AbslracL The Ni 2p and 0 Is XPS of NiO single crystals were measured using monochromatic Al K a radiation. Different treatmenu of the crystals allow us to give a description of the influence of defects on these spectra. The Ni 2p3p spectrum of in-sirucleaved NiO exibiu clearly visible features at 854.1 eV, 855.6 eV and 861 eV The intensity ratio changes after ion bombardment and an additional peak at 852.2 eV appears. ApaR from a slight increase in linewidth the 0 1s spectrum remains unchanged. The 0 Is spectrum from an in-sirueleaved NiO single crystal exhibits only a single peak at 529.4 eV which can be fitted by a Gaussian line profile. Further measurements allow us to attribute the well known 0 Is satellite at 531.2 eV to emission [ram oxygen-containing species adsorbed at defects.
We have investigated the adsorption of NO on a thin NiO(100) film of several layers thickness grown on top of a Ni{100) surface in comparison with data of an in vacuo cleaved NiO(100) single crystal. The layer exhibits a high defect density. We demonstrate via application of several surface-sensitive electron-spectroscopic techniques [i.e., x-ray photoelectron spectroscopy (XPS), angle-resolved ultraviolet photoelectron spectroscopy (ARUPS), near-edge x-ray-absorption fine structure (NEXAFS), and high-resolution electron-energy-loss spectroscopy (HREELS)j that this layer has similar occupied (ARUPS) and unoccupied (NEXAFS) states as a bulk NiO(100) sample. In spite of its limited thickness, the band structure of the film exhibits dispersions perpendicular to the surface compatible with bulk NiO(100). It is shown that the electronic structure of the oxygen sublattice can be described in a band-structure picture while for the Ni sublattice electron localization eftects lead to a breakdown of the band-structure picture. NO on NiO desorbs at 220 K. This indicates weak chemisorption. The NO coverage is close to 0.2 relative to the number of Ni surface atoms as determined by XPS. HREELS reveals that there is only one species on the surface documented by the observation of only one bond-stretching frequency. NEXAFS data on the system and a comparison with previous data on the system NO/Ni(100) indicate that the molecular axis of adsorbed NO is tilted by an angle of approximately 45' relative to the surface normal. The N 1s XP spectra of the weakly chemisorbed species show giant satellites similar to the previously observed cases for weak chemisorption on metal surfaces. This is the first observation of an intense satellite structure for an adsorbate on an insulator surface, which shows that there must be sufficient screening channels even on an insulating surface. A theoretical assignment of the peaks is discussed. We compare the spectroscopic properties of the NO species on the thin-film oxide surface, which is likely to contain a certain number of defects, with NO adsorbed on a basically defect-free bulk oxide surface by thermal-desorption (TDS) and XP spectra. TDS and XP spectra of the bulk system are basically identical as compared with the oxide film, indicating that the majority of species adsorbed on the film is not adsorbed on defects but rather on regular NiO sites. Results of ab initio oxide cluster calculations are used to explain the bonding geometry of NO on regular NiO sites.
The electronic structure of cubic KNb03 and KTa03 has been calculated using the self-consistent, scalar-relativistic linear-mufBn-tin-orbital method. The calculated density of states (DOS) shows a strong similarity for both materials and is in good accordance with measured photoelectron spectra (PES). The projected DOS reveals a strong d-band character for the valence band, which is due to an evident hybridization of 0 2p states with the unoccupied Nb(Ta) d states T. his is also confirmed by PES data, if one makes use of the Cooper minimum for d bands. The calculation underestimates the band gaps by about 5070, a result that is known also from other band calculations for insulators within density-functional theory. Ground-state properties are obtained &om totalenergy calculations. Lattice constants agree within a few percent with experimental ones. The bulk modulus for KTaOs (2.25 Mbar) is in good agreement with experiment, while for KNbOs (2.47 Mbar) it is nearly twice as large as the experimental value. Cohesive energies are found to be -42.2 eV for KNbOs and -44.5 eV for KTaOs (per unit cell). Corresponding experimental values do not seem to exist in standard literature.
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