A. E. Bocquet

Electronic structure of 3d transition metal compounds: systematic chemical trends and multiplet effects

Abstract The local electronic structure of 3d transition metal compounds is characterized by a few parameters, namely the ligand p to cation d charge transfer energy Δ, the dd Coulomb repulsion energy U , and the pd transfer integrals T . Values for these parameters deduced from the cluster model analysis of cation core level photoemission spectra are shown to exhibit systematic chemical trends as functions of cation atomic number, ligand, and cation valence. Physical properties of these compounds such as the magnitudes of the band gaps, pd covalency and the character of doped carriers, however, are not necessarily smooth functions of those variables but depend also on the nominal d electron number n due to the multiplet effects leading to the stabilization of the Hunds rule ground state. As an illustrative example, the electronic structure of valence-control Mn and Fe oxides is discussed.

Systematics in the electronic structure of 3d transition-metal compounds

Abstract We have studied a wide range of 3d transition-metal compounds by a cluster configuration-interaction analysis of the metal 2p core-level photoemission spectra. Deduced values for the charge-transfer energy Δ and the d-d Coulomb repulsion U, defined with respect to the multiplet-averaged energies, show a smooth variation as functions of cation atomic number, ligand and cation valence. Many physical properties, however, show apparently irregular variation, which we attribute to d-d exchange or multiplet effects, which also reflect upon Δeff and Ueff defined with respect to the lowest multiplet energies.

Electronic structure and electron-phonon interaction in transition metal oxides with d0 configuration and lightly doped compounds

Abstract While photoemission spectroscopy has been used as a powerful technique to study the electronic structure of solids, it also reflects the dynamical as well as the static properties of the lattice through electronphonon interaction. In this paper, we present the results of photoemission studies on insulating and lightly carrier-doped oxides and discuss their relevance to the lattice properties of the solids. First, the electronic structure of perovskite-type Ti oxides as well as that of V oxides with empty metal 3 d bands is deduced and discussed in relation to recent microscopic theories of ferroelectricity. Next, we point out that some spectral features of these oxides, when lightly doped with electrons, indicate a sign of strong coupling between the doped electrons and lattice distortion and hence the polaronic nature of the charged carriers.

Predictions for core-level X-ray photoemission spectra of divalent and trivalent 3d transition-metal compounds

Abstract Core-level metal 2p X-ray photoemission spectra are calculated by a configuration-interaction cluster model for divalent and trivalent transition-metal (TM) compounds, with TM cations ranging from Mn to Cu octahedrally coordinated by six ligand anions, for a wide range of model parameter values for Δ (charge-transfer energy), U (d-d coulombic repulsion energy) and T (ligand p-metal d hybridization strength). Predictions for the positions and intensities of the satellite structures as these parameters are varied are plotted to form a practical look-up table for the various cluster model solutions found from experimentally determined spectra. In addition, the net d-electron occupation numbers for the ground states described by the model parameters are also calculated.

Electronic structure of 3d transition metal pyrites (M = Fe, Co or Ni) by analysis of the M 2p core-level photoemission spectra

We have reinterpreted the metal 2p core-level x-ray photoemission spectra of the 3d transition-metal pyrites , and using a new version of the single-impurity cluster model which includes the effects of charge transfer to the conduction band. By accounting for the effective screening of the strong core-hole potential involving the unfilled S 3p orbitals on the ligand sites, we can successfully model the highly asymmetric lineshapes and weak satellite structures found for and . Reduced screening in leads to the appearance of strong satellite structures and the spectrum is best interpreted using the screening channel to the narrow upper Hubbard band. These results indicate the importance of the empty ligand orbitals when interpreting the physical properties of the pyrite-type chalcogenides.

Electronic Structure of Ce(Pd1-xCux)3 (0≤x≤0.133) Studied by Photoemission and Bremsstrahlung Isochromat Spectroscopy

We report on X-ray photoemission (XPS), Bremsstrahlung isochromat (BIS) and 4 f resonant ultraviolet photoemission (4 f RPES or UPS) spectroscopy of Ce(Pd 1- x Cu x ) 3 (0≤ x ≤0.133). With increasing x , the 4 f 1 BIS peak clearly observable in CePd 3 is strongly depressed, the 4 f 2 BIS peak is found to shift to lower energies and the intensity of the 3 d 9 4 f 0 component of the Ce 3d core XPS is found to become weaker. These experimental results are discussed in comparison with the spectra calculated by using the impurity Anderson model. It is found that lowering of the 4 f level energy e f along with reduction of the 4 f hybridization strength V with x can qualitatively reproduce the experimental result. It is also strongly suggested that the 4 f -4 f Coulomb energies are different for the 4 f BIS and 3 d XPS final states.

Strontium-doped Lanthanum Manganese Oxides Studied by XPS

La1−xSrxMnO3 is a perovskite-type magnetic material. LaMnO3 and SrMnO3 are antiferromagnetic insulators while the system becomes a ferromagnetic metal in a certain Sr concentration. In ∼0.2<x<∼0.5, it shows a so-called colossal magnetoresistance (CMR) which is a phenomenon of a huge resistivity drop when a magnetic field is applied. This property has a potential application to the magnetic storage industry. The aim of this research was to understand the electronic structure of this system using photoemission spectroscopy and to elucidate the origin of the CMR phenomenon. The samples were polycrystalline and were prepared by solid-state reaction. XPS measurements were carried out using a Mg Kα source at liquid nitrogen temperature (∼80 K). In order to obtain a fresh, clean surface, the samples were scraped with a diamond file in situ, and surface aging was monitored by observing the intensity of a tail at the higher binding-energy side of the O 1s peak. The surface typically lasted for 1–2 h.

Systematic Aspects of the Electronic Structure of 3d Transition-Metal Compounds

Systematic changes in the electronic structure of 3d transition-metal compounds with varying chemical compositions are studied by photoemission spectroscopy and configuration-interaction cluster-model analysis. The changes consist of (i) the smooth variation of model parameters such as the charge-transfer energy Δ and the d-d Coulomb repulsion energy U with atomic number and valence and (ii) the apparently irregular multiplet corrections to Δ and U, which are functions of the nominal electron number. These systematics are reflected upon the band gaps, covalency and character of doped carriers in transition-metal oxides and chalcogenides, and upon the optical absorption spectra and the ionization energies of substitutional transition-metal impurities in semiconductors.

The inclusion of charge transfer to the conduction band in a cluster model analysis of some 3d transition metal chalcogenides

We have extended the single impurity cluster model to include the effects of charge transfer to the conduction band. By introducing a few well defined parameters to describe the interaction between the single impurity and the conduction band we find an improved agreement for the asymmetric line shapes of the metal 2p core-level photoemission spectra of the 3d-transition metal pyrites FeS2 and CoS2 and of NiAs-type NiS.

The inclusion of charge transfer to the conduction band in a cluster model analysis of core level line shapes

We have extended the single-impurity cluster model to include the effects of charge transfer to the conduction band for the interpretation of the core level photoemission spectra of highly correlated, small-gap semiconducting and metallic transition metal compounds. The model includes the charge fluctuations between the local transition metal site and the conduction band and has been successfully applied to reinterpret the spectra of some 3d transition metal chalcogenides, reproducing well the large asymmetry of the main peaks, as well as the other satellite structures.