C. Demangeat

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.

Tight-binding calculation of force constants in α palladium hydrides☆

Abstract A formal derivation of the force constants in pure transition metals and in transition-metal-based interstitial alloys is derived from a tight-binding description of the electronic structure of the system. This model takes into account all contributions to the energy and the self-consistent renormalization due to the energy variation on the impurity site. Charge transfer effects, i.e. electronic rearrangement around the displaced atoms, appear only up to first order in the displacement. The model is applied to pure palladium and α-PdH, and the difficulties involved in a numerical estimation are briefly outlined.

Elastic binding energy in α-palladium hydrides

Abstract We point out that the asymptotic form for the elastic energy of a pair of interstitials at distance R in a transition metal is uselessfor R less than a few lattice parameters. Explicit calculation for a pair of hydrogen atoms in α-palladium hydrides displays this fact.

Magnetism of epitaxial Ru and Rh monolayers on graphite

Abstract Recently, Pfandzelter et al. (1995) reported the first observation of monolayer ferromagnetism of a 4d metal, namely in a Ru monolayer grown on graphite. Using the tight-binding linear-muffin-tin-orbital (TB-LMTO) method we have calculated the electronic and magnetic structure of epitaxial Ru and Rh monolayers on graphite with the experimentally determined atomic density. Monolayers of the other 4d elements were found to be non-magnetic already in the free-standing limit. The magnetic structure of the Ru and Rh monolayers is studied as a function of metal-graphite interlayer distance h . They become magnetic at h = 4.5 a.u. (Ru) and h = 4.8 a.u. (Rh) in a first-order transition. In the assumed p (2 × 2) super-structure, the moments on the “hollow” site atoms are up to four times bigger than those on the “on-top” site atoms. For h > 5.4 a.u. (Ru) and h > 5.1 a.u. (Rh) the site dependence vanishes and the moments of the free monolayers are approximately reached (1.9 μ B and 1.2 μ B , respectively).

Spin-polarization of thin Mn films on Fe(107)

Abstract We have carried out a real-space calculation using a d-band tight-binding Hamiltonian to study the magnetic behaviour of thin body-centred-tetragonal (bct) Mn films on Fe(107). The surface roughness in the Mn/Fe(107) system gives rise to a completely different polarization map compared to the usual abrupt interface. For one monolayer of Mn, two stable solutions have been obtained, which display a vanishing average surface magnetization. Results for two Mn monolayers are also reported.

Binding energy of a pair of hydrogen atoms in α-iron

Abstract The interaction energy between hydrogen atoms at tetrahedral positions in α-iron is investigated using the tight-binding method of band structure calculation based on the extended Huckel formalism. Nearest and next-nearest pair interactions are shown to be considerably repulsive whereas the most probable stable situation appears to be the third nearest neighbouring position.

The dipole force tensor in α-PdH

Abstract The dipole force tensor P in α-PdH is derived from a tight-binding description of the electronic structure of hydrogen at the octahedral position in palladium. The effect of the relaxation is described by a pseudopotential which arises from the partial cancelling of the attractive term (band contribution) and the repulsive terms (ion-ion and electron-electron contributions). The calculation is based on a rigidly moving wavefunction basis where the electronic structure of the unrelaxed alloy is determined by taking into account a perturbing potential up to the nearest neighbours of the hydrogen atom. Expressions for the distribution of forces and for the dipole force tensor are derived in terms of the variation of the hopping integrals and the energy levels. The estimation of P and of the forces up to the third-nearest-neighbour shell of the hydrogen atom are presented.

Charge transfer in α-palladium hydrides

Abstract A simple tight binding approximation is used to describe the electronic structure of a single hydrogen impurity in α-palladium hydrides. The position of the hydrogen bound state, below the bottom of the sp bands of pure palladium. depends on the value of the intra-atomic Coulomb correlation U λ 88 on the hydrogen site λ. Charge transfer from metal to metalloid is shown to decrease when U λ 88 is increased.

Calculation for core-s-XPS of transition metals on graphite

A few years ago, the appearance of surface magnetism of vanadium upon graphite was pointed out experimentally and was assumed to be quenched after exposure to CO pollutant: only for freshly evaporated V clusters on graphite, a satellite structure had been found in the V 3s-XPS spectrum and it was attributed to the presence of magnetic moments on the V surface. More recently the appearance of magnetism was observed (by means of other techniques) in a Ru monolayer grown laterally on the C(0001) graphite surface. In the present paper, we extend the impurity Anderson model for the calculation of transition metal core-s-XPS spectra by taking into account the exchange terms, especially between the core hole spin and the transition metal magnetic moment. Our results on core-s-XPS are discussed in terms of the various parameters entering the present model Hamiltonian and a new interpretation of the experimental V 3s-XPS spectrum is proposed.

Cluster model for the electronics structure of point defects in SmS compounds

Abstract First we regenerate the ab initio band structure of spd states in pure SmS three-centre-Slater-Koster method, the f states being represented by a simplified periodic Anderson model. Then we study the electronic structure of such a compound containing either a samarium interstitial or a substitutional oxygen. In both cases we consider the whole cluster formed by the samarium atoms around the point defect: we calculate and discuss the “local valence transitions” in relation with recent experiments.