M. Abbate

Oxygen 1s and cobalt 2p X-ray absorption of cobalt oxides

The oxygen 1s and cobalt 2p X-ray absorption spectra of CoO, Li-doped CoO and LiCoO2 have been measured with 0.1 eV resolution. The cobalt 2p spectra are analysed with a ligand-field multiplet model and the inclusion of chase-transfer effects is discussed. The oxygen 1s spectra are interpreted as transitions to empty oxygen p states and it is concluded that the effects of correlations in the 3d band possibly are too small to be detectable. The symmetries and the electronic configurations of the cobalt ions in the oxides are determined. It is concluded that, in contrast to for example NiO and La2CuO4, the doping-induced states are possibly of 1A1 symmetry, which would imply that the quasi-particles have spin 3/2 and are most likely trapped.

Soft X-ray absorption spectroscopy of vanadium oxides

Abstract We present the V2p and O1s X-ray absorption spectra of V 2 O 3 , VO 2 and V 2 O 5 . The V2p spectra show strong multiplet effects caused by large 2p-3d and 3d-3d Coulomb and exchange interactions. The spectra exhibit noticeable chemical shifts and completely different multiplets. The shapes of the multiplet are related to the symmetry and spin of the ground state. The V2p spectrum of V 2 O 3 can be simulated by an atomic-multiplet calculation projected in the appropriate crystal field. The O1s spectra are related to unoccupied Op character mixed in the conduction band. The spectra reflect V3d bands near the edge and V4sp bands at higher energies. The V3d bands are split by crystal field effects. Changes in the relative intensities are related to differences in the metal-ligand hybridization. The extra dispersion of the V3d bands in V 2 O 3 is attributed to larger metal-metal interactions. The O1s spectrum of VO 2 compares favorably to a symmetry-projected band structure calculation. Finally, the O1s spectra of VO 2 taken at room temperature (insulator phase) and at T ≈ 120°C (metallic phase) show clearly the splitting of the V3d | band across the metal-insulator transition.

The interaction of N with Ti and the oxidation of TiN studied by soft X-ray absorption spectroscopy

Abstract The N1s XAS spectrum of a reference TiN sample is compared to an N p-projected DOS. Good overall agreement is found except for a small discrepancy at the edge which could be due to core-hole effects. Teh N1s XAS spectra of N 2 chemisorbed on Ti (3000 L) and N + implanted in Ti (250 eV) compare well with TiN; this indicate a close chemical similarity between them. The differences in the spectra are attributed to disorder in the implanted layer and to the two-dimensional character of the chemisorbed layer. The N1s XAS spectra of TiN oxidized at different temperatures indicate that O progressively displaces N to form TiO 2 . The results are consistent with a complete TiN/TiO 2 phase separation and rule out the possible formation of oxynitride. The appearance of a narrow peak in the spectrum of TiN oxidized at 300°C suggests that some of the N atoms displaced during the oxidation process remain relatively unbound within the TiO 2 matrix. At 500°C this peak disappears; this is attributed to N migration towards the surface followed by thermal desorption.

The bremsstrahlung isochromat spectra of d0 transition-metal oxides

Abstract We present and discuss the bremsstrahlung isochromat spectra (BIS) of four d0 transition-metal oxides; namely ZrO2, HfO2, Nb2O5, and Ta2O5. The spectra are related to the density of unoccupied states in the conduction band. They give directly the magnitude of the crystal-field splitting and the dispersion of the metal-d bands. The trends observed in the spectra are discussed and compared to those found in the O 1s x-ray absorption spectra (XAS) of the same materials.

Evolution of the spectral function in Mott-Hubbard systems across metal-insulator transitions

Abstract The electronic structure of Mott-Hubbard-type Ti and V oxides have been studied by photoemission and X-ray absorption spectroscopy. The evolution of the spectra across the Mott transition indicates characteristic transfer of spectral weight from the incoherent to the coherent part of the spectral function. Across the metal-insulator transitions induced by carrier doping, new spectral weight is shown to fill the band gaps of the parent compounds.

Resonant photoemission at the 2p thresholds of Fe, Co, and Ni metal

Resonant photoemission studies have been performed at the 2p soft-x-ray absorption thresholds of Fe, Co, and Ni metal. At resonance, the photoelectron intensity exhibits a strong enhancement in the region of the two-hole satellites, which is shown, however, to be mostly due to an incoherent superposition of a photoemission signal with a more intense signal from anL3M4,5M4,5 Coster-Kronig decay of the core-ionized state. The resonant enhancement of the valence-band photoemission satellite in Ni metal amounts to a factor of ≃12. In contrast, no photoemission satellites are observed for Fe and Co metal.

Chemical study of passivating chromium oxide films by soft X-ray absorption spectroscopy

Chromium oxide coatings deposited on stainless-steel surfaces were studied by X-ray absorption spectroscopy (XAS) using synchrotron radiation. Migration of manganese from the steel substrate into the film was found to be significant when γ-Cr2O3 was developed by thermal treatment of an originally amorphous coating. The passivating efficiency of the film was then largely improved. The presence of other oxidation states of chromium was not confirmed by XAS.

Electronic structure and metal-insulator transitions in Ti and V oxides

Abstract The electronic structures of various Ti and V oxides have been studied by photoemission and X-ray absorption spectroscopy. For LaxSr1−xTiO3, electron doping induces new photoemission feature within the band gap of SrTiO3. For the 3d1 systems, as the Hubbard gap collapses and U/W becomes small, a new spectral feature grows around the Fermi level.