Fe Clusters Research Articles

BH-DFTB/DFT calculations for iron clusters

We present a study on the structural, electronic, and magnetic properties of Fen(n = 2 − 20) clusters by performing density functional tight binding (DFTB) calculations within a basin hopping (BH) global optimization search followed by density functional theory (DFT) investigations. The structures, total energies and total spin magnetic moments are calculated and compared with previously reported theoretical and experimental results. Two basis sets SDD with ECP and 6-31G** are employed in the DFT calculations together with BLYP GGA exchange-correlation functional. The results indicate that the offered BH-DFTB/DFT strategy collects all the global minima of which different minima have been reported in the previous studies by different groups. Small Fe clusters have three kinds of packing; icosahedral (Fe9−13), centered hexagonal antiprism (Fe14−17, Fe20), and truncated decahedral (Fe17(2), Fe18−19). It is obtained in a qualitative agreement with the time of flight mass spectra that the magic numbers for the small Fe clusters are 7, 13, 15, and 19 and with the collision induced dissociation experiments that the sizes 6, 7, 13, 15, and 19 are thermodynamically more stable than their neighboring sizes. The spin magnetic moment per atom of Fen(n = 2 − 20) clusters is between 2.4 and 3.6 μB for the most of the sizes. The antiferromagnetic coupling between the central and the surface atoms of the Fe13 icosahedron, which have already been reported by experimental and theoretical studies, is verified by our calculations as well. The quantitative disagreements between the calculations and measurements of the magnetic moments of the individual sizes are still to be resolved.

Finite temperature magnetic properties of small Fe chains and clusters on Pt(111)

The magnetic properties of Fe chains and clusters on Pt(111) are investigated in the framework of a functional-integral theory of itinerant magnetism. The considered nanostructures show a ferromagnetic (FM) ground state with nearly saturated Fe local magnetic moments ${\ensuremath{\mu}}_{\text{Fe}}^{0}\ensuremath{\simeq}3.15{\ensuremath{\mu}}_{\text{B}}$. In addition, small moments ${\ensuremath{\mu}}_{\text{Pt}}^{0}\ensuremath{\simeq}0.1--0.3{\ensuremath{\mu}}_{\text{B}}$ are induced at the Pt substrate, which depend sensitively on the number of Fe atoms in their nearest-neighbor (NN) shell. The spin-fluctuation (SF) energies $\mathrm{\ensuremath{\Delta}}{F}_{l}(\ensuremath{\xi})$ at the different atoms $l$ are calculated as a function of the local exchange fields ${\ensuremath{\xi}}_{l}$, by using a real-space recursive expansion of the local Green's functions. Results for the temperature dependence of the average magnetization per atom ${\overline{\ensuremath{\mu}}}_{N}$, local magnetic moments ${\ensuremath{\mu}}_{l}$, and spin correlation functions ${\ensuremath{\gamma}}_{lk}$ are derived. At the Fe atoms the dominant magnetic excitations are fluctuations of the local-moment orientations. The spin-flip energies $\mathrm{\ensuremath{\Delta}}{F}_{l}(\ensuremath{\xi})$ in the deposited Fe clusters are found to be about $50%$ smaller than in free-standing clusters of comparable size. This results in flatter SF-energy landscapes and in a weaker stability of the FM order at $Tg0$. The effective exchange interactions between the Fe local moments, which are derived from the electronic calculations, reveal competing FM and antiferromagnetic couplings at different distances. In contrast to Fe, the main spin excitations at the Pt atoms are fluctuations of the size of the induced local magnetic moments. The interplay between the different types of spin excitations and their effect on the temperature-dependent magnetic properties is discussed.

Reversible Hydrogen Uptake by BN and BC3 Monolayers Functionalized with Small Fe Clusters: A Route to Effective Energy Storage.

In an effort to design new functionalized nanostructures for clean energy storage, DFT calculations of Fen (n = 1-3) clusters on BC3 and BN monolayers are performed. The stability of the systems was considered by calculating the binding energies of the monolayers with Fen clusters on one or both sides. All the clusters bound strongly to both the monolayers and transferred electron density to the sheets. The cationic Fe clusters were then able to adsorb multiple H2 molecules through electrostatic and van der Waals interactions. The average adsorption energies per H2 in the case of maximum coverage were calculated to be -0.389 and -0.358 eV for systems with one Fe on both sides of BC3 and BN monolayers, respectively. In these cases four H2 molecules were adsorbed to the Fe atoms on both sides of the monolayer. These adsorption energies are such that there is potential for adsorption/desorption at ambient conditions. The results provide insights into an efficient and reversible storage of H2 by using Fen-functionalized BC3 and BN monolayers.

Enhanced photocatalytic performance of hierarchical Bi2O2CO3 microflowers by Fe(III) modification

Hierarchical Bi2O2CO3 microflowers with Fe(III) clusters grafted on the surfaces were synthesized through a simple impregnation technique. The as-prepared samples were characterized by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and UV–vis diffuse reflectance spectroscopy. The photocatalytic activity was evaluated by the decomposition of methyl orange in an aqueous solution under visible-light (λ>420nm) and simulated sunlight illumination. The results show that the Fe(III) clusters are just deposited on the surfaces rather than doped in the lattices of Bi2O2CO3. It is found that the morphologies and crystal structures of Bi2O2CO3 microflowers were not changed after modification of Fe(III) clusters. The photocatalytic activities of the Fe(III)-modified Bi2O2CO3 were markedly increased by the modification of Fe(III) clusters under visible light and simulated sunlight irradiation. The enhanced photocatalytic performance can be attributed to the direct interfacial charge transfer from the valence band of Bi2O2CO3 to the surface Fe(III) clusters, which offer a new channel for the efficient generation of the hole in the valence band of Bi2O2CO3 substrate.

Clustering of Fe atoms in liquid Li and its effect on the viscosity of liquid Li

The clustering processes of Fe atoms in liquid Li at different temperatures and the effect from the Fe clusters on the viscosity of liquid Li are investigated using molecular dynamics simulation combined with the embedded atom method. The clustering processes are vividly captured by the microstructure evolution snapshots and the details are uncovered by the cluster analysis results. The cluster analysis results indicate the higher the temperature the faster the clustering process, and the temperature-dependent mixing enthalpy of Li–Fe (solute) dilute solution also suggests that high temperature is beneficial to Fe atoms’ clustering. In addition, our results show that the Fe clusters can dramatically increase the viscosity of liquid Li by lowering the diffusivity of the Li atoms around it, and the larger the clusters the larger the viscosity increment.

Structural optimization of Fe nanoclusters based on multi-populations differential evolution algorithm

Structural optimization of Fe nanoclusters is of importance to their applications since their magnetic and catalytic properties are strongly dependent on their structures. In this article, the lowest-energy structures of Fe nanoclusters have been investigated using a multi-populations differential evolution algorithm. Finnis–Sinclair potentials and Johnson potentials have been, respectively, used to describe the interatomic interactions. The algorithm performance has been first evaluated by comparing the results of Finnis–Sinclair potentials with Cambridge Cluster Database. Two lower-energy structures of Fe n clusters (N = 83 and 84) have been discovered. Our results show that many of the structural configurations optimized with the two potentials are different, indicating that the potential model plays an important role in structural optimization. However, the structures of Fe clusters exhibit the similar growth patterns as the cluster increasing for the two potentials, that is, their lowest-energy structures contain many icosahedra, and the number of the icosahedral rings increases with their sizes. Furthermore, the stability analysis by investigating the finite difference and the second finite difference of energy indicates that the two potentials predict the same relative stability at the magic numbers. The comparison of optimized structures of Fe n clusters at magic numbers (N = 13, 19, 23, 26, 29, 55), respectively, using Finnis–Sinclair and Johnson potentials.

A Triazolate-Supported Fe3(μ 3 -O) Core: Crystal Structure, Fluorescence, and Hirshfeld Surface Analysis

One new trinuclear Fe(III) cluster with a Fe3(μ 3-O) core was synthesized by the method of hydrothermal synthesis. The complex [Fe3(μ 3-O)(pmta)6(H2O)3]FeCl4·2H2O (1) (where Hpmta is 5-methyl-1-phenyl-1H-1,2,3-triazole-4-carboxylic acid) was characterized by single-crystal X-ray diffraction methods, elemental analyses, IR spectroscopy, fluorescence and Hirshfeld surface analysis. For complex 1, the crystal structure is extended into 3D structure through N–H···O and O–H···O hydrogen bonds. According to the 3D Hirshfeld surface and 2D fingerprint plots, the main interactions in the cluster are the H···H, N···H and C···H contacts.

Improved ferromagnetism and ferroelectricity of La and Co co-doped BiFeO3 ceramics with Fe vacancies

In this study, La and Co co-doped BiFeO3 with Fe vacancies ((Bi0.9La0.1)(Fe0.95Cox)O3(x=0, 0.01, and 0.05)) ceramics were prepared by the solid state method using a rapid liquid process. X-ray diffraction characterization indicates that all the samples exhibit the rhombohedral structure with a minor secondary phase such as Bi46Fe2O72. The XPS analysis revealed the variation in the valency of Fe ions because of the doping. The results of electrical measurement showed that the doping significantly reduced dielectric constant, dielectric loss, and leakage current, but clearly enhanced the ferroelectricity. The magnetization results indicated that the doping of Co ions could effectively improve the ferromagnetism. The remnant magnetization (Mr) and saturation magnetization (Ms) increase upon increasing x from 0 to 0.05, and for x=0.05, the values of Mr and Ms at room temperature reach 0.65 and 3emu/g, respectively. It is proposed that the Fe-deficiency, Co–Fe superexchange action and clusters lead to the improved ferromagnetism.

CEMS study of defect annealing in Fe implanted AlN

An AlN thin film grown on sapphire substrate was implanted with 45 keV 57Fe and 56Fe ions at several energies to achieve a homogeneous concentration profile of approximately 2.6 at.%. in the AlN film. Conversion electron Mossbauer Spectroscopy data were collected after annealing the sample up to 900 °C. The spectra were fitted with three components, a single line attributed to small Fe clusters, and two quadrupole split doublets attributed to Fe substituting Al in the wurtzite AlN lattice and to Fe located in implantation induced lattice damage. The damage component shows significant decrease on annealing up to 900 °C, accompanied by corresponding increases in the singlet component and the substitutional Fe.

Graphene decorated with Fe nanoclusters for improving the hydrogen sorption kinetics of MgH2 – experimental and theoretical evidence

Graphene decorated with Fe clusters is proposed to be a possible alternative catalyst for the hydrogenation and dehydrogenation reactions of MgH2.

Influence of Fe-Doping on the Structural and Magnetic Properties of ZnO Nanopowders, Produced by the Method of Pulsed Electron Beam Evaporation

The nanopowders (NPs) ZnO-Zn-Fe and ZnO-Fe with the various concentrations of Fe (xFe) (0≤xFe≤0.619mass.%) were prepared by the pulsed electron beam evaporation method. The influence of doping Fe on structural and magnetic properties of NPs was investigated. X-ray diffraction showed that powders contain fine-crystalline and coarse-crystalline ZnO fractions with wurtzite structure and an amorphous component. Secondary phases were not found. The magnetic measurements made at room temperature, using the vibration magnetometer and Faraday’s scales, showed ferromagnetic behavior for all powders. Magnetization growth of NPs ZnO-Zn and ZnO-Zn-Fe was detected after their short-term annealing on air at temperatures of 300–500°C. The growth of magnetization is connected with the increase in the concentration of the phase ZnO with a defective structure as the result of oxidation nanoparticles (NPles) of Zn. The scanning transmission electron microscopy (STEM) showed a lack of Fe clusters and uniform distribution of atoms dopant in the initial powder ZnO-Zn-Fe. A lack of logical correlation between magnetization and concentration of a magnetic dopant of Fe in powders is shown.

Density functional theory study of Mo-doped M@(BN)48 (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) clusters

The structure and magnetic properties of Mo-doped M@(BN)48 (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) clusters were calculated at BPW91/LanL2DZ level. The magnetic nature of the clusters M@(BN)48 significantly changed when doping with Mo atom, except for Co@(BN)48. Only the magnetic moment for the CrMo@(BN)48 cluster was decreased to zero. Thus, M@(BN)48 clusters can be selected as the model system to detect Mo atom by the change of the magnetic moment.

Adsorption of Fe atoms on strongly-correlated NiO(001) surfaces with surface oxygen vacancies: Oxide support effects

The adsorption properties of Fe atoms on strongly-correlated NiO(001) surfaces with surface O vacancies (F s centers) were studied by using density functional theory combined with on-site Coulomb repulsion U. The adsorption of Fe on the F s center of NiO(001) is 0.57 eV more stable than that on the regular surface O sites of NiO(001). This demonstrates the significant role of the F s centers in the adsorption of Fe atoms and the subsequent growth of Fe clusters on NiO(001) surfaces. This is in sharp contrast with the behavior observed for Fe atoms adsorbed on MgO(001) with F s centers, where the surface O vacancies do not play a role as nucleation sites for the growth of Fe clusters due to the blind nature of Fe atoms to O vacancies. An analysis of the electronic properties and charge rearrangement for adsorption of the Fe atoms on defect-free and O-defective NiO(001) was performed. The charge states of the Fe atoms on strongly-correlated NiO(001) exhibit features different from those of the Fe atoms on MgO(001).

The spin and orbital contributions to the total magnetic moments of free Fe, Co, and Ni clusters.

We present size dependent spin and orbital magnetic moments of cobalt (Con (+), 8 ≤ n ≤ 22), iron (Fen (+), 7 ≤ n ≤ 17), and nickel cluster (Nin (+), 7 ≤ n ≤ 17) cations as obtained by X-ray magnetic circular dichroism (XMCD) spectroscopy of isolated clusters in the gas phase. The spin and orbital magnetic moments range between the corresponding atomic and bulk values in all three cases. We compare our findings to previous XMCD data, Stern-Gerlach data, and computational results. We discuss the application of scaling laws to the size dependent evolution of the spin and orbital magnetic moments per atom in the clusters. We find a spin scaling law "per cluster diameter," ∼n(-1/3), that interpolates between known atomic and bulk values. In remarkable contrast, the orbital moments do likewise only if the atomic asymptote is exempt. A concept of "primary" and "secondary" (induced) orbital moments is invoked for interpretation.

Iron-, Cobalt-, and Nickel-Catalyzed Asymmetric Transfer Hydrogenation and Asymmetric Hydrogenation of Ketones.

Chiral alcohols are important building blocks in the pharmaceutical and fine chemical industries. The enantioselective reduction of prochiral ketones catalyzed by transition metal complexes, especially asymmetric transfer hydrogenation (ATH) and asymmetric hydrogenation (AH), is one of the most efficient and practical methods for producing chiral alcohols. In both academic laboratories and industrial operations, catalysts based on noble metals such as ruthenium, rhodium, and iridium dominated the asymmetric reduction of ketones. However, the limited availability, high price, and toxicity of these critical metals demand their replacement with abundant, nonprecious, and biocommon metals. In this respect, the reactions catalyzed by first-row transition metals, which are more abundant and benign, have attracted more and more attention. As one of the most abundant metals on earth, iron is inexpensive, environmentally benign, and of low toxicity, and as such it is a fascinating alternative to the precious metals for catalysis and sustainable chemical manufacturing. However, iron catalysts have been undeveloped compared to other transition metals. Compared with the examples of iron-catalyzed asymmetric reduction, cobalt- and nickel-catalyzed ATH and AH of ketones are even seldom reported. In early 2004, we reported the first ATH of ketones with catalysts generated in situ from iron cluster complex and chiral PNNP ligand. Since then, we have devoted ourselves to the development of ATH and AH of ketones with iron, cobalt, and nickel catalysts containing novel chiral aminophosphine ligands. In our study, the iron catalyst containing chiral aminophosphine ligands, which are expected to control the stereochemistry at the metal atom, restrict the number of possible diastereoisomers, and effectively transfer chiral information, are successful catalysts for enantioselective reduction of ketones. Among these novel chiral aminophosphine ligands, 22-membered macrocycle P2N4 exhibited extraordinary enantioselectivities when combined with iron(0) cluster Fe3(CO)12. A broad scope of ketones including aromatic, heteroaromatic, and β-ketoesters can be reduced smoothly with excellent enantioselectivities (up to 99% ee) approaching or exceeding those achievable with the noble metal catalysts. Notably, the chiral iron-based catalyst proved to be highly efficient for both ATH as well as AH of various ketones. Until now, such "universal" catalyst is very rare. Preliminary studies suggest that the AH reaction most likely involved iron particles as the active catalytic species. These research results point to a new direction in developing viable effective nonprecious metal catalysts for asymmetric reduction and probably for other asymmetric catalytic reactions as well.

Effect of structure on the magnetic anisotropy ofL10FePt nanoparticles

We carry out a systematic theoretical investigation of Magneto Crystalline Anisotropy (MCA) of L10 FePt clusters with alternating Fe and Pt planes along the (001) direction. We calculate the structural relaxation and magnetic moment of each cluster by using ab initio spin-polarized density functional theory (DFT), and the MCA with both spin-polarized DFT (including spin-orbit coupling self-consistently) and the torque method. We find that the MCA of any composite structure of a given size is enhanced with respect to that of the same-sized pure Pt or pure Fe cluster as well as to that of any pair of Fe and Pt atoms in bulk L10 FePt. This enhancement results from the hybridization we observe between the 3d orbital of the Fe atoms and the 5d orbital of their Pt neighbors. This hybridization, however, affects the electronic properties of the component atoms in significantly different ways. While it somewhat increases the spin moment of the Fe atoms, it has little effect on their orbital moment; at the same time, it greatly increases both the spin and orbital moment of the Pt atoms. Given the fact that the spin-orbit coupling (SOC) constant of Pt is about 7 times greater than that of Fe, this Fe-induced jump in the orbital moment of the Pt atoms produces the increase in MCA of the composite structures over that of their pure counterparts. That any composite structure exhibits higher MCA than bulk L10 FePt results from the lower coordination of Pt atoms in the cluster, whether Fe or Pt predominates within it. We also find that bipyramidal clusters whose central layer is Pt have higher MCA than their same-sized counterparts whose central layer is Fe. This results from the fact that Pt atoms in such configurations are coordinated with more Fe atoms than in the latter. By thus participating in more instances of hybridization, they contribute higher orbital moments to the overall MCA of the unit.

Structure, spectroscopy, stability, and bridge exchange in the M3O4 incomplete-cubane complexes, [M(III)3(Sal-AHA)3(μ-OR)]− (M = Fe, Ga)

The [Ga(III)3(3,5-diCl-Sal-AHA)3(μ-OCH3)]− anion crystallizes with one K+ and one H3O+ counter ion between each pair of anions. The structure is analogous to the previously reported [Fe(III)3(3,5-diCl-Sal-AHA)3(μ-OCH3)]−, with each containing a M3O4 incomplete cubane at the base of a phenyl-derived pocket that surrounds a triply bridging methoxy group. The EPR spectrum of [Fe(III)3(3,5-diCl-Sal-AHA)3(μ-OCH3)]− at 4.5K is complex with features near geff=2, 4.3, and above 5. Only the 4.30 feature maintains intensity much above 30K, and it persists to above 150K. Room temperature magnetic susceptibility measurements indicate an average spin of S=5/2 per trimer in both solid and solution. Both the Ga and Fe clusters are stable in methanol. However, the Ga complex slowly decomposes in 40% water or in the presence of EDTA, while the Fe cluster does not. Experiments with a mixed Fe/Ga solution show that the order of stability of the [M(III)3(3,5-diCl-Sal-AHA)3(μ-OCH3)]−, clusters in the presence of EDTA is M(III)3=Fe3>Fe2Ga>FeGa2>Ga3. Metal or chelate exchange occurs very slowly (weeks to months) if at all. However, the μ3-methoxy bridge can be replaced by deuterated methoxy or a μ3-hydroxy bridge within hours to days.

Field effects on spontaneous magnetization reversal of bulk BiFe0.5Mn0.5O3, an effective strategy for the study of magnetic disordered systems

We report a comprehensive study of the spontaneous magnetization reversal (MRV) performed on the disordered polycrystalline perovskite BiFe0.5Mn0.5O3, an intriguing compound synthesized in high pressure–high temperature conditions. In disordered systems, the origin of MRV is not completely clarified, yet. In BiFe0.5Mn0.5O3, compositional disorder involves the ions on the B-site of the perovskite determining the presence of mesoscopic clusters, characterized by high concentrations of iron or manganese and thus by different resultant magnetization. This leads to the observation of two singular fields H1 and H2 dependent on the degree of inhomogeneity, unpredictably changing from sample to sample due to synthesis effects. These fields separate different magnetic responses of the system; for applied fields H < H1, the Fe and Mn clusters weakly interact in a competitive way, giving rise to MRV, while for an intermediate field regime the energy of this weak interaction becomes comparable to the energy of the system under field application. As a consequence, the zero field cooled magnetization thermal evolution depends on the sample degree of inhomogeneity. In this field regime, applied field Mössbauer spectroscopy indicates that the iron rich clusters are highly polarized by the field, while the largest part of the material, consisting of AFM clusters characterized by axial anisotropy and uncompensated moments, shows soft or hard magnetism depending on T. Above the higher singular field, the M(T) curves show the trend expected for a classical antiferromagnetic material and the competitive character is suppressed. The MRV phenomenon results to be highly sensitive on both the thermal and magnetic measurement conditions; for this reason the present work proposes a characterization strategy that in principle has a large applicability in the study of disordered perovskites showing similar phenomenology.

Multiplet features and magnetic properties of Fe on Cu(111): From single atoms to small clusters

The observation of sharp atomiclike multiplet features is unexpected for individual 3d atoms adsorbed on transition-metal surfaces. However, we show by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism that individual Fe atoms on Cu(111) exhibit such features. They are reminiscent of a low degree of hybridization, similar to 3d atoms adsorbed on alkali-metal surfaces. We determine the spin, orbital, and total magnetic moments, as well as magnetic anisotropy energy for the individual Fe atoms and for small Fe clusters that we form by increasing the coverage. The multiplet features are smoothened and the orbital moment rapidly decreases with increasing cluster size. For Fe monomers, comparison with density functional theory and multiplet calculations reveals a d 7 electronic configuration, owing to the transfer of one electron from the 4s to

Direct imaging of structural heterogeneity of the melt-spun Fe85.2Si2B8P4Cu0.8 alloy

A structural heterogeneity of the melt-spun Fe85.2Si2B8P4Cu0.8 alloy has been studied by spherical aberration (Cs) corrected high-resolution transmission electron microscopy. Hollow-cone illumination imaging revealed that the density of coherent scattering regions in the as-quenched Fe85.2Si2B8P4Cu0.8 alloy is much higher than that in the Fe76Si9B10P5 bulk metallic glass. According to the Cs-corrected TEM, crystalline atomic clusters, typically of ∼1 nm in diameter, are densely distributed in an amorphous matrix of Fe85.2Si2B8P4Cu0.8 alloy. Observation of four-fold and six-fold atomic arrangements of these clusters implies existence of Fe clusters with the body centered cubic structure. These Fe clusters must be responsible for the formation of ultrahigh-density α-Fe nanocrystals produced by post-annealing.