Delocalization Of 3d Electrons Research Articles

Delocalization of d-electrons induced by cation coupling in ultrathin Chevrel-phase NiMo3S4 nanosheets for efficient electrochemical water splitting

Chevrel-phase metal sulfides are known to be promising materials for energy conversion and storage applications. However, a detailed understanding of the intrinsic kinetic mechanisms of electrocatalytic bifunctional hydrogen and oxygen evolution reactions (HER/OER) on NiMo3S4-based Chevrel-phases is lacking. Herein, novel ultrathin self-assembled nanosheets of NiMo3S4 are coupled with transition metal atoms (M/N-NiMo3S4; where M = Co, Fe, and Cu) were formed by a facile hydrothermal approach. Notably, the Co/N-NiMo3S4 electrocatalyst exhibits excellent performance in terms of ultralow overpotentials of 78, 208, 282, and 307 mV at 10, 100, 500, and 1000 mA cm−2 for the HER; and 186, 204, and 225 mV at 50, 100, and 300 mA cm−2 for the OER, respectively. Experimental and first principle calculations demonstrate that Co atoms coupling with edge Ni atoms results in d‐electron delocalization on Co/N-NiMo3S4, signifying the efficient charge transfer to improve overall water electrolysis. In addition, an upshift in the d‐band center of Co/N-NiMo3S4 can optimize the free energies of a variety of reaction intermediates for water adsorption and dissociation; thereby facilitating the robust alkaline overall water electrolysis at 1.47 V. This work therefore greatly deepens the understanding of the bifunctional hydrogen and oxygen evolution reaction of Chevrel-phase electrocatalysts.

Spin-Enhanced C–C Coupling in CO 2 Electroreduction with Oxide-Derived Copper

Spin-Enhanced C–C Coupling in CO ₂ Electroreduction with Oxide-Derived Copper

Pressure-induced high-spin/low-spin disproportionated state in the Mott insulator FeBO3

The pressure-induced Mott insulator-to-metal transitions are often accompanied by a collapse of magnetic interactions associated with delocalization of 3d electrons and high-spin to low-spin (HS-LS) state transition. Here, we address a long-standing controversy regarding the high-pressure behavior of an archetypal Mott insulator FeBO3 and show the insufficiency of a standard theoretical approach assuming a conventional HS-LS transition for the description of the electronic properties of the Mott insulators at high pressures. Using high-resolution x-ray diffraction measurements supplemented by Mössbauer spectroscopy up to pressures ~ 150 GPa, we document an unusual electronic state characterized by a “mixed” HS/LS state with a stable abundance ratio realized in the Roverline{3 }c crystal structure with a single Fe site within a wide pressure range of ~ 50–106 GPa. Our results imply an unconventional cooperative (and probably dynamical) nature of the ordering of the HS/LS Fe sites randomly distributed over the lattice, resulting in frustration of magnetic moments.

Synthesis and characterization of graphene modified by iron oxide nanoparticles

The process of interaction of graphene with iron oxide nanoparticles was investigated. First, graphene oxide (GO) modified with magnetite Fe3O4 nanoparticles was successfully synthesized. Raman and Mössbauer spectroscopy revealed that the magnetite Fe3O4 in combination with GO became non-stoichiometric, and the maghemite phase γ-Fe2O3 appears. Subsequent reduction of graphene oxide by thermal treatment leads to an increase in the fraction of maghemite content and, in addition, the hematite phase α-Fe2O3 appears in the sample annealed at above 500 °C. Meanwhile, the core-shell nanocomposites of FexOy/G appear, were FexOy consists of a mixture of the Fe3O4, γ-Fe2O3 and α-Fe2O3 phases. The content of each phase can be varied by the annealing temperature. Magnetic, Mössbauer and Raman spectroscopy measurements indicate that graphene can interact with iron oxide. Charge-transfer from iron to graphene can occur due to delocalization of 3d electrons, which reduces the overall magnetic moment of the charge-transfer complexes. These properties can have potential applications in electronic such as supercapacitors, advanced anode materials for lithium-ion batteries, magnetically targeted drug delivery, photothermic therapy, and magnetic resonance imaging.

The influence of charge transfers effects in monazite-type LaVO4 and perovskite-type LaVO3 prepared by sol-gel acrylamide polymerization

Core-hole spectroscopy such as X-ray absorption spectroscopy (XAS) is useful to determine the electronic structure of strongly correlated and strongly hybridized compounds such as vanadates. Monazite-type LaVO4 and perovskite-type LaVO3 are good candidates to elucidate the electronic structure through the vanadium L2,3 edge. LaVO4 was prepared by sol-gel acrylamide polymerization and solid-state reaction. LaVO3 was obtained by reduction of LaVO4 using Zr as gatherer. Monoclinic crystal phase for LaVO4 and orthorhombic crystal phase for LaVO3 were confirmed by the Rietveld refinement of X-ray diffraction patterns. XAS comparison between Vanadium L2,3 edge confirms the presence of V5+ for the monazite and V3+ for the orthorhombic perovskite. Multiplet calculations including crystal field and charge transfer effects (CTM) were performed in order to elucidate the tetragonal (D4h symmetry) parameters Dq, Ds and Dt, the charge transfer energy Δ, and d-d Coulomb repulsion energy U parameters. CTM confirms for LaVO3 the strong V 3d–O 2p hybridization with a significant contribution of covalent character due to the delocalization of 3d electrons. For LaVO4 this work suggest the reclassification of this band insulator as charge transfer insulator that shows a significant contribution of ionic character.

Superconductivity in transition metals.

A qualitative account of the occurrence and magnitude of superconductivity in the transition metals is presented, with a primary emphasis on elements of the first row. Correlations of the important parameters of the Bardeen-Cooper-Schrieffer theory of superconductivity are highlighted with respect to the number of d-shell electrons per atom of the transition elements. The relation between the systematics of superconductivity in the transition metals and the periodic table high-lights the importance of short-range or chemical bonding on the remarkable natural phenomenon of superconductivity in the chemical elements. A relationship between superconductivity and lattice instability appears naturally as a balance and competition between localized covalent bonding and so-called broken covalency, which favours d-electron delocalization and superconductivity. In this manner, the systematics of superconductivity and various other physical properties of the transition elements are related and unified.

Magnetic and spectroscopic characterizations of high-spin cobalt(II) complex with soft-scorpionate ligand

For the purpose of investigating the effect of sulfur ligands on the metal complex electronic states, a soft scorpionate ligand, which bears three sulfur coordination sites on a molecule, was used to afford a novel high-spin cobalt(II) complex, [CoII(Tbz)2] (Tbz− = hydrotris(2-mercaptobenzothiazolyl)borate), having an octahedral [CoIIS6] core. Based on the electronic spectrum of the complex consisting of two d-d bands, the ligand-field splitting and the Racah parameter were estimated as Δ0/hc = 8000 cm− 1 and B/hc = 560 cm− 1, respectively. The latter proved remarkable nephelauxetic effect arising from delocalization of d-electrons over the ligands. These parameters were successfully utilized in the simulation of the magnetic susceptibility.

Effect of high pressure on the crystal structure and electronic properties of magnetite below 25 GPa

We report results from high-pressure single-crystal X-ray diffraction and Mössbauer absorption experiments on magnetite. Based on high-quality diffraction data, we have obtained accurate information on the crystal structure of magnetite below 25 GPa, which enables an unambiguous interpretation of the Mössbauer data using constrained area ratios and a full transmission integral fit that avoids area distortion due to thickness effects. Based on our analysis, all aspects of the electronic and magnetic properties of magnetite reported previously below 25 GPa at ambient temperature can be explained solely by the enhanced delocalization of 3d electrons of iron atoms. For instance, we present evidence that the compression-induced metallization changes the sign of the charge carrier spin polarization at 15 GPa.

Many-body effect in the resonant Ti L 23–M 23V Auger-electron spectroscopy spectra and Auger-photoelectron coincidence spectroscopy spectra of Ti oxides

Recently Danger et al. [J. Danger, H. Magnan, D. Chandesris, P. Le Fèvre, S. Bourgeois, J. Jupille, A. Verdini, R. Gotter, A. Morgante, Phys. Rev. B 64 (2001) 045110] and Le Fèvre et al. [P. Le Fèvre, J. Danger, H. Magnan, D. Chandesris, J. Jupille, S. Bourgeois, M.-A. Arrio, R. Gotter, A. Verdini, A. Morgante, Phys. Rev. B 69 (2004) 155421] showed the absence of resonant Raman scattering feature in the Ti L 23–M 23V resonant Auger-electron spectroscopy (RAES) spectra of Ti oxides measured across the Ti 2p edges. They attributed the absence to the covalent character of the Ti–O bond which allows an effective delocalization of 3d electrons. It is shown by a many-body theory that when the time scale of relaxation of the resonantly excited core-hole state to the fully relaxed core-hole state is much shorter than that of core-hole decay, any sizeable Raman scattering is absent in the RAES spectra measured across the Ti 2p edges. The relaxation width depends on the hybridization strength and the charge transfer (CT) energy between the two states. The L 2–L 3V Coster–Kronig (CK) decay widths of TiO 2 and TiO 2− x are determined from the L 23–M 23V Auger-photoelectron coincidence spectroscopy (APECS) spectra reported in the aforementioned papers. They are about 0.18 and 0.35 eV, respectively. The CK-decay width in the reduced Ti oxide increases compared to that of TiO 2 in rutile because of filling of the 3d states just below the Fermi level in the former.

Spin polarization effect for Ta 2 molecule

Density functional theory (DFT) (B3p86) has been used to optimize the structure of the molecule Ta2. The result shows that the ground state of molecule Ta2 is a 7-multiple state and its electronic configuration is 7Σu+, which shows the spin polarization effect for molecule Ta2 of transition metal elements for the first time. Meanwhile, spin pollution has not been found because the wavefunction of the ground state does not mix with those of higher states. So, the fact that the ground state of molecule Ta2 is a 7-multiple state indicates a spin polarization effect of molecule Ta2 of the transition metal elements, i.e. there exist 6 parallel spin electrons and the non-conjugated electrons are greatest in number. These electrons occupy different space orbitals so that the energy of molecule Ta2 is minimized. It can be concluded that the effect of parallel spin of the Ta2 molecule is larger than the effect of the conjugated molecule, which is obviously related to the effect of d-electron delocalization. In addition, the Murrell–Sorbie potential functions with parameters for the ground state 7Σu+ and other states of the Ta2 molecule are derived. The dissociation energy De, equilibrium bond length Re and vibration frequency ωe for the ground state of Ta2 molecule are 4.5513eV, 0.2433 nm and 173.06 cm−1, respectively. Its force constants f2, f3 and f4 are 1.5965×102aJ.nm−2, –6.4722×103aJ.nm−3 and 29.4851×104aJ.nm−4, respectively. Other spectroscopic data ωe χe, Be and αe for the ground state of Ta2 are 0.2078 cm−1, 0.0315 cm−1 and 0.7858×10−4 cm−1, respectively.

Neutral and Cationic VIV/VV Mixed-Valence Alkoxo-polyoxovanadium Clusters [V6O7(OR)12]n+ (R = −CH3, −C2H5): Structural, Cyclovoltammetric and IR-Spectroscopic Investigations on Mixed Valency in a Hexanuclear Core

The alkoxo-polyoxovanadium clusters [V6O7(OR)12]n+ (R = -CH3, -C2H5) are fully alkylated polyoxometalate derivatives comprising a hexavanadate core with the vanadium ions organized in an octahedral fashion, a classic isopolyoxometalate structure (Lindqvist) which as an entity is not known for vanadium. The clusters are highly redox-active compounds, displaying a large number of thermodynamically stable redox isomers of which the chemical syntheses and structural characterization of the neutral and cationic V(IV)/V(V) mixed-valence species [V(IV)(4-n)V(V)(2+n)O7(OR)12]n+ [SbCl6]n (R = -CH3, n = 0, 1; R = -C2H5, n = 0, 1, 2) are presented here. Neutral and positively charged clusters remain exceptional in the field of polyoxometalate chemistry. Results obtained from cyclic voltammetry, infrared spectroscopy, and from valence sum calculations conducted on X-ray structural data classify these clusters as class II mixed-valence compounds. Their highly symmetrical molecular structures make them particularly interesting as model compounds for the investigation of intervalence charge transfer and electron delocalization in the hexanuclear core. Furthermore, the large number of isostructural redox isomers affords a high variability in d-electron content. Accordingly, a dependency could clearly be established between the extent of electron delocalization and the V(IV)/V(V) ratio in a cluster species. A further interesting observation concerns the neutral ethoxo compound [V(IV)4V(V)2O7(OC2H5)12] (3) which exhibits a crystallographic phase transition accompanied by the conversion from a structure at 173 K with fully localized valencies to a room-temperature modification displaying complete d-electron delocalization.

Properties of Nanogranular Metal–Dielectric Composites in Strong Electric Fields and the Cluster Electronic States

The electrical resistance of granular structures with ferromagnetic and nonferromagnetic metal nanoparticles embedded in concentrations below the percolation threshold was studied in strong electric fields. More specifically, amorphous silicon dioxide containing nanoparticles of a Co41Fe39B20 alloy [(a-SiO2)100− x(Co41Fe39B20)x structure] and amorphous hydrogenated carbon with embedded copper nanoparticles, a-C: H(Cu), were investigated. The (a-SiO2)100−x(Co41Fe39B20)x structures revealed changes in the electrical resistance and magnetoresistance after being subjected to a strong electric field. The changes could have reversible or irreversible character and depended on the electrical prehistory of the sample. A strong electric field caused not only a decrease in the electrical resistance but also a decrease in the magnetoresistance, although the magnetization of the sample remained unchanged. The temperature dependences of the current in a-C: H(Cu) films exhibited conductivity peaks under a decrease in temperature in strong electric fields and transitions from the insulating to conducting state; after the field was removed, there occurred reverse transitions and conductivity relaxation, as well as pronounced changes in the dielectric permittivity and an increase in dielectric losses with increasing temperature. A model of cluster electronic states (CESs) is proposed to account for the experimental findings. These states are created by electrons of the metal grains and matrix defects near the Fermi surface. The observed features find explanation in a change in the CES structure. A strong electric field does not bring about d-electron delocalization, and the fraction of d electron wave functions in a CES is small.

Relative stability of missing-row reconstructed (1 1 0) surfaces of noble metals

The missing-row reconstruction has been observed experimentally on clean Au, Pt and Ir, while Cu and Ag are dormant cases in that the reconstruction can be provoked only by modest alkali coverage. The experimentally observed deconstruction and roughing transitions of Au(1 1 0) and Pt(1 1 0) are interesting realizations of two-dimensional Ising and XY models. The detailed understanding of the structures and energies of the missing-row configurations is also important for the constructions of many-atom model such as the embedded-atom potentials. We have carried out density-functional molecular-dynamics calculations for the missing-row reconstructed (1×2), (1×3), and (1×∞) surfaces. The calculation results show that the delocalization of d-electrons and the accumulation of sp-electrons near the surface associated with the multilayer relaxation are important factors in stabilizing the experimentally observed (1×2) missing-row structure for Au and Pt. For Ir(1 1 0), a faceted (3 3 1) surface is shown to be the lowest energy configuration, in agreement with experimental observation. These results shed light on the relative stability of the missing-row configurations, and are useful for the construction of improved semi-empirical potentials, especially for the long range interaction.

Factors affecting metal-metal bonding in the face-shared d(3)d(3) bioctahedral dimer systems, MM'Cl(9)(5-) (M, M' = V, Nb, Ta).

Density functional theory (DFT) calculations have been used to investigate the d(3)d(3) bioctahedral complexes, MM'Cl(9)(5-), of the vanadium triad. Broken-symmetry calculations upon these species indicate that the V-containing complexes have optimized metal-metal separations of 3.4-3.5 A, corresponding to essentially localized magnetic electrons. The metal-metal separations in these weakly coupled dimers are elongated as a consequence of Coulombic repulsion, which profoundly influences (and destabilizes) the gas-phase structures for such dimers; nevertheless, the intermetallic interactions in the V-containing dimers involve significantly greater metal-metal bonding character than in the analogous Cr-containing dimers. These observations all show good agreement with existing experimental (solid state) results for the chloride-bridged, face-shared dimers V(2)Cl(9)(5-) and V(2)Cl(3)(thf)(6)(+). In contrast to the V-containing dimers, complexes featuring only Nb and Ta have much shorter intermetallic distances (approximately 2.4 A) consistent with d-electron delocalization and formal metal-metal triple bond formation; again, good agreement is found with available experimental data. Calculations on the complexes V(2)(mu-Cl)(3)(dme)(6)(+), Nb(2)(mu-dms)(3)Cl(6)(2-), and Ta(2)(mu-dms)(3)Cl(6)(2-), which are closely related to compounds for which crystallographic structural data exist, have been pursued and provide an insight into the intermetallic interactions in the experimentally characterized complexes. Analysis of the contributions from d-orbital overlap (E(ovlp)) stabilization, as well as spin polarization (exchange) stabilization of localized d electrons (E(spe)), has also been attempted for the MM'Cl(9)(5-) dimers. While E(ovlp) clearly dominates over E(spe) as a stabilizing factor in those dimers containing only Nb and Ta metal atoms, detailed assessment of the competition between E(ovlp) and E(spe) for V-containing dimers is obstructed by the instability of triply bonded V-containing dimers against Coulombic explosion. On the basis of the periodic trends in E(ovlp) versus E(spe), the V-triad dimers have a greater propensity for metal-metal bonding than do their Cr-triad or Mn-triad counterparts.

Direct evidence for the electronic phase inhomogeneity in HoNi 2B 2C

We substitute a few parts per million of 57 Co for Ni. The Mössbauer spectra of HoNi 2( 57 Co) B 2 C reveal the presence of two components, one of which is truly metallic as indicated by delocalization of 3d electrons. In the other component, the electrons are delocalized to a lesser extent. The relative ratio of the components can vary from sample to sample.

XPS study of transition metal electronic structure in crystalline and liquid states

The valence band photoemission spectra of Ni and Cu metals in solid and liquid states are measured using a magnetic spectrometer. In the liquid state, fine structure spectra are displayed more clearly: the valence band width is narrower in the liquid state than in the solid state. Narrower features are specific to the spectra of isolated atoms. In a liquid, the icosahedral clusters can play the role of ‘giant’ collective isolated atoms. The difference in the fine structure of the valence band XPS-spectra of transition metals between liquid and crystalline state can be explained: first, by the change of the local order, the chemical bonding, and the delocalization of d-electrons during the disordering process; and, second, by the enhancement of overlap of the d-electron orbitals of nearest neighbour atoms in clusters of the liquid compared to the crystalline metal.

Anomalous behaviors in high temperature superconductors: Lattice effect and d-electron delocalization effect of the normal state on the Bi, Tl, and Y-system superconductors

The dynamic studies of the hyperfine interactions and the lattice vibrational behavior are performed by means of temperature-dependent Mossbauer spectroscopy on the Bi-, Tl-, and Y-based superconductors, respectively. The temperature-dependent Mossbauer recoil-free factor f show that, above, but relatively close to, the transition temperature, there is an anomalous lattice effect which is related to the softening of phonon vibrational modes for Bi-2223, Tl-1223, and Y-123 superconductors. Moreover, for such three systems, the anomalous decrease of the Mossbauer isomer shift above Tc reveal the delocalization effect of d-electrons. Since the lattice effect is accompanied by d-electron delocalization effect at the almost same temperature region, it is believed that the structural instability involved in phonon softening is strongly interrelated to the change of the electronic states, and the phase separation behavior.

On the Positronium Spin Exchange Reactions Promoted by 3d Aqua Ions

In order to ascertain possible relationships between the rate constants, KSE, of positronium, Ps, spin exchange /SE/ reactions promoted by 3d aqua ions and the electron configuration of the ion, the KSE's of Mn aq 2+ and Co aq 2+ aqua ions were measured together with that of [Cr(NH3)6]3+. The KSE's values obtained are discussed together with those of the Cr aq 3+ , Fe aq 2+ , Ni aq 2+ and [Ni(NH3)6]2+ ions previously measured. It was found that Cr aq 3+ and Mn aq 2+ KSE's are ∼1 M−1ns−1, while those of Fe aq 2+ , Co aq 2+ and Ni aq 2+ ions are ∼2.5 M−1ns−1. Thus the KSE values of 3d aqua ions do not depend solely on the number of their unpaired electrons as it was found for the 4f aqua ions. The trend observed parallels that of the electron delocalization from the metal atom; the delocalization occurs only with Fe aq 2+ , Co aq 2+ and Ni aq 2+ ions, but not with the Cr aq 3+ and Mn aq 2+ aqua ions. The trend of the KSE's of aqua, ammine and perhaps of chloro complexes parallels that of ligand capabilities to cause d-electron cloud expansion, and thus d-electron delocalization from metal atoms.

On the postronium spin exchange reactions promoted by 3d aqua ions

In order to ascertain possible relationships between the rate constants, KSE, of positronium, Ps, spin exchange /SE/ reactions promoted by 3d aqua ions and the electron configuration of the ion, the KSE's of Mn aq 2+ and Co aq 2+ aqua ions were measured together with that of [Cr(NH3)6]3+. The KSE's values obtained are discussed together with those of the Cr aq 3+ , Fe aq 2+ , Ni aq 2+ and [Ni(NH3)6]2+ ions previously measured. It was found that Cr aq 3+ and Mn aq 2+ KSE's are ∼1 M−1ns−1, while those of Fe aq 2+ , Co aq 2+ and Ni aq 2+ ions are ∼2.5 M−1ns−1. Thus the KSE values of 3d aqua ions do not depend solely on the number of their unpaired electrons as it was found for the 4f aqua ions. The trend observed parallels that of the electron delocalization from the metal atom; the delocalization occurs only with Fe aq 2+ , Co aq 2+ and Ni aq 2+ ions, but not with the Cr aq 3+ and Mn aq 2+ aqua ions. The trend of the KSE's of aqua, ammine and perhaps of chloro complexes parallels that of ligand capabilities to cause d-electron cloud expansion, and thus d-electron delocalization from metal atoms.

Electron delocalization in rhodium (1+) bis diisocyanobiphenyl chloride polymers with direct rhodium-rhodium interactions

It is shown in the present work that hexagonal and tetragonal forms of Rh(I) bis-4,4′-diisocyanobiphenyl chloride polymers containing stacks of rhodium ions in the integral + 1 state show d-electron delocalization in one dimension along Rh +Rh + chains at and below ambient temperature at atmospheric pressure.