Crystal Field Multiplet Research Articles

Electronic Structure of the Complete Series of Gas-Phase Manganese Acetylacetonates by X-ray Absorption Spectroscopy.

Metal centers in transition metal-ligand complexes occur in a variety of oxidation states causing their redox activity and therefore making them relevant for applications in physics and chemistry. The electronic state of these complexes can be studied by X-ray absorption spectroscopy, which is, however, due to the complex spectral signature not always straightforward. Here, we study the electronic structure of gas-phase cationic manganese acetylacetonate complexes Mn(acac)1-3+ using X-ray absorption spectroscopy at the metal center and ligand constituents. The spectra are well reproduced by multiconfigurational wave function theory, time-dependent density functional theory as well as parameterized crystal field and charge transfer multiplet simulations. This enables us to get detailed insights into the electronic structure of ground-state Mn(acac)1-3+ and extract empirical parameters such as crystal field strength and exchange coupling from X-ray excitation at both the metal and ligand sites. By comparison to X-ray absorption spectra of neutral, solvated Mn(acac)2,3 complexes, we also show that the effect of coordination on the L3 excitation energy, routinely used to identify oxidation states, can contribute about 40-50% to the observed shift, which for the current study is 1.9 eV per oxidation state.

Stoichiometry driven tuning of physical properties in epitaxial Fe3-xCrxO4 thin films

Tuning magnetic and electronic transport properties in spinel oxides requires a faithful description between chemical composition and cation site-occupation. Here this challenge is addressed using Fe3-xCrxO4 thin films grown by oxygen-assisted molecular-beam epitaxy within a wide range of composition (0.0 ≤ x ≤ 1.2). Spectroscopic measurements (e.g., X-ray magnetic circular dichroism), refined by theoretical simulations (e.g., crystal field multiplet), are performed to establish a quantitative link between chromium content, Fe2+/Fe3+ site-occupation and macroscopic physical properties of the layers. It is found that Fe3-xCrxO4 thin films (i) delay the transition from inverse to normal spinel configuration with increasing chromium content and (ii) promote collinear spin structure, at odds with bulk material. As a result, strong antiferromagnetic interactions are preserved between spins in tetrahedral and octahedral spinel sublattices, so that chromium-rich thin films exhibit Curie temperatures above room temperature and higher magnetization. Electron hopping is also favored by this singular cation distribution and electronic band gap is smaller than expected for these thin films. The cation site-occupation is therefore a key feature to consider for applications of Fe3-xCrxO4 thin films in spintronics and photocatalysis, as it enables manipulation of magnetic properties (Curie temperature and magnetization) and band gap engineering.

Energy Levels in Pentacoordinate d5 to d9 Complexes

Energy levels of pentacoordinate d5 to d9 complexes were evaluated according to the generalized crystal field theory at three levels of sophistication for two limiting cases of pentacoordination: trigonal bipyramid and tetragonal pyramid. The electronic crystal field terms involve the interelectron repulsion and the crystal field potential; crystal field multiplets account for the spin–orbit interaction; and magnetic energy levels involve the orbital– and spin–Zeeman interactions with the magnetic field. The crystal field terms are labelled according to the irreducible representations of point groups D3h and C4v using Mulliken notation. The crystal field multiplets are labelled with the Bethe notations for the respective double groups D’3 and C’4. The magnetic functions, such as the temperature dependence of the effective magnetic moment and the field dependence of the magnetization, are evaluated by employing the apparatus of statistical thermodynamics as derivatives of the field-dependent partition function. When appropriate, the formalism of the spin Hamiltonian is applied, giving rise to a set of magnetic parameters, such as the zero-field splitting D and E, magnetogyric ratio tensor, and temperature-independent paramagnetism. The data calculated using GCFT were compared with the ab initio calculations at the CASSCF+NEVPT2 level and those involving the spin–orbit interaction.

Niobium speciation in minerals revealed byL2,3-edges XANES spectroscopy

AbstractThe systematic mineralogy of niobium (Nb) is complex, with more than one hundred species dominated by multicomponent oxides of similar chemistry. The determination of Nb speciation in solids (i.e., the distribution between the phases and the crystal-chemical environment of Nb) is thus a challenge in geological contexts. Here, we present the first Nb L2,3-edges X-ray absorption near-edge structure (XANES) measurements on various Nb minerals and synthetic oxides with geological relevance. The interpretation of Nb L2,3-edges XANES spectra in the light of crystal-field theory shows the sensitivity of spectra to local site symmetry and electronic environment around Nb atoms. Crystal-field multiplet simulations give estimates of the 10Dq crystal-field parameter values for Nb5+, which range from 2.8 to 3.9 eV depending on Nb coordination and Nb—O distances. Rather than a 10Dq vs. R–5 relationship (where R represents the average Nb-O bond distance) expected in a point-charge model, we find a R–3 dependence with the crystal-field splitting for reference materials with octahedrally coordinated Nb. Complementary ligand-field multiplet simulations provide evidence of charge transfer between Nb and O. The contribution of the ionic and covalent characters to the Nb-O bonds is equivalent, unlike more ionic 3d metal–O bonds. This systematic characterization of the L2,3-edges XANES spectral properties of Nb provides information on the mechanisms by which Nb5+ substitutes for Fe3+, Ti4+, or Ce4+ in oxides common in geological contexts. Whereas the substitution of Nb5+ for Ce4+ does not modify the local structure of the cation site in cerianite, the substitution of Nb5+ for Ti4+ in rutile and anatase results in an increase of the cation-ligand distance and a decrease in the symmetry of the cation site. Conversely, the substitution of Nb5+ for Fe3+ in hematite and goethite results in a smaller cation site distortion. Our study demonstrates the usefulness of L2,3-edges XANES spectroscopy to determine Nb speciation in minerals to understand the processes of enrichment of this critical metal.

Atomic scale crystal field mapping of polar vortices in oxide superlattices

Polar vortices in oxide superlattices exhibit complex polarization topologies. Using a combination of electron energy loss near-edge structure analysis, crystal field multiplet theory, and first-principles calculations, we probe the electronic structure within such polar vortices in [(PbTiO3)16/(SrTiO3)16] superlattices at the atomic scale. The peaks in Ti L-edge spectra shift systematically depending on the position of the Ti4+ cations within the vortices i.e., the direction and magnitude of the local dipole. First-principles computation of the local projected density of states on the Ti 3d orbitals, together with the simulated crystal field multiplet spectra derived from first principles are in good agreement with the experiments.

Extraction of the local coordination and electronic structures of FeO6 octahedra using crystal field multiplet calculations combined with STEM-EELS

The material properties of metal oxides are sensitive to even low levels of cation doping, and the local atomic structures around such trace elements in a crystal will differ from the bulk structure due to the different ionic size and electron configuration. The present work explores the extraction of the local structure of a minority component contained in an atomic column by analyzing the Fe L2,3 energy-loss near-edge structure of atomic-resolution monochromated electron energy-loss spectra, using crystal field multiplet calculations. The crystal field splitting of the Fe 3d state and the local coordination structure of FeO6 octahedra in iron oxides in real space with known structures were determined. The results closely agreed with structures obtained from neutron diffraction and the Fe 3d DOS generated by density functional theory calculations. The previously unknown local structure of FeO6 octahedra present at octahedral sites as a minor component (14.4%) in the brownmillerite structure of Ca2Fe1.07Mn0.93O5 was determined. This structure was found to be unique and differed from the results obtained using neutron diffraction, which reflect those of mainly MnO6 octahedra. The present method is evidently an effective means of investigating the local structures around trace elements.

Crystal field effects on the photoemission spectra in Cr2O3 thin films: From multiplet splitting features to the local structure

Changes in the shape of X-ray photoemission (XPS) spectra can be related to changes in the local structure of a transition metal. By combining Crystal Field Multiplet calculations and well-controlled molecular beam epitaxy growth of α-Cr2O3(0001) thin films on α-Al2O3(0001) substrates, we prove that it is possible to link the features of Cr 2p XPS spectra with local distortions of CrO6 octahedra and d-orbitals reorganization. Hence, we show that the splitting of the Cr 2p3/2 envelope is related to the degeneracy of the t2g orbital triplet, which corresponds to a fully relaxed structure. Conversely, the broad unstructured Cr 2p3/2 envelope relies on splitting of t2g orbitals and it is the fingerprint of large trigonal distortions of CrO6 octahedra. Then, using the Cr 2p XPS as a structural tool for α-Cr2O3, we show that the Cr2O3 protective layer formed by oxidation of polycrystalline Ni30Cr alloy exhibits in-plane strains at early oxidation stages and grows preferentially along the c-axis.

Crystal Field Effects on the Photoemission Spectra in Cr2o3 Thin Films: From Multiplet Splitting Features to the Local Structure

Changes in the shape of X-ray photoemission (XPS) spectra can be related to changes in the local structure of a transition metal. By combining Crystal Field Multiplet calculations and well-controlled molecular beam epitaxy growth of a-Cr2O3(0001) thin films on a-Al2O3(0001) substrates, we prove that it possible to link the features of Cr 2p XPS spectra with local distortions of CrO6 octahedra and d-orbitals reorganization. Hence, we show that the splitting of the Cr 2p3/2 envelope is related to the degeneracy of the t2g orbital triplet, which corresponds to a fully relaxed structure. Conversely, the broad unstructured Cr 2p3/2 envelope relies on splitting of t2g orbitals and it is the fingerprint of large trigonal distortions of CrO6 octahedra. Then, using the Cr 2p XPS as a structural tool for a-Cr2O3, we show that the Cr2O3 protective layer formed by oxidation of polycrystalline Ni30Cr alloy exhibits in-plane strains at early oxidation stages and grows preferentially along the c-axis.

Crystal-field levels of Nd3+ in a new langasite compound Nd3CrGe3Be2O14

We report on the crystal-field (CF) levels of Nd3+ in Nd3CrGe3Be2O14, a representative of recently synthesized Be-containing langasites. They are the first in the family to contain two magnetic subsystems, 1D chromium and Kagome-like rare-earth ones. High-resolution broad-band temperature-dependent spectra allow us to find all five CF Kramers doublets of the ground 4I9/2 CF multiplet and 38 levels of the excited 4I11/2, 13/2, 15/2, 4F3/2, 4F5/2 + 2H9/2, 4F7/2 + 4S3/2 and 4F9/2 CF multiplets. These data form a basis for a future exploration of this interesting two-sublattice magnet with frustrated interactions. Optical transitions between Kramers doublets of Nd3+ indicate magnetic activity of 1D-chromium chains at T < 20 K.

Strain analysis from M-edge resonant inelastic X-ray scattering of nickel oxide films.

Electronic structure modifications due to strain are an effective method for tailoring nano-scale functional materials. Demonstrated on nickel oxide (NiO) thin films, Resonant Inelastic X-ray Scattering (RIXS) at the transition-metal M2,3-edge is shown to be a powerful tool for measuring the electronic structure modification due to strain in the near-surface region. Analyses from the M2,3-edge RIXS in comparison with dedicated crystal field multiplet calculations show distortions in 40 nm NiO grown on a magnesium oxide (MgO) substrate (NiO/MgO) similar to those caused by surface relaxation of bulk NiO. The films of 20 and 10 nm NiO/MgO show slightly larger differences from bulk NiO. Quantitatively, the NiO/MgO samples all are distorted from perfect octahedral (Oh) symmetry with a tetragonal parameter Ds of about -0.1 eV, very close to the Ds distortion from octahedral (Oh) symmetry parameter of -0.11 eV obtained for the surface-near region from a bulk NiO crystal. Comparing the spectra of a 20 nm film of NiO grown on a 20 nm magnetite (Fe3O4) film on a MgO substrate (NiO/Fe3O4/MgO) with the calculated multiplet analyses, the distortion parameter Ds appears to be closer to zero, showing that the surface-near region of this templated film is less distorted from Oh symmetry than the surface-near region in bulk NiO. Finally, the potential of M2,3-edge RIXS for other investigations of strain on electronic structure is discussed.

Impact of tetrahedral and square planar geometry of Ni(II) complexes with (pseudo)halide ligands to magnetic properties

Four tetracoordinate Ni(II) complexes have been prepared, structurally characterized, and subjected to magnetometric studies. The complexes [Ni(PPh3)2(NCS)2], [Ni(dppp)(NCS)2], and [Ni(dppm)Br2] are planar and thus diamagnetic. The complex [Ni(biqu)Br2] is quasi-tetrahedral, with the geometry close to C2v symmetry, and paramagnetic. While on one side it resembles a prolate bisphenoid (the angle N-Ni-N = 83 deg), on the second side it mimics an oblate bisphenoid (Br-Ni-Br = 126 deg). It exhibits a zero-field splitting of the ground term 3A2 into three crystal-field multiplets that can be described by D and E parameters within the spin Hamiltonian formalism. The ab initio calculations confirm this interpretation; however, the evaluation of the spin-Hamiltonian parameters meets difficulties owing to the quasi-degeneracy of the electronic terms.

1s2p Resonant Inelastic X-ray Scattering Magnetic Circular Dichroism as a probe for the local and non-local orbitals in CrO2

We have determined the magnetic ground state of the half-metal CrO2 based on 1s2p Resonant Inelastic X-ray Scattering Magnetic Circular Dichroism (RIXS-MCD) experiments. The two-dimensional RIXS-MCD map displays the 1s X-ray absorption spectrum combined with the 1s2p X-ray emission decay, where there is a large MCD contrast in the final state involving the 2p core hole.Our measurements show that the Cr K pre-edge structure is dominated by dipolar contributions and the quadrupole peak is invisible in direct K pre-edge absorption. Using RIXS-MCD, we reveal that the quadrupole 1s3d pre-edge has a large MCD contrast, which appears at lower energy with respect to the K pre-edge maximum.We use crystal field multiplet calculations to model the excitonic RIXS-MCD spectral shape in tetragonal (D4h) symmetry. The RIXS-MCD is strongly sensitive to the ground state distortion of the Cr4+ sites. The calculations of the RIXS-MCD maps suggest that the 3d spin–orbit interaction is fully quenched (ζ3d=0meV) and the ground state electron configuration must contain a 3B2g (D4h) contribution, which is required to explain the appearance of the Magnetic Circular Dichroism (MCD) in the Cr K pre-edge. This is in apparent contrast with the compressed tetragonal distortion.

Local vs Nonlocal States in FeTiO3 Probed with 1s2pRIXS: Implications for Photochemistry.

Metal–metal charge transfer (MMCT) is expected to be the main mechanism that enables the harvesting of solar light by iron–titanium oxides for photocatalysis. We have studied FeTiO3 as a model compound for MMCT with 1s2pRIXS at the Fe K-edge. The high-energy resolution XANES enables distinguishing five pre-edge features. The three first well distinct RIXS features are assigned to electric quadrupole transitions to the localized Fe* 3d states, shifted to lower energy by the 1s core–hole. Crystal field multiplet calculations confirm the speciation of divalent iron. The contribution of electric dipole absorption due to local p-d mixing allowed by the trigonal distortion of the cation site is supported by DFT and CFM calculations. The two other nonlocal features are assigned to electric dipole transitions to excited Fe* 4p states mixed with the neighboring Ti 3d states. The comparison with DFT calculations demonstrates that MMCT in ilmenite is favored by the hybridization between the Fe 4p and delocalized Ti 3d orbitals via the O 2p orbitals.

Study of the Structural Characteristics of 3d Metals Cr, Mn, Fe, Co, Ni, and Cu Implanted in ZnO and TiO2—Experiment and Theory

Herein we systematically study a range of dopants (Cr, Fe, Ni, Cu, and an MnCo alloy) in ZnO and TiO2 using several X-ray spectroscopic techniques. We identify the dopant’s local environment and interaction with the host lattice by employing crystal field multiplet calculations and hence clarify their potential applicability for spintronic technologies. Our density functional theory (DFT) calculations predict a decreasing probability of direct cation (Zn/Ti) substitution by dopant atoms as atomic number increases, as well as a much greater likelihood of metallic clustering in TiO2. Our spectroscopic measurements confirm that in all cases, except Mn, metallic clusters of dopant atoms form in the TiO2 crystal lattice, thus making it unfit for spintronic capabilities. On the other hand, in ZnO, the dopants substitute directly into zinc sites, which is promising for spintronic technologies.

MHz pump and probe combined with XAS-XES

We implemented a MHz pump and probe scheme on beamline ID26 of the European Synchrotron Radiation Facility. The laser runs at 1.4 MHz in the ESRF 16b mode and thus pumps every fourth pulse with ca. 15 uJ per pulse and 350 fs pulse length. The beamline hosts an X-ray emission spectrometer and thus allows combining resonant inelastic X-ray scattering with a MHz pump and probe schemes. The scattered X-rays are recorded with an avalanche photodiode in single photon counting mode. We measured the transient spectra of the spin cross-over transition in [Fe(bpy)3]Cl2 of the non-resonant Ka lines and of 1s2p resonant inelastic X-ray scattering (RIXS) at the K absorption pre-edge of Fe. The Ka transient spectrum can be readily modeled using crystal field multiplet calculations because the spectra mainly depend on the Fe spin state. The 1s2p RIXS is richer in information because it also probes the unoccupied molecular orbitals and a theoretical interpretation is more challenging.

Strongly correlated transition metal compounds investigated by soft X-ray spectroscopies and multiplet calculations

Direct probe of Mott–Hubbard (MH) to charge-transfer (CT) insulator transition in the MF2 (M=Cr–Zn) family of compounds was observed by combining F K and M L X-ray emission spectra (XES). This transition is evident as a crossover of the F-2p and M-3d occupied states. By combining F K XES data with F K edge X-ray absorption (XAS) data we directly obtained values for the M-3d Hubbard energy (Udd) and the F-2p to M-3d charge-transfer energy (ΔCT). Our results are in good agreement with the Zaanen–Sawatzky–Allen theory. We also present three examples where X-ray absorption at the transition metal L2,3 edges is used to study the oxidation state of various strongly correlated transition metal compounds. The metal oxidation state is obtained by direct comparison with crystal field multiplet calculations. The compounds are CrF2, members of the La1−xSrxCoO3 family, and the MVO3 (M = La and Y) perovskites.

Valence states of iron ions in nanostructured yttrium iron garnet Y3Fe5O12 studied by means of soft X-ray absorption spectroscopy

Nanostructured samples of yttrium iron garnet Y3Fe5O12 obtained by plastic deformation method (high-pressure torsion) were studied with help of soft X-ray absorption spectroscopy using Fe 2p and O 1s spectra. Experimental spectra were compared with crystal field multiplet calculations for Fe ions. Some amount of Fe2+ ions in nanostructured Y3Fe5O12 was found. The concentration of Fe2+ ions was found to be increased with the increase of the degree of plastic deformation.

Theory of topological Kondo insulators

We examine how the properties of the Kondo insulators change when the symmetry of the underlying crystal field multiplets is taken into account. We employ the Anderson lattice model and consider its low-energy physics. We show that in a large class of crystal field configurations, Kondo insulators can develop a topological nontrivial ground state. Such topological Kondo insulators are adiabatically connected to noninteracting insulators with unphysically large spin-orbit coupling, and as such may be regarded as interaction-driven topological insulators. We analyze the entanglement entropy of the Anderson lattice model of Kondo insulators by evaluating its entanglement spectrum. Our results for the entanglement spectrum are consistent with the surface state calculations. Last, we discuss the construction of the maximally localized Wannier wave functions for generic Kondo insulators.

Direct determination of zero-field splitting in Co(II) complexes by FAR infrared spectroscopy

Three centrosymmetric, mononuclear cobalt(II) complexes possessing large zero-field splitting parameter D have been studied by means temperature-dependent FAR infrared spectroscopy. From 15 temperature-dependent absorption peaks referring to the transition between the six lowest crystal-field multiplets the value of the D-parameter has been directly read off. These data are comparable with those values extracted by fitting the magnetic susceptibility and magnetization data.

Magnetostructural D Correlations in Hexacoordinated Cobalt(II) Complexes

The magnetostructural D correlation for hexacoordinated cobalt(II) complexes is outlined. The structural and magnetic properties of a series of mononuclear cobalt(II) complexes with the general formulas [Co(II)(L)(6)]X(2), [Co(II)(L)(2)X(2)], and [Co(II)(L)(2)(H(2)O)(2)(car)(2)] have been investigated where the coordination sphere is formed by nitrogen/oxygen-donor heterocycle (L), carboxylato (car), aqua, and chlorido ligands. The chromophores of these compounds involve {CoN(6)}, {CoO(6)}, {CoO(4)O'(2)}, {CoN(2)O(2)O'(2)}, and {CoN(2)O(2)Cl(2)}. All complexes were subjected to magnetochemical investigation down to 2 K (SQUID susceptibility and magnetization measurements). Most of the studied complexes show magnetic behavior typical for zero-field-splitting systems. The magnetism of the complex [Co(H(2)O)(6)](6-OHnic)(2) reflects the presence of the magnetic angular momentum in the ground-state crystal-field term. The obtained values of the magnetic anisotropy (D or δ) have been correlated with the structural distortion of the coordination polyhedron. This correlation can be understood with the help of crystal-field theory, where the magnetic anisotropy parameters are related to the splitting of the lowest crystal-field multiplets.