Börje Johansson

Electronic structure of the actinide metals

Abstract Some recent experimental photoelectron spectroscopic results for the actinide metals are reviewed and compared with the theoretical picture of the basic electronic structure that has been developed for the actinides during the last decade. In particular the experimental data confirm the change from itinerant to localized 5f electron behaviour calculated to take place between plutonium and americium. From experimental data it is shown that the screening of deep core-holes is due to 5f electrons for the lighter actinide elements and 6d electrons for the heavier elements. A simplified model for the full LMTO electronic structure calculations is introduced. In this model the spd and 5f electronic contributions are treated as separable entities. It is shown that the model reproduces quite well the results from the full treatment. The equilibrium volume, cohesive energy and bulk modulus are calculated and compared with experiment. Also the 5f delocalization transition under pressure in Am is treated.

Auger Energy Shifts for Metallic Elements

A previous treatment of core-level binding-energy shifts for metals is extended to Auger energy shifts. In comparison to single core-hole shifts, the new information given by the Auger shift is contained in the shift of the two core-hole final state. An accurate expression for the shift of the two core-hole ionization energy between the free atom and the metal is derived. In the actual calculation of the shift a (Z + 2) equivalent core approximation is applied, but where important corrections are included. Comparison between theory and experiment is performed for the metallic elements Na-Al, Ni-Ga, Pd-Sn, Au, Tl-Bi, Ba and Yb, and in all cases a good agreement is obtained. A rather detailed compilation of atomic and solid phase double-hole energies is presented in the Appendices. To elucidate the break-down of the concept of metallic screening, non-conducting elements like Si, Ge-Se, Sb and Te are also considered. In addition, atom-metal two core-hole energy shifts are calculated for the 4d transition series and an abrupt change in the energy shift is found when we proceed from the d elements to the elements beyond. The influence of the atomic structure on the shift is considered for the 4dn5s2, 4dn+15s and 4dn+2 configurations. The metallic renormalization of the atomic core-hole Coulomb correlation energy is calculated for the 4d elements and the chemical shift of this quantity is shown to be closely related to the change of the Auger parameter. The present formalism provides a most suitable framework for treating chemical shifts in metallic systems. As a special example the situation at the surface of a metal is considered and surface shifts are predicted both for the two-hole and the Auger energies. As for the single core-holes, these surface shifts change sign as we proceed through a transition series due to the bonding-antibonding division of the d-band.

Regularity of the cohesive energy of the lanthanide elements

Abstract A systematic treatment of the experimental cohesive energies Ecohexp for the lanthanide elements is undertaken. Even after corrections for differences in the valence configuration of the free atoms (fn+1s2 or fnds2) have been performed, a large number of apparently irregular variations in the modified cohesive energy ECohIII occur on going through the series. It is shown that these irregularities are due to multiplet coupling between the (4f)n and 5d shell electrons in the free atom. The magnitude of this coupling is derived and a new cohesive energy function E coh ∗ is introduced by removing the intershell multiplet coupling energy from the atomic reference state. This new function shows regular behaviour throughout the series and it is not possible to identify a nephelauxetic effect. The function P(M) introduced by Nugent, Burnett and Morss (J. Chem Thermodyn., 5 (1973) 665) is similarly affected by such purely atomic effects, and therefore a more appropriate function P ∗ ( M ) is defined and studied. As for E coh ∗ , the behaviour of P ∗ ( M ) does not permit an unambiguous identification of the nephelauxetic effect. The implication of these results for the understanding of the cohesive energies of the heavier actinides is briefly discussed. It is stressed that estimations of thermochemical data based on the function P(M) are much less accurate than previously believed.

Chemically shifted surface core-levels and surface segregation in EuAu and YbAu alloys

Abstract Chemically shifted surface core-level binding energies are observed for the rare-earth alloy component in Eu1−xAux and Yb1−xAux. Furthermore, these shifts are different from the chemical shifts of the bulk 4f levels. The surface core-levels are used to identify surface segregation of the lanthanide metal in the present alloys.

A New High-Pressure Phase and the Equation of State of YbH2

High-pressure X-ray diffraction studies have been performed on YbH2 up to 28 GPa. A first order phase transition from an orthorhombic structure to a collapsed hexagonal structure with c/a = 1.34 has been observed at about 15 GPa. The transition is accompanied by a 5.2% decrease in volume. Fitting the V(P) data to Murnaghans equation of state yields the bulk modulus B0 = 40.2 GPa and its pressure derivative B0 = 4.75 for the orthorhombic phase. For the hexagonal phase we find the bulk modulus to be B = B0 = 138 GPa independent of pressure, i.e. B0 = 0.

Double-hole binding energy shifts for the 5d transition metals

Abstract Recent experimental M 45 N 67 N 67 Auger energies for Ta, W, Ir, Pt, Au, Hg and Pb are analyzed in terms of the complete screening picture. For the shift in the double-hole ( N 67 N 67 ) binding energy between the free atom and the metal a good agreement is found between theory and experiment. The variation of the shift throughout the 5 d series and beyond reflects different types of screening of the two core holes, i.e. a large shift in the case of d -type screening and a considerably smaller shift for sp -type screening.