Ni 3d Electrons Research Articles

Defects in oxide overlayers at nickel single-crystal surfaces

The chemisorption of oxygen at Ni(100) and Ni(210) surfaces has been studied by X -ray photoelectron spectroscopy in the temperature range 77-300 K. O(1s) spectra distinguished between the formation of O - and O 2- species while Ni(2p) spectra revealed the formation of Ni 2+ and Ni 3+ states. The binding energies of the latter are 854.7 and 856 eV respectively. These values were established by the generation of difference spectra in the Ni(2p) spectral region and assignments confirmed by studies of bulk nickel oxides of different stoichiometry. There were two distinct régimes of oxygen interaction; for low oxygen coverages, θ < 0.6, there was no evidence for the higher oxidation states (Ni 2+ or Ni 3+ ). For θ > 0.6 the Ni(2p) difference spectra indicated that both states were formed, their relative proportions being dependent on temperature and coverage. It is suggested that the initial stage of oxygen chemisorption involves only the Ni(4s4p) band while the Ni(3d) electrons participate in oxide formation, which is reflected by the emergence of Ni 2+ and Ni 3+ states. Ni (L 2, 3 M 4, 5 M 4, 5 ) Auger spectra and also recent theoretical work support this conclusion. The mixed valence states, Ni 2+ and Ni 3+ , are a characteristic feature not only of oxygen interaction with Ni(210) and Ni(100) at low temperature but also of the surface of bulk nickel oxides. The latter unexpectedly exhibit a high relative proportion of Ni 3+ states. It is suggested that Ni 3+ is formed in a surface redox reaction from Ni 2+ with the simultaneous generation of O - . Angular dependent studies confirm that the Ni 3+ and O - species are at the surface relative to Ni 2+ and O 2- .

NMR Study of Hydrogen Adsorbed on Copper-Nickel Alloys

The adsorption states of hydrogen on Cu100-xNix (x=0 ∼10) alloys were studied by nuclear magnetic resonance NMR, and the proton resonance shift was found to decrease and the line width to increase with increasing Ni concentration x. Steep changes in the shift and the line width were observed in the range of low Ni concentration (x<1). The decrease in the shift is attributable to the rapid exchange of H adatoms between sites in the Cu environment and sites of Ni or Cu atoms in contact with the Ni atoms. Since H adatoms on the latter sites are spin-polarized by Ni 3d-electrons via conduction s-electrons, their shifts are smaller than those for H adatoms on the former sites.

The dissociation of H2 on the Ni(100) surface

The dissociation of H2 on the (100) surface of Ni is investigated using a cluster model. The mechanism for dissociation of H2 directly above a Ni atom has little to no barrier and involves the Ni 3d electrons; elimination of the Ni 3d interaction with the H2 increases the barrier to more than 50 kcal/mol. The dissociation at the bridge site, treated without the Ni 3d interaction, leads to a barrier of about 30 kcal/mol, leading to the conclusion that the dissociation of H2 at any site on a Ni(100) surface requires strong 3d participation. The results are quantitatively different if the Ni 4p orbitals are not included. The effects of cluster size on the results are also discussed.

ELECTRONIC STRUCTURE OF NiSi2

The electronic structure of NiSi2 was calculated by using the self-consistent LMTO method. Our result was found to be in resonable agreement with the results of AEUPS photoemission experiment and other calculations. The change in the Ni electron configuration was found to be responsible for the Ni3p3/2 core level binding energy shift; the decrease of Ni3d electrons weakened the screening effect for the Ni3p electrons. The result of calculation showed that the ionicity plays a minor role in chemical bond.

Intermediate valence state of cerium in CeNi

Lattice parameter analysis and studies of thermal expansion show that the orthorhombic CeNi compound is, like CeSn 3, an intermediate valence compound in which the cerium valence state varies from 3.5 to 3.3 between 4 and 300 K. This compound behaves as an enhanced Pauli paramagnet in which the magnetic susceptibility along the c axis passes through a maximum at around 140 K. Magnetic susceptibility, resistivity and heat capacity measurements are characteristic of an almost magnetic Fermi liquid in which spin fluctuations are present. Unlike the other cerium intermediate valence compounds which are generally cubic, large anisotropic effects due to the local surroundings are observed in CeNi because of its orthorhombic symmetry. All these properties can be understood in the light of the band structure of this type of alloy in which a large hybridization occurs between the Ni 3d electrons and the Ce 5d electrons.

D-Band structure and atomic arrangement in NiZr compounds

In this paper we investigate the electronic structure of various compounds in the NiZr system ; namely NiZr 2, NiZr, Ni 21Zr 8 and Ni 5Zr. The obtained results suggest, in particular for low Ni concentration (≤ 50 at % Ni), that the hydridization between the d orbitals of each species can be neglected in first approximation. For NiZr 2 and NiZr, the contribution of Ni 3d electrons and Zr 4d electrons can be separated. In the investigated compounds the structure of Ni d states seems to be closely related to the number and the symmetry of Ni neighbours of a Ni atom. The characteristics of Ni quasi-atomic states are estimated ; for NiZr 2 the observed narrow Ni d band can be considered as specific of linear chains of Ni atoms. For the other compounds, an additional contribution to the Ni d band appears, its intensity increases progressively with the Ni content, leading to an increase of the band width ; the origin of such a contribution is discussed in terms of hybridization between Ni d orbitals.