J.C. Parlebas

Theory of XPS and BIS spectra in Ce2O3 and CeO2

The spectra of 3 d photoemission, valence photoemission and BIS in Ce 2 O 3 are analyzed theoretically by using the filled band Anderson model. The calculated spectra are in satisfactory agreement with experimental data by choosing appropriately the parameter values in the model. The characteristic features of these spectra are discussed in comparison with those of CeO 2 , for which some new analysis is also made to obtain better agreement with experimental results.

Electronic changes induced by vacancies on spectral and elastic properties of titanium carbides and nitrides

Abstract Substoichiometric carbides TiC0.49, TiC0.78 and nitrides TiN0.45, TiN0.61 have been synthesized by multiple energy ion implantation of C+ and N+ in polycrystalline titanium. Here, we review theoretical and experimental results concerning the electronic structure of these compounds with various degrees of vacancies. The valence band X-ray photoemission spectra (v-XPS) as well as core level XPS suggest important changes of the electronic structure in comparison to the corresponding stoichiometric compounds. New structures appear near the Fermi level in the experimental v-XPS spectra in agreement with tight-binding-linear-muffin-tin-orbitals (TB-LMTO) calculations of the electronic densities of states (DOS). These structures, absent in stoichiometric compounds, are interpreted as resulting from Ti–Ti bond states between neighbouring titanium atoms through vacancies induced by substoichiometry along the whole range of vacancy concentration. Moreover, these bond modifications explain the experimental changes of Young’s modulus, observed by nanoindentation, in substoichiometric titanium carbides and nitrides.

Band structure of substoichiometric titanium nitrides and carbonitrides: spectroscopical and theoretical investigations

Recently, substoichiometric titanium nitrides (x = 0.45 and 0.61) and carbonitrides (x = y = 0.35) have been synthesized by multiple-energy ion implantation at the surface of pure titanium. Changes in the binary and ternary alloy electronic properties versus defect impurity and vacancy concentrations have been studied by means of core Ti 2p and valence x-ray photoemission spectra (XPS). Moreover, self-consistent tight-binding linear-muffin-tin-orbital calculations with empty spheres in place of the missing nitrogen and/or carbon atoms give, among other information, the total density of occupied states, for comparison with the observed valence XPS value.

Many Body Effect in Inner Shell Photoemission and Photoabsorption Spectra of La Compounds

Theoretical analysis is made for spectra of the 3 d core photoemission (3 d -XPS) and 2 p core photoabsorption ( L 3 -XAS) in insulating La compounds La 2 O 3 , LaF 3 and LaCl 3 by using the single-site Anderson model. In order to give a consistent interpretation for 3 d -XPS and L 3 -XAS, it is essentially important to take into account, in the final state of L 3 -XAS, the Coulomb interaction U f d (between 4 f and 5 d electrons) and - U d c (between a 5 d electron and a core hole) in addition to - U f c (between a 4 f electron and a core hole). A similar analysis is also presented for the intermetallic cornpound LaPd 3 .

A Resonant Photoemission Model for Copper Metal

We study the satellite near 3 p -threshold in the resonant photoemission associated with the two- d -hole bound state in metals such as copper. The satellite is considered to originate from Auger transition resulting from the super Coster-Kronig decay of the 3 p -core hole. The copper density of states is reproduced by a two hybridized 3 d -4 s -band model. We test the influence of the hybridization upon the photoelectron spectrum and we discuss the importance of the s -screening around the d -hole pair with respect to the satellite position and relative intensity of the satellite and the Auger emission. Comparison is made with available experimental data.

Non-stoichiometric niobium nitrides: structure and properties

Abstract Non-stoichiometric niobium nitride films of various nitrogen compositions were deposited on glass and silicon (111) substrates, heated to 150 °C, by direct current reactive magnetron sputtering from a niobium metal target in an argon–nitrogen atmosphere. The argon gas flow was fixed at 20 sccm and the nitrogen gas flow was changed from 0 to 8 sccm. Composition, structure, electrical and optical properties of the films were studied using respectively, energy dispersion spectroscopy, X-ray diffraction, four probes method and optical spectrometry. Thickness was measured by X-ray reflectometry. The structures found were hexagonal for the 2 sccm sample and of NaCl type for 4 sccm samples and beyond. The film thickness indicates that the deposition rate decreases as the nitrogen gas flow increases. Electrical resistivity increases with the nitrogen content for samples having NaCl structures. Moreover, optical spectrometry shows that samples with NaCl structure have a high reflectivity in infrared region while reflectivity remains low in the ultraviolet region.

Theoretical and experimental investigations on elastic properties of substoichiometric titanium nitrides: influence of lattice vacancies

Abstract Substoichiometric titanium nitrides have been synthesized by multiple energy ion implantation at the surface of pure titanium. Elastic properties have been studied by nanoindentation. Self consistent Full Potential Linear Muffin Tin Orbital (FP-LMTO) calculation with empty spheres in place of the missing nitrogen atoms gives the corresponding bulk modulus and lattice parameter values. The results, both experimentally and theoretically, show the same evolution of the lattice parameter and the bulk modulus, as a function of vacancy concentration. The agreement between experimental and theoretical values is discussed in relation with the vacancy effect.

Experimental and theoretical study of the electronic structure of ion-beam-synthesized substoichiometric titanium carbides

Abstract Non-stoichiometric titanium carbide layers TiC x with x =0.26, 0.49 and 0.78 have been synthesized by multiple-energy carbon-ion implantation at the surface of pure titanium. Changes in the binary alloy electronic properties versus defect concentration are studied by core Ti2p and valence X-ray photoemission spectroscopy (XPS). A self-consistent Tight Binding Linear Muffin Tin Orbital (TB-LMTO) calculation with empty spheres in place of the missing carbon atoms (i.e. C 1− x ) gives the total density of occupied states in comparison with observed XPS.

Electronic structure and ordering of hydrogen in f.c.c. transition metals

Abstract The heats of formation of hydrogen atoms at octahedral and tetrahedral interstitial sites in f.c.c. transition metals (palladium and nickel) were estimated using a generalized tight-binding Slater-Koster fit to the band structure calculated from first principles for the host combined with one extra s orbital for each isolated impurity atom. The chemical binding energies of nearest-neighbouring and next-nearest-neighbouring pairs of hydrogen atoms in α-Pd hydride were found to be repulsive. We also calculated chemical binding energies between d element substitutional impurities in palladium (such that δZ ⩽ −1) and hydrogen atoms as the first and the second neighbours. These theoretical studies are compared with existing experimental results.

PHENOMENOLOGICAL INTERPRETATION OF MCXD IN L2,3 XAS OF TRIVALENT CERIUM COMPOUND

Abstract We use a model interpretation for the observed MCXD (Magnetic Circular X-ray Dichroism) in L 2,3 XAS (X-ray Absorption Spectra) of CeRu 2 Ge 2 as a typical example of normal trivalent cerium compounds. Even in this simplest case of Ce compounds, our analysis needs detailed information on the 4f states, which affect indirectly the electric-dipole transition (2p-5d) through the 4f-5d exchange interaction. We find that the tetragonal crystal field in addition to the spin-orbit coupling for the 4f states is a key interaction to reproduce the MCXD spectra based on an atomic model, in which the polarization effect on the 5d ground state as well as an enhancement of the transition dipole matrix element due to a contraction in the radial part of the 5d orbitals are phenomenologically included.