization data of isolated NPs were generated using atomistic spin dynamics (ASD) simulations for various nanoparticle shapes (cubes, spheres, octahedra, cones, cylinders, ellipsoids, flakes, and pyramids, with or without nanovoids) and FeNi distributions (random, core-shell, onion, sandwich, and Janus with different boundary planes). More than 1600 NPs were created and split into training and testing sets (70%–30% split), with features including the number of Ni/Fe surface and core atoms, potential energy distributions, pair correlation functions, and coordination distributions. Several machine-learning algorithms, including Random Forest (RF), Elastic Net, Support Vector Regression (SVR), and Gradient Boosting Regression (CatBoost), were applied to predict the average magnetic moment per atom of these NPs. The best-performing models, CatBoost and RF, achieved R2 scores of up to 0.86, demonstrating their accuracy in predicting NP magnetization. Feature analysis highlighted the significance of the interface between Fe and Ni clusters, Fe–Fe interactions, and the presence of Fe on the surface as critical contributors to overall magnetization. Random alloy spherical NPs without porosity exhibited the highest ⟨μ⟩ ∼ 1.6μB due to reduced Ni–Ni interactions. Applying machine-learning methods significantly reduces computational time and memory requirements compared to traditional ASD simulations. This allows for rapid prediction of NPs with desired magnetic properties, making them suitable for various technological applications.

Mechanochemical synthesis of nitrogen-modified microscale zero-valent iron for efficient Cr(VI) removal: The role of lattice strain

Lattice engineering of Fe0 based on heteroatom doping holds great promise in water purification owing to their tunable geometric and electronic structure. However, there is limited understanding of how nitrogen atom in Fe0 lattice structure affects its local microenvironment and corresponding reactivity toward target contaminants. Herein, microscale zero-valent iron with interstitial N doping (N-mZVI) was successfully prepared by a facile mechanochemical method for heavy metal pollution control. N-mZVI exhibited excellent reactivity for Cr(VI) elimination, and both removal efficiency and conversion rate could reach 100 %, which was 8.06 and 2.73 times higher than that of pristine ball milled mZVI, respectively. Besides, the rate constant of various heavy metals sequestration (Cr(VI), Ni(II), Pb(II), Cu(II), and Cd(II)) over N-mZVI was 61.3 ∼ 115.6-fold faster than that of pristine ball milled mZVI. N-mZVI was also demonstrated to be pH-universal, long-term stability, and environmental applicability. Experimental results and theoretical calculations indicated that tensile strain induced by interstitial N doping expanded Fe0 lattice and weakened the lattice Fe-Fe interaction, thus significantly accelerating electron transport dynamics in Fe0 core. Meanwhile, iron nitride species on the surface of N-mZVI promoted electron transfer from iron shell layer to heavy metals. This work comprehensively investigates the structure–property relationships of lattice engineering-based N-mZVI, and provides a new insight into rational design of modified mZVI with high reactivity.

Sequestration of Labile Organic Matter by Secondary Fe Minerals from Chemodenitrification: Insight into Mineral Protection Mechanisms.

Labile organic matter (OM) immobilized by secondary iron (Fe) minerals from chemodenitrification may be an effective way to immobilize organic carbon (OC). However, the underlying mechanisms of coupled chemodenitrification and OC sequestration are poorly understood. Here, OM immobilization by secondary Fe minerals from chemodenitrification was investigated at different C/Fe ratios. Kinetics of Fe(II) oxidation and nitrite reduction rates decreased with increasing C/Fe ratios. Despite efficient sequestration, the immobilization efficiency of OM by secondary minerals varied with the C/Fe ratios. Higher C/Fe ratios were conducive to the formation of ferrihydrite and lepidocrocite, with defects and nanopores. Three contributions, including inner-core Fe-O and edge- and corner-shared Fe-Fe interactions, constituted the local coordination environment of mineral-organic composites. Microscopic analysis at the molecular scale uncovered that labile OM was more likely to combine with secondary minerals with poor crystallinity to enhance its stability, and OM distributed within nanopores and defects had a higher oxidation state. After chemodenitrification, high molecular weight substances and substances high in unsaturation or O/C ratios including phenols, polycyclic aromatics, and carboxylic compounds exhibited a stronger affinity to Fe minerals in the treatments with lower C/Fe ratios. Collectively, labile OM immobilization can occur during chemodenitrification. The findings on OM sequestration coupled with chemodenitrification have significant implications for understanding the long-term cycling of Fe, C, and N, providing a potential strategy for OM immobilization in anoxic soils and sediments.

Effects of strain and defects on hydrogen diffusion and trapping in [formula omitted]-iron using a developed Fe-H reactive force field

Strain and defects significantly affect the diffusion and trapping of hydrogen (H), which plays a critical role in the initiation of hydrogen embrittlement phenomena in α-iron. In this work, based on a newly developed Fe-H reactive force field, the molecular dynamic models with perfect lattice and defects across various strain regions are presented for studying diffusion and aggregation of H atoms in α-iron. At first, the effect of gradient strain on H diffusion and trapping are elucidated. Then, H diffusion and trapping behaviors at defects (including vacancies, edge dislocations, Σ3[11̄0](112) and Σ5[001](310) grain boundaries) undergoing various strain are investigated. The simulation results demonstrate that the gradient strain leads to a gradient distribution of H solution energies, which drives a directional diffusion and aggregation of H atoms from compressive to tensile strain region. The charge transfer from nearest neighbor Fe atoms to H atoms weakens the Fe-Fe interactions, consequently resulting in the local stress relaxation within the H-enrich region. Regarding the H diffusion and trapping at defects, the results show that tensile strain decreases the difference in H solution energies between the bulk structure and defects, which facilitates H diffusion and escape. Conversely, compressive strain amplifies the energy difference, leading to the enhancement of H trapping and inhibition of H transport. This study contributes to an enhanced comprehension of interactions between hydrogen and defects under strain from an atomic scale.

Enhancing Intrinsic Magnetic Hardness by Modulating Antagonistic Interactions in the Rare-Earth-Free Magnetic Solid Solution Hf2 Fe1-δ Ru5-x Irx+δ B2.

The quinary members in the solid solution Hf2 Fe1-δ Ru5-x Irx+δ B2 (x=1-4, VE=63-66) have been investigated experimentally and computationally. They were synthesized via arc-melting and analyzed by EDX and X-ray diffraction. Density functional theory (DFT) calculations predicted a preference for magnetic ordering in all members, but with a strong competition between ferro- and antiferromagnetism arising from interchain Fe-Fe interactions. The spin exchange and magnetic anisotropy energies predicted the lowest magnetic hardness for x=1 and 3 and the highest for x=2. Magnetization measurements confirm the DFT predictions and demonstrate that the antiferromagnetic ordering (TN =55-70 K) found at low magnetic fields changed to ferromagnetic (TC =150-750 K) at higher fields, suggesting metamagnetic behavior for all samples. As predicted, Hf2 FeRu3 Ir2 B2 has the highest intrinsic coercivity (Hc =74 kA/m) reported to date for Ti3 Co5 B2 -type phases. Furthermore, all coercivities outperform that of ferromagnetic Hf2 FeIr5 B2 , indicating the importance of AFM interactions in enhancing magnetic anisotropy in these materials. Importantly, two members (x=1 and 4) maintain intrinsic coercivities in the semi-hard range at room temperature. This study opens an avenue for controlling magnetic hardness by modulating antagonistic AFM and FM interactions in low-dimensional rare-earth-free magnetic materials.

Influence of Al and Si concentration on the structural, electronic, magnetic, and mechanical properties of Fe2CoAl1−xSix (x = 0, 0.25, 0.5, 0.75, and 1.0) full heusler alloys: First principles calculations and Monte Carlo simulation

A comprehensive investigation into the structural, electronic, magnetic, mechanical, and thermomagnetic properties of Fe2CoAl1−xSix (x = 0, 0.25, 0.5, 0.75, and 1.0) full Heusler alloy is performed using density functional theory and Monte Carlo simulation. Pristine and doped structures are found to adopt the XA prototype, and the lattice parameter is observed to decrease with increasing Si concentration. An increase in the bulk modulus is noticed with the increase of Si concentration, the resulted values range from 160 to 240 GPa. The electronic properties reveal a band gap in the spin-down states of the pristine and doped structures using mBJ approximation. The magnetic moments of the doped structures are found to be similar to those of the pristine structures, and the exchange interaction parameters highlight the Fe-Fe interaction as the strongest in the doped structures. The resulting Curie temperature using Monte Carlo simulation ranged between 792 K and 1029 K. Finally, understanding the magnetic properties of these Heusler alloys could pave the way for spintronics and high-temperature magnetic applications.

Theoretical Exploration of Properties of Iron-Silicon Interface Constructed by Depositing Fe on Si(111)-(7×7).

Exploring the properties of magnetic metal on the semiconductor surface is of great significance for the application of magnetic recording materials. Herein, DFT calculations are carried out to explore the properties of the iron-silicon interface structures (nFe/DASF) formed by depositing n Fe atoms on the reconstructed Si(111)-(7×7) surface (DASF). The stable nFe/DASF structures are studied in the cases of the adsorption and permeation of Fe atoms on the DASF. In both cases, Fe atoms are not very dispersed and prefer binding with Si atoms rather than the adsorbed Fe atoms, because the Fe-Si interaction is stronger than the Fe-Fe interaction. As the n value increases, the average binding energy (Eb_ave) of Fe generally firstly becomes more negative and then becomes less negative, with the presence of a 7Fe wheel as a stable geometry on the upmost surface. The presence of the 7Fe wheel is attributed to the enhanced Fe-Si interaction in this wheel compared to other geometries. CO adsorption occurs at the central Fe site of the 7Fe wheel which is greatly influenced by the surrounding Si atoms but is little influenced by the additional Fe atoms in the interlayer.

Symbiotic synergy enabling moderate oxo-hydroxy adsorption capacity for high-selectivity oxygen reduction

Accurately regulating the adsorption-desorption behaviors of oxygenated intermediates on the active centers is crucial to control the catalytic activity and selectivity in the oxygen reduction reaction (ORR) domain, but remains a great challenge. Here, we propose a universal strategy of symbiotically growing atomically metal sites on the electronic state-rich metallic nanocrystals to tailor the electron structure of the catalytic active centers, enabling moderate adsorption capacity of oxo-hydroxy on the active centers toward high four-electron ORR selectivity. As a proof-of-concept experiment, three typical catalysts of atomically-dispersed Fe sites (Fe-SA/NC), symbiotic Fe complex by coupling atomically-dispersed Fe sites onto Fe nanocrystals (SBT-Fe-NC@SA), and Fe nanoparticles (Fe-NP/NC) were designed to flexibly alter the kinetic pathways of ORR. In situ synchrotron characterizations identified that a self-evolved Fe site adsorbed on the oxygen-bridge enhances the dinuclear Fe-Fe interaction under working conditions for SBT-Fe-NC@SA, which regulates the oxo-hydroxy adsorption capacity to be moderate for rapid breakage of O-O bond. As a result, the well-designed SBT-Fe-NC@SA exhibits the largest efficiency of four-electron ORR pathway and ultrahigh mass activity at the half-wave potential, tens of times those of control counterparts. These findings provide a unique perspective for optimally regulating intermediates adsorption capacity of active sites toward superior activity.

To Study the Structural, Optical and Magnetic Properties of Ni-Fe Doped ZnO Diluted Magnetic Semiconductors

The iron and Nickel co-doped in zinc oxide nano-particle, Zn1-x-yFexNiyO {(x=0, y=0), (x=0.05, y=0), (x=0, y=0.05), (x=0.03, y=0.02)} were prepared by Sol-Gel technique. This type of synthesis is used for the manufacture of a Field Effect Transistor (FET) detector which is extremely sensitive, secure, and capable of reproducing. It is also used for the detection of the Piperidine (CH2)5NH (hexahydro pyridone) chemical.
 The well-closed pack spherical nano-particles of the identified crystalline structure of the usual wurtzite hexagonal phase were achieved by characterization of X-Ray Diffraction (XRD). The blue shift was well observed of Co-doped samples as compared to un-doped or single-doped ZnO nanoparticle samples by UV-visible absorption spectrum. The Fourier Transform Infrared Spectroscopy (FTIR) reveals the existence of chemical bonding and functional groups are established and the occurrence of iron Fe in modes associated with iron Fe in addition to ZnO. The room temperature ferromagnetism was achieved in single or co-doped (Fe-Ni) with ZnO nanoparticles and is confirmed by magnetic characterization (VSM). It was perceived that magnetic saturation was improved in Fe-Ni Co-doped samples relative to pure or single doped. Also, the characterizations of samples show that the exchange interaction of Ni and Fe dominates over Ni-Ni and Fe-Fe interactions.

Metal-Metal-Bonded Fe4 Clusters with Slow Magnetic Relaxation.

Reaction of FeBr2 with Li(N═CtBu2) (0.5 equiv) and Zn0 (2 equiv) results in the formation of the formally mixed-valent cluster [Fe4Br2(N═CtBu2)4] (1) in moderate yield. The subsequent reaction of 1 with Na(N═CtBu2) results in formation of [Fe4Br(N═CtBu2)5] (2), also in moderate yield. Both 1 and 2 were characterized by zero-field 57Fe Mössbauer spectroscopy, X-ray crystallography, and superconducting quantum interference device magnetometry. Their tetrahedral [Fe4]6+ cores feature short Fe-Fe interactions (ca. 2.50 Å). Additionally, both 1 and 2 display S = 7 ground states at room temperature and slow magnetic relaxation with zero-field relaxation barriers of Ueff = 14.7(4) and 15.6(7) cm-1, respectively. Moreover, AC magnetic susceptibility measurements were well modeled by assuming an Orbach relaxation process.

Potentials for Describing Interatomic Interactions in γFe-Mn-C-N System

Potentials for describing interatomic interactions in a γFe-Mn-C-N multicomponent system, modified Hadfield steel, where face-centered cubic (f.c.c.) iron is the main component, are proposed. To describe the Fe-Fe interactions in austenite, it is proposed to use Lau EAM potential. For all other interactions, Morse potentials are proposed, the parameters of which were found from various experimental characteristics: in particular, the energy of dissolution and migration of an impurity in an f.c.c. iron crystal, the radius of atoms, their electronegativity, mutual binding energy, etc. The found potentials are intended for modeling the atomic structures and processes occurring at the atomic level in Hadfield steel using relatively large computational cells by the molecular dynamics method.

A New Look at Molecular and Electronic Structure of Homoleptic Diiron(II,II) Complexes with N,N-Bidentate Ligands: Combined Experimental and Theoretical Study.

Paddlewheel-type binuclear complexes featuring metal-metal bonding have been the subject of widespread interest due to fundamental concern in their electronic structures and potential applications. Here, we explore the molecular and electronic structures of diiron(II,II) complexes with N,N'-diarylformamidinate ligands. While a paddlewheel-type diiron(II,II) complex with N,N'-diphenylformamidinate ligands (DPhF) exhibits the centrosymmetric [Fe2 (μ-DPhF)4 ] structure, a minor alteration in the ligand system, i. e., switching from phenyl to p-tolyl N-substituted formamidinate ligand (DTolF), resulted in the isolation of an unprecedented non-centrosymmetric [Fe(μ-DTolF)3 Fe(κ2 -DTolF)] complex. Both complexes were characterized using single-crystal X-ray diffraction, magnetic measurements, 57 Fe Mössbauer spectroscopy, and cyclic voltammetry along with high-level ab-initio calculations. The results provide a new view on a range of factors controlling the ground-state electronic configuration and structural diversity of homoleptic diiron(II,II) complexes. Model calculations determined that the Mayer bond orders for Fe-Fe interactions are significantly lower than 1 and equal to 0.15 and 0.28 for [Fe2 (μ-DPhF)4 ] and [Fe(μ-DTolF)3 Fe(κ2 -DTolF)], respectively.

Impact of chemical disorder on magnetic exchange interactions in L10−FeNi

We investigate the effect of chemical disorder on the magnetic exchange couplings and the Curie temperature $({T}_{\text{c}})$ in $\mathrm{L}{1}_{0}$-ordered FeNi using first-principles-based calculations. We use supercells to model chemical disorder, to account for the specific symmetry-broken local chemical environments around the individual atoms. We find a very strong variation of the most dominant first-nearest neighbor Fe-Fe interaction for different inequivalent Fe-Fe pairs, ranging from around 5 to 37 meV, compared to a coupling strength of 27 meV in the ordered state. To estimate the influence of such strong variations of the magnetic coupling constants on the Curie temperature of the disordered or partially ordered state, we study a simple Heisenberg model with random Gaussian-distributed nearest-neighbor couplings on an fcc lattice. Our Monte Carlo simulations for this model indicate that strongly varying exchange couplings, such as those obtained for FeNi, can lead to a reduction of ${T}_{\text{c}}$ of around 10 % relative to the one obtained using only the average coupling.

Thickness induced magnetic anisotropic properties of Tb-Fe-Co thin films

The influence of Tb25Fe61Co14 thin film thicknesses varying from 2 to 300 nm on the structural and magnetic properties has been systematically investigated by using of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, magnetization, and magneto-optic Kerr effect microscopy measurements. Thin film growth mechanism is pursued and controlled by ex-situ X-ray refractometry measurements. X-ray diffraction studies reveal that the Tb25Fe61Co14 films are amorphous regardless of thin films thicknesses. The magnetic properties are found to be strongly related to thickness and preferred orientation. With an increase in film thickness, the easy axis of magnetization is reversed from in-plane to out-of-plane direction. The change in the easy axes direction also affects the remanence, coercivity and magnetic anisotropy values. The cause for the magnetic anisotropy direction change from in-plane to out-of-plane can be related to the preferred orientation of the thin film which depends on the large out-of-plane coercivity and plays an important role in deciding the easy axes direction of the films. According to our results, up to the 100 nm in-plane direction is dominated over the whole system under major Fe-Fe interaction region, after that point, the magnetic anisotropy direction change to the out-of-plane under major Tb-Fe/Tb-Co interaction region and preferred orientation dependent perpendicular magnetic anisotropic properties become more dominated with 2.7 kOe high coercive field values.

Simple model of nitrogen protrusions forming nanostripes within hexagonal FeN on Cu(001)

Hexagonal FeN monolayers on Cu(001) show a stripe pattern composed of dark and bright stripes as observed recently by scanning tunneling microscopy (STM). The nanostripes are formed by more or less protruded nitrogen atoms in the center of iron triangles. Here a simple model is proposed to interpret those results. It uses a planar Fourier analysis with two different wave vectors to calculate small displacements of Fe atoms caused by lattice mismatch stress. The area of displaced Fe atom triangles is used as a measure for the protrusion of N atoms in their center. Proper selection of the wave vectors allows to visualize the dominant protrusions of (3×3) ordered nitrogen atoms. The wave vectors selected are related to Fe configurations which minimize lattice mismatch stress. Hard spheres are used to model Cu-Fe and Fe-Fe interactions in search of a minimal configuration energy. The model explains key elements of the stripe configuration. The model is kept simple, so just the most basic features of the STM experiments are covered. The limits of the model are discussed and ways for extending it are proposed.

Synthesis and Characterization of a Linear Triiron(II) Extended Metal Atom Chain Complex with Fe-Fe Bonds.

Extended metal atom chain (EMAC) complexes of first-row transition metals with metal-metal bonds have the potential to elicit unique magnetic properties and reactivities. Until now, the library of EMAC complexes with late-first-row transition metals was incomplete because of the omission of a triiron species with Fe-Fe bonding. Herein we report the synthesis and preliminary investigation of the first linear, triiron(II) complex containing close Fe-Fe interactions. The complex is supported by three dianionic 2,6-bis[(trimethylsilyl)amido]pyridine ligands (L), with an overall composition of Fe3L3, and pseudohelical ligand coordination stabilizing the local trigonal-planar geometry at each iron. Fe3L3 was characterized by X-ray diffraction, 1H NMR, cyclic voltammetry, electronic absorption, and Mössbauer spectroscopies. Evans method analysis indicated a large uncompensated spin and an S = 6 ground state, suggesting ferromagnetic coupling in the triiron chain, likely due to direct exchange.

Novel magnetic and transport properties of FePP@GNP bilayers formed by Fe-porphyrin embedded into graphene nanopieces

A series of FePP@GNP bilayer systems formed by Fe-porphyrin (FePP) embedded into graphene nanopieces (GNP) are predicted to possess novel magnetic and transport properties by using density functional theory (DFT) and non-equilibrium Green's function (NEGF) calculations. All these systems exhibit a ferromagnetic (FM) ground state. The packing styles and the sizes of the tailored pieces can both affect the conductivity. Electron transport in BL(FePP@GNP) mainly depends on the inter-plane channel. The inter-plane channel mainly derives from the Fe-Fe interaction. Polarized transport character is found for all considered systems. For small piece junctions, the computed spin filter efficiency (SFE) within the bias range of −1.0 V–1.0 V can be greater than 80%. Furthermore, large piece junctions exhibit evident negative differential resistance (NDR) behavior. In addition, an interesting current flip behavior is observed for the large AB’-stacking bilayer. These novel features of BL(FePP@GNP) systems may have implications to nanoelectronic design.

Spin dynamics and magnonic characteristics of a magnetically ordered fcc Fe-Ni alloy monolayer on an fcc Ni slab substrate

We investigate the surface spin dynamics and magnonic characteristics of a magnetically ordered fcc Fe-Ni alloy monolayer on a Ni magnetic slab substrate. The calculations are performed using the Heisenberg Hamiltonian representation for the magnetic ground state of the system. Using the Spin Polarized Relativistic Korringa-Kohn-Rostoker (SPRKKR) method, the relevant magnetic exchange parameters are computed, taking into account the nearest and next nearest Fe-Fe interactions. This establishes the appropriate Heisenberg Hamiltonian. The Phase Field Matching Theory (PFMT) is applied to compute the spin dynamics and surface magnonics of the system, and thus determine the spin wave eigenmodes localized at the surface but propagating in its plane with surface group velocities; these intrinsic eigenmodes constitute basic elements for the magnonic characteristics of the system. The inclusion of the Fe-Fe interactions alters the highest magnonic mode compared to the case when such interactions are absent. The localized densities of states (LDOS) for the irreducible representative magnetic sites at the surface nanostructure are extracted from our computed PFMT Green’s functions. The model is general, and can be applied to different ultrathin layered magnetic alloys on magnetically ordered substrates.

On the correlation between glass forming ability (GFA) and soft magnetism of Ni-substituted Fe-based metallic glassy alloys

The present work aims to investigate the mechanism of glass formation and correlates it to the soft magnetism of Ni-substituted Fe-B-Nb alloys. The impact of ferrous Ni on the mechanism of glass formation and soft magnetism of Fe-based bulk metallic glasses was critically analyzed. By quantifying glass forming ability and soft magnetic characteristic for varying degrees of substitution, we observe a maximum in glass forming ability together with a minimum coercivity, which we suggest is due to an increased atomic packing density of the glassy phase. Interestingly, a monotonic increase of Curie temperature with increasing substitution of Ni was observed, which could be attributed to a reduction of an antiferromagnetic Fe-Fe interaction in the glassy iron-rich matrix. The overarching goal of this study is to explore the underlying mechanisms of enhanced glass forming ability, improved soft magnetic properties, and increased Tc of Ni-substituted Fe-based glassy alloys.

Structural and Electronic Rearrangements in Fe2S2, Fe3S4, and Fe4S4 Atomic Clusters under the Attack of NO, CO, and O2.

We report results, based on density functional theory-generalized gradient approximation calculations, that shed light on how NO, CO, and O2 interact with Fe2S2, Fe3S4, and Fe4S4 clusters and how they modify their structural and electronic properties. The interest in these small iron sulfide clusters comes from the fact that they are at the protein cores and that elucidating fundamental aspects of their interaction with those light molecules which are known to modify their functionality may help in understanding complex behaviors in biological systems. CO and NO are found to bind molecularly, leading to moderate relaxations in the clusters, but nevertheless to changes in the spin-polarized electronic structure and related properties. In contrast, dissociative chemisorption of O2 is much more stable than molecular adsorption, giving rise to significant structural distortions, particularly in Fe4S4 that splits into two Fe2S2 subclusters. As a consequence, oxygen tends to strongly reduce the spin polarization in Fe and to weaken the Fe-Fe interaction inducing antiparallel couplings that, in the case of Fe4S4, clearly arise from indirect Fe-Fe exchange coupling mediated by O. The three molecules (particularly CO) enhance the stability of the iron-sulfur clusters. This increase is noticeably more pronounced for Fe2S2 than for the other iron-sulfur clusters of different compositions, a result that correlates with the fact that in recent experiments of CO reaction with FemSm (m = 1-4), the Fe2S2CO product results as a prominent one.