Ligand Field Multiplet Calculations Research Articles

Magnetic Hysteresis in a Monolayer of Oriented 6 nm CsNiCr Prussian Blue Analogue Nanocrystals.

Prussian blue analogue nanocrystals of the CsINiII[CrIII(CN)6] cubic network with 6 nm size were assembled as a single monolayer on highly organized pyrolytic graphite (HOPG). X-ray magnetic circular dichroism (XMCD) studies, at the Ni and Cr L2,3 edges, reveal the presence of an easy plane of magnetization evidenced by an opening of the magnetic hysteresis loop (coercive field of ≈200 Oe) when the magnetic field, B, is at 60° relative to the normal to the substrate. The angular dependence of the X-ray natural linear dichroism (XNLD) reveals both an orientation of the nanocrystals on the substrate and an anisotropy of the electronic cloud of the NiII and CrIII coordination sphere species belonging to the nanocrystals' surface. Ligand field multiplet (LFM) calculations that reproduce the experimental data are consistent with an elongated tetragonal distortion of surface NiII coordination sphere responsible for the magnetic behavior of monolayer.

Structure rearrangements induced by lithium insertion in metal alloying oxide mixed spinel structure studied by x-ray absorption near-edge spectroscopy

Carbon-coated ZnFe2O4 (ZFO-C) spinel ferrite nanoparticles can be used in electrodes for Li-ion batteries and are known to show capacities larger than those calculated for an ideal spinel structure. In this work, the local structure evolution and reordering of this material upon lithium insertion are studied using K-edge and L-edge x-ray absorption near edge spectroscopy (XANES). XANES simulations corresponding to different lithiation stages are performed using full multiple scattering (Fe, Zn K-edge, Zn L-edge) and ligand field multiplet (LFM) calculations (Fe L-edge). XANES simulations are compared with experimental spectra obtained on ZFO-C nanoparticles previously characterized by electrochemical measurements. It is shown that a satisfactory agreement for the XANES Fe and Zn K-edges of pristine ZFO-C bulk nanoparticles can be obtained introducing a mixed spinel structure with Fe and Zn partially occupying tetrahedral and octahedral sites. Upon lithiation, changes in the XANES spectra are interpreted introducing displacements of the cations as an effect of occupation of Li into empty lattice sites. In particular, comparison of the simulations with the XANES data indicates that reversible Li insertion is accompanied by a migration of the Zn and Fe atoms from tetrahedral to octahedral sites. Furthermore, by studying L-edge XANES spectra, we show that the relocation and valence change of metal ions occur at earlier lithiation stages at the surface of the active material, gradually extending to the bulk for larger Li uptakes.

Ab initio calculation of ligand field multiplet parameters for transition metal L-edge spectra

A simple method for non-empirical ligand field multiplet calculations for transition metal L-edge spectra is presented. Ligand field splittings and anisotropic scaling factors for Coulomb integrals are obtained from density functional theory. The method is applied to transition metal monoxide solids and nickel and cobalt phthalocyanines molecules and good agreement with experiment is obtained.

Tabletop Femtosecond M-edge X-ray Absorption Near-Edge Structure of FeTPPCl: Metalloporphyrin Photophysics from the Perspective of the Metal.

Iron porphyrins are the active sites of many natural and artificial catalysts, and their photoinduced dynamics have been described as either relaxation into a vibrationally hot ground state or as a cascade through metal-centered states. In this work, we directly probe the metal center of iron(III) tetraphenyl porphyrin chloride (FeTPPCl) using femtosecond M2,3-edge X-ray absorption near-edge structure (XANES) spectroscopy. Photoexcitation at 400 nm produces a (π,π*) state that evolves in 70 fs to an iron(II) ligand-to-metal charge transfer (LMCT) state. The LMCT state relaxes to a vibrationally hot ground state in 1.13 ps, without involvement of (d,d) intermediates. The tabletop extreme-ultraviolet probe, combined with semiempirical ligand field multiplet calculations, clearly distinguishes between metal-centered and ligand-centered excited states and resolves competing accounts of Fe(III) porphyrin relaxation. This work introduces tabletop M-edge XANES as a valuable tool for measuring femtosecond dynamics of molecular transition metal complexes in the condensed phase.

X-ray magnetic circular dichroism measured at the Fe K-edge with a reduced intrinsic broadening: x-ray absorption spectroscopy versus resonant inelastic x-ray scattering measurements

X-ray magnetic circular dichroism is measured at the Fe K pre-edge in yttrium iron garnet using two different procedures that allow reducing the intrinsic broadening due to the 1s corehole lifetime. First, deconvolution of XMCD data measured in total fluorescence yield (TFY) with an extremely high signal-to-noise ratio enables a factor of 2.4 to be gained in the XMCD intensity. Ligand field multiplet calculations performed with different values of intrinsic broadening show that deconvolving such high quality XMCD data is similar to reducing the lifetime broadening from a 1s corehole to a 2p corehole. Second, MCD is measured by resonant inelastic x-ray scattering spectroscopy as a function of incident energy and emission energy. Selection of a fixed emission energy, instead of using the TFY, allows enhancing the MCD intensity up to a factor of ∼4.7. However, this significantly changes the spectral shape of the XMCD signal, which cannot be interpreted any more as an absorption spectrum.

Large Orbital Magnetic Moment Measured in the [TpFe(III)(CN)3](-) Precursor of Photomagnetic Molecular Prussian Blue Analogues.

Photomagnetism in three-dimensional Co/Fe Prussian blue analogues is a complex phenomenon, whose detailed mechanism is not yet fully understood. Recently, researchers have been able to prepare molecular fragments of these networks using a building block synthetic approach from mononuclear precursors. The main objective in this strategy is to isolate the smallest units that show an intramolecular electron transfer to have a better understanding of the electronic processes. A prior requirement to the development of this kind of system is to understand to what extent electronic and magnetic properties are inherited from the corresponding precursors. In this work, we investigate the electronic and magnetic properties of the FeTp precursor (N(C4H9)4)[TpFe(III)(CN)3], (Tp being tris-pyrazolyl borate) of a recently reported binuclear cyanido-bridged Fe/Co complex. X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements at the Fe L2,3 edges (2p → 3d) supported by ligand field multiplet calculations have allowed to determine the spin and orbit magnetic moments. Inaccuracy of the spin sum rule in the case of low-spin Fe(III) ion was demonstrated. An exceptionally large value of the orbital magnetic moment is found (0.9 μB at T = 2 K and B = 6.5 T) that is likely to play an important role in the magnetic and photomagnetic properties of molecular Fe/Co Prussian blue analogues.

Tracking the signature of low symmetry environments in the XAS K pre-edge

For Co-containing crystalline compounds measured by XAS, the influence of the local symmetry on the K pre-edge features is studied using advanced Ligand Field Multiplet (LFM) calculations, which accurately take into account the p-d hybridization. The LFM theory is used to calculate the K pre-edge spectra of Co2+ in various environments and the absolute intensities of electric-quadrupole and -dipole transitions involved in the pre-edge. This enables to reproduce the spectra for cubic (Oh, Td) and lower symmetries (C4v, D3h), and allows quantitative derivation of the local p-d mixing.

Co Polyoxometalates and a Co3O4 Thin Film Investigated by L-Edge X-ray Absorption Spectroscopy

We have performed cobalt L-edge X-ray absorption spectroscopy (XAS) on important materials for photoactive catalysis, namely nanoscale cobalt polyoxometalates (Co POM) and a Co3O4 thin film. A set of Co POM analogues were studied that vary according to the position and number of cobalts within the POM structure, metal valence state, oxygen ligand coordination geometry and heteroatom identity. Ligand field multiplet calculations simulate experimental XAS spectra in well-defined model systems provided by the Co POMs and extended to a Co3O4 thin film, thereby characterizing atomic multiplet and ligand field effects, including the ligand field parameter, structural distortions, and electron–electron interactions for Co2+ and Co3+ ions in both Oh and Td environments. The ligand field parameter, 10Dq, is determined to within an accuracy of ±0.1 eV, the spectra are sensitive to small structural distortions that further split d-levels (0.16 eV), and the strength of electron–electron interactions is found to within ±5% of the atomic value. We also find that the electronic structure parameters and the XAS spectra do not vary among POMs with pronounced differences in catalytic activity, and therefore X-ray spectroscopies even more sensitive to the 3d electronic structure (such as resonant inelastic X-ray scattering (RIXS)) should be used to differentiate the more active catalysts.

Magnetism of Ho and Er atoms on close-packed metal surfaces.

We investigated the magnetic properties of individual Ho atoms adsorbed on the (111) surface of Pt, which have been recently claimed to display single ion magnetic behavior. By combining x-ray absorption spectroscopy and magnetic dichroism measurements with ligand field multiplet calculations, we reveal a ground state which is incompatible with long spin relaxation times, in disagreement with former findings. A comparative study of the ground state and magnetic anisotropy of Ho and Er on Pt(111) and Cu(111) emphasizes the different interaction of the 4f orbitals with localized and delocalized substrate states. In particular, we find a striking rotation of the magnetization easy axis for Er, which changes from out of plane on Pt(111) to in plane on Cu(111).

Spin Tuning of Electron-Doped Metal–Phthalocyanine Layers

The spin state of organic-based magnets at interfaces is to a great extent determined by the organic environment and the nature of the spin-carrying metal center, which is further subject to modifications by the adsorbate-substrate coupling. Direct chemical doping offers an additional route for tailoring the electronic and magnetic characteristics of molecular magnets. Here we present a systematic investigation of the effects of alkali metal doping on the charge state and crystal field of 3d metal ions in Cu, Ni, Fe, and Mn phthalocyanine (Pc) monolayers adsorbed on Ag. Combined X-ray absorption spectroscopy and ligand field multiplet calculations show that Cu(II), Ni(II), and Fe(II) ions reduce to Cu(I), Ni(I), and Fe(I) upon alkali metal adsorption, whereas Mn maintains its formal oxidation state. The strength of the crystal field at the Ni, Fe, and Mn sites is strongly reduced upon doping. The combined effect of these changes is that the magnetic moment of high- and low-spin ions such as Cu and Ni can be entirely turned off or on, respectively, whereas the magnetic configuration of MnPc can be changed from intermediate (3/2) to high (5/2) spin. In the case of FePc a 10-fold increase of the orbital magnetic moment accompanies charge transfer and a transition to a high-spin state.

Femtosecond M2,3-Edge Spectroscopy of Transition-Metal Oxides: Photoinduced Oxidation State Change in α-Fe2O3

Oxidation-state-specific dynamics at the Fe M2,3-edge are measured on the sub-100 fs time scale using tabletop high-harmonic extreme ultraviolet spectroscopy. Transient absorption spectroscopy of α-Fe2O3 thin films after 400 nm excitation reveals distinct changes in the shape and position of the 3p → valence absorption peak at ∼57 eV due to a ligand-to-metal charge transfer from O to Fe. Semiempirical ligand field multiplet calculations of the spectra of the initial Fe3+ and photoinduced Fe2+ state confirm this assignment and exclude the alternative d–d excitation. The Fe2+ state decays to a long-lived trap state in 240 fs. This work establishes the ability of time-resolved extreme ultraviolet spectroscopy to measure ultrafast charge-transfer processes in condensed-phase systems.

Origin of Low Energy d–d Excitations Observed on Wet Chemically Prepared Cobalt Bearing Nanoparticles by 2p3d Resonant X-ray Emission Spectroscopy

The 2p3d resonant X-ray emission spectroscopic (RXES) measurements on 8.4 and 5.0 nm cobalt and 3.6 nm cobalt–nickel nanoparticles coated with oleate molecules are provided. The spectra reveal low energy resonant Raman features at 0.3 and 0.75 eV. In combination with time-dependent density functional theory (TD-DFT) and ligand field multiplet (LFM) calculations, these are ascribed to d–d excitations of cobalt ions in a low symmetry ligand field. Two different chemical environments of the ion may cause the transitions. In the first model cobalt ions in the nanoparticle outer atomic layer, resulting from the adsorbate binding, cause the excitations. These are transitions from a mixture of 4B1g plus 4Eg to 4Eg at 0.3 eV. At 0.75 eV transitions to 4B2g and 4A1g take place. In the alternative model the excitations occur in a cobalt molecular species that might coexist with the nanoparticles. Here the transitions are from 4A2g to 4Eg and to 4Eg plus 4B2g at 0.3 and 0.75 eV, respectively. On the basis of two-dim...

A multispectroscopic study of 3d orbitals in cobalt carboxylates: the high sensitivity of 2p3d resonant X-ray emission spectroscopy to the ligand field.

Determination of the ligand coordination number and symmetry of a transition metal ion is important to understand reaction mechanisms in inorganic chemistry. The problem can be addressed through diffraction techniques or by spectroscopy. Here we limit ourselves to the latter and note that, historically, the problem has been studied with UV/Vis, or optical absorption or electronic spectroscopy.1 Recently however the field of resonant X-ray emission spectroscopy (RXES) developed at a high pace.2 Here we show that metal 2p3d RXES is highly sensitive to the metal ion ligand field. We present a comparison of UV/Vis, 2p X-ray absorption spectroscopy (XAS), and 2p3d RXES on a series of cobalt(II) carboxylates. The X-ray data were acquired at the state-of-the-art ADRESS beamline.3 We show that 2p XAS and UV/Vis have a limited discriminative power compared to 2p3d RXES. Through ligand field multiplet (LFM) calculations we show that 2p3d RXES allows the most judicious analysis of the ligand field. While previous 2p3d RXES studies on metal oxides revealed its d–d sensitivity,4 this is the first such observation on inorganic complexes. More importantly, the notion that 2p3d RXES measures element-selective, more as well as more intense d–d excitations than UV/Vis, and that this allows a more reliable determination of the ligand field, is novel. 2p3d RXES will allow unraveling reaction mechanisms of important 3d-metal-mediated chemical processes.

Mixed-valence behavior and strong correlation effects of metal phthalocyanines adsorbed on metals

We investigate the hierarchy of local correlation and hybridization effects in metal-organic molecules adsorbed on metals. Using x-ray magnetic circular dichroism and ligand field multiplet calculations, we demonstrate that the 3$d$ electronic ground state of monolayer metal-phthalocyanine (CoPc, FePc) on Au(111) is given by the coherent superposition of two charge states, ${d}^{n}E+{d}^{n+1}$, where $E$ represents a substrate electron antiferromagnetically coupled to the central metal ion and ${d}^{n}$ the many-body ionic orbital configuration of the unperturbed molecule. These results differ from previous models of hybrid metal-organic systems and provide a consistent description of their magnetic moments and Kondo physics in terms of spin and orbital multiplicity.

Identifying local dopant structures and their impact on the magnetic properties of spintronic materials

The local structures of transition-metal dopant atoms in semiconductor host lattices of dilute magnetic semiconductors are determined using resonant inelastic x-ray scattering. Ligand field multiplet calculations are employed to analyze experimental data and show when dopant atoms substitute into the lattice and when they form metallic or oxide clusters. For ZnO doped with Mn, low-temperature ferromagnetism is shown to be intrinsic, but can be quenched by the formation of MnO clusters. For CeO${}_{2}$ doped with Co, room-temperature ferromagnetism is due to clusters of Co, while substitution into the lattice leads to paramagnetic behavior.

Photomagnetic CoFe Prussian Blue Analogues: Role of the Cyanide Ions as Active Electron Transfer Bridges Modulated by Cyanide−Alkali Metal Ion Interactions

X-ray absorption spectra at the Co L(2,3)-edges were analyzed by means of ligand field multiplet calculations in different states of three photomagnetic CoFe Prussian blue analogues of chemical formula Cs(2)Co(4)[Fe(CN)(6)](3.3) x 11 H(2)O, Rb(2)Co(4)[Fe(CN)(6)](3.3) x 11 H(2)O and Na(2)Co(4)[Fe(CN)(6)](3.3) x 11 H(2)O. These simulations of the experimental spectra allowed the quantification of the crystal field parameter (10Dq). This determination led us (i) to evidence different behaviors of the Co(III)(LS) and Co(II)(HS) ions in the three-dimensional structure related to their electronic configurations, (ii) to propose an approach based on the electronic density distribution along the Co-NC-Fe linkage to account for the energy position of the states implied in the switching properties of the compounds, and (iii) to explain the different photomagnetic properties observed as a function of the size of the inserted alkali cation by competing interactions between the cyanide ion and the transition metal ions within the CoFe cyanide bimetallic network on the one hand and the cyanide ion and the alkali metal ions on the other hand.

Giant spin and orbital moment anisotropies of a Cu-phthalocyanine monolayer

The magnetism of a Cu-phthalocyanine (CuPc) monolayer on Ag(100) was investigated using x-ray magnetic circular dichroism (XMCD) and ligand-field multiplet calculations. Contrary to other metal-Pc adsorbed on metals, we show that the local CuPc moment survives the interaction with the electronic states of the substrate and presents enhanced susceptibility with respect to bulk powder samples. Our measurements reveal extraordinary orbital moment anisotropy (500%) and an anisotropic spin dipole moment up to twice the isotropic spin in a metal-organic layer. A complete description of the orbital, spin, and spin-orbit operators is provided based on the XMCD sum rules.

A Star-Shaped Heteronuclear CrIIIMnII3 Species and Its Precise Electronic and Magnetic Structure: Spin Frustration Studied by X-Ray Spectroscopic, Magnetic, and Theoretical Methods

Molecular magnets incorporate transition-metal ions with organic groups providing a bridge to mediate magnetic exchange interactions between the ions. Among them are star-shaped molecules in which antiferromagnetic couplings between the central and peripheral atoms are predominantly present. Those configurations lead to an appreciable spin moment in the nonfrustrated ground state. In spite of its topologically simple magnetic structure, the [Cr(III)Mn(II)(3) (PyA)(6)Cl(3)] (CrMn(3)) molecule, in which PyA represents the monoanion of syn-pyridine-2-aldoxime, exhibits nontrivial magnetic properties, which emerge from the combined action of single-ion anisotropy and frustration. In the present work, we elucidate the underlying electronic and magnetic properties of the heteronuclear, spin-frustrated CrMn(3) molecule by applying X-ray magnetic circular dichroism (XMCD), as well as magnetization measurements in high magnetic fields, density functional theory, and ligand-field multiplet calculations. Quantum-model calculations based on a Heisenberg Hamiltonian augmented with local anisotropic terms enable us not only to improve the accuracy of the exchange interactions but also to determine the dominant local anisotropies. A discussion of the various spin Hamiltonian parameters not only leads to a validation of our element selective transition metal L edge XMCD spin moments at a magnetic field of 5 T and a temperature of 5 K but also allows us to monitor an interesting effect of anisotropy and frustration of the manganese and chromium ions.

Correlation of O (1s) and Fe (2p) near edge x-ray absorption fine structure spectra and electrical conductivity of La1−xSrxFe0.75Ni0.25O3−δ

A -site substitution of La3+ by Sr2+ in polaron conducting ABO3-type perovskite La1−xSrxFe0.75Ni0.25O3−δ causes oxidation of Fe3+ toward Fe4+ and formation of conducting electron holes, as evidenced by Fe (2p) and O (1s) near edge x-ray absorption fine structure spectra. Hole doping is reflected by linear variation of the prepeak ratio eg(↑)/[t2g(↓)+eg(↓)] of oxygen spectra, along with increased conductivity. The significant increase in conductivity due to NiO doping in La1−xSrxFeO3−δ is caused by increased overlap between Fe (3d) and O (2p) and charge transfer from the O (2p) to the Ni (3d) states, as concluded from near edge x-ray absorption fine structure spectra and ligand field multiplet calculations.

Valence and spin states, and the metal-insulator transition in ferromagneticLa2−xSrxMnNiO6(x=0,0.2)

Electronic structures of a ferromagnetic insulator ${\text{La}}_{2}{\text{MnNiO}}_{6}$ and Sr-doped ${\text{La}}_{1.8}{\text{Sr}}_{0.2}{\text{MnNiO}}_{6}$ have been investigated by employing soft x-ray synchrotron-radiation spectroscopies, first-principles band-structure calculations, and ligand-field multiplet calculations. Our study provides evidence that ${\text{La}}_{2}{\text{MnNiO}}_{6}$ has the [high-spin (HS) ${\text{Mn}}^{4+}\text{\ensuremath{-}}{^{3}A}_{2}\text{ }{\text{Ni}}^{2+}$] configuration while ${\text{La}}_{1.8}{\text{Sr}}_{0.2}{\text{MnNiO}}_{6}$ has [HS ${\text{Mn}}^{4+}\text{\ensuremath{-}}{^{3}A}_{2}\text{ }{\text{Ni}}^{2+}$ low-spin ${\text{Ni}}^{3+}$]. It is directly observed that the spin moments of Mn and Ni ions are coupled ferromagnetically. The metallic spectral weight is observed for $x=0.2$ in $\text{O}\text{ }1s$ x-ray absorption spectroscopy, which is consistent with the half-metallic ground state predicted by band-structure calculations.