The results of a systematic XPS study, under high controlled conditions, of different basic oxides of transition metals, alkali and alkaline-earth metals are presented; the XPS data of some hydroxides and peroxides are also reported. Variations of the O 1s binding energies are analysed and one point of interest is the large
 binding energy scale obtained for O 1s peaks all associated with a ‘‘2−
’’ formal charge. Through extended
 Huckel theory-tight binding (EHT-TB) calculations, attempts are made to rationalize the observed variations.
 The results illustrate
 the significant
 differences
 between real
 charges on oxygen atoms in transition metal and alkaline-earth oxides.

Systematic XPS studies of metal oxides, hydroxides and peroxides

Giant and isotropic magnetoresistance as huge as −53% was observed in magnetic manganese oxide La0.72Ca0.25MnOz films with an intrinsic antiferromagnetic spin structure. We ascribe this magnetoresistance to spin‐dependent electron scattering due to spin canting of the manganese oxide.

31p-S-4 Giant Magnetoresistance in Magnetic Manganese Oxide with Intrinsic Antiferromagnetic Spin Structure

Cobalt oxides comprise two readily accessible cation oxidation states: Co 2+ and Co 3+ , which are thermodynamically competitive under common ambient conditions, and redox mechanisms connecting the two states are largely responsible for their success in partial oxidation catalysis. In our studies, CoO(1 0 0), Co 3 O 4 (1 1 0), and Co 3 O 4 (1 1 1) single crystal substrates have been investigated with X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), and low energy electron diffraction (LEED) for their surface reactivity toward O 2 and H 2 O and for their stability under reducing UHV conditions. There is facile inter-conversion between CoO and Co 3 O 4 stoichiometry at the oxide surface which, despite the compositional variability, remains well ordered in long-range structure. Surface impurities, however, can pin the surface at either CoO or Co 3 O 4 compositional extremes. Contrary to reports of a pressure gap that creates difficulty in oxide hydroxylation under UHV, it is pos sible to hydroxylate both cobalt monoxide and spinel oxide substrates with H 2 O, provided sufficient activation is available to dis sociate the water molecule.

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Cobalt oxide surface chemistry: The interaction of CoO(1 0 0), Co3O4(1 1 0) and Co3O4(1 1 1) with oxygen and water

The present work reports the synthesis and the characterization of cobalt oxide thin films obtained by chemical vapor deposition (CVD) on indium tin oxide (ITO) substrates, using a cobalt(II) β-diketonate as precursor. The complex is characterized by electron impact mass spectrometry (EI-MS) and thermal analysis in order to investigate its decomposition pattern. The depositions are carried out in a cold wall reactor in the temperature range 350−500 °C at different oxygen pressures, to tailor film composition from CoO to Co3O4. The crystalline nanostructure is evidenced by X-ray diffraction (XRD), while the surface and in-depth chemical composition is studied by X-ray photoelectron (XPS) and X-ray excited auger electron spectroscopy (XE-AES). Atomic force microscopy (AFM) is employed to analyze the surface morphology of the films and its dependence on the synthesis conditions. Relevant results concerning the control of composition and microstructure of Co−O thin films are presented and discussed.

Composition and Microstructure of Cobalt Oxide Thin Films Obtained from a Novel Cobalt(II) Precursor by Chemical Vapor Deposition

X-ray absorption and dichroism of transition metals and their compounds

We have performed resonance photoemission, angle-resolved photoemission, and core-level photoemission studies of single-crystalline CoO. On the one hand, strong correlation effects among the {ital d} electrons are observed, as signaled by a strong reduction of Co 3{ital d} bandwidths and satellites in both the valence band and the cation core levels. On the other hand, the oxygen states are found to be very bandlike, as indicated by strong dispersions of oxygen states in the valence band and the lack of oxygen satellites. We give estimations of 30 and 1.5 for {ital U}/{ital W} (Coulomb interaction divided by bandwidth) ratio of Co 3{ital d} bands and O 2{ital p} bands, respectively. By comparing the experimental and theoretical {ital E} versus {bold k} relation, we show that the density-functional band calculation works well for the oxygen bands but not for the Co bands. We argue that CoO is not a band insulator, but a charge-transfer insulator. We have also observed the effects of local magnetic order on the electronic structure. Finally, we suggest a guideline on calculating the band structure of CoO: introducing a mechanism that reduces the Co 3{ital d} bandwidth by 25% while still retaining the other essential features ofmore » the band calculation.« less

Photoemission study of CoO

We extend the charge-transfer model with the core-hole--3d-electron Coulomb attraction [van der Laan et al., Phys. Rev. B 23, 4369 (1981)] to the 2p core-level photoemission satellite structures of cobalt, iron, and manganese dihalides. This model was found to account for the positions and intensities of satellites and main peaks very well with reasonable values of parameters. These parameter values show the expected trends not only along the ligand series from fluorine to bromine but also along the transition-metal series from copper to manganese. This gives us confidence that the charge-transfer mechanism is responsible for the satellite structures in the 2p core-level photoemission spectra of heavy-transition-metal compounds, and that the screening response is important even for insulators in the presence of a core hole. It also suggests the core-level photoemission spectra, if properly understood, can be used to obtain parameters on the valence-electronic structures.

Charge-transfer satellites in the 2p core-level photoelectron spectra of heavy-transition-metal dihalides.

We have studied the electronic structures of rare-earth trihalides ${\mathrm{LaF}}_{3}$, ${\mathrm{LaCl}}_{3}$, ${\mathrm{CeF}}_{3}$, ${\mathrm{CeCl}}_{3}$, and ${\mathrm{GdF}}_{3}$ by x-ray photoemission spectroscopy (XPS), bremsstrahlung isochromat spectroscopy (BIS), and electron-energy-loss spectroscopy (EELS). The double peak structures of the rare-earth 3d core-level XPS can be understood within the Anderson impurity model as being due to the charge transfer from the ligand 2p level to the rare-earth unfilled 4f level. The parameters obtained from fitting these core-level spectra using the Gunnarsson-Sch\"onhammer-model approach are consistent with the valence-band XPS and the conduction-band BIS spectra. In the higher binding-energy side (10--40 eV) of 3d XPS, several more satellites are observed. Through comparing with other core-level XPS and EELS, we find that they are mainly loss structures arising from the interband transition, the rare-earth 5p excitation, and the charge-transfer transition, but there are some contributions from the ``intrinsic'' plasmon excitations as well.

Electron-spectroscopy study of rare-earth trihalides.

We have analyzed the satellite structures of the transition-metal 2${\mathit{p}}_{3/2}$ core-level photoemission spectra of NiO, CoO, and FeO within the charge-transfer cluster model. The spectra can be fitted with reasonable values of the parameters of the model, which show the expected trend along the transition-metal series as well as along the ligand series including halogen and oxygen. The values of the d-d electron correlation energy U and the charge-transfer energy \ensuremath{\Delta} from the ligand to the metal 3d level in the valence electronic structures are then estimated from these parameter values. We confirm that the large-U value is important for the insulating properties of these monoxides, but the band gap is not determined by U, but rather by \ensuremath{\Delta}. This supports the recent proposal that these oxides are not Mott-Hubbard insulators in the original sense, but belong to the charge-transfer-type insulators. We also calculate the magnitudes of the band gaps with our parameter values, and they are in fair agreement with experimental values and show the correct trend.

S.-J. Oh

Papers

Photoemission study of CoO

Charge-transfer satellites in the 2p core-level photoelectron spectra of heavy-transition-metal dihalides.

Electron-spectroscopy study of rare-earth trihalides.

Electronic structures of NiO, CoO, and FeO studied by 2p core-level x-ray photoelectron spectroscopy.

Structure, magnetic and transport properties of La1−xBixMnO3