Electronic, spectroscopic and elastic properties of early transition metalcompounds

Photoemission experiments on early 3d transition metal compounds (TMC), inv olving both valence bands and core levels of the 3d elements, are reviewed. Extensive use is made of ab initio schemes as well as simple models and th e emphasis is put on understanding the results of experiments more than on the experimental details and methods. Compounds of the TMs are first analyz ed in terms of ab initio band structure calculations, which are shown to be usually sufficient as far as the interpretation of valence photoemission s pectra is concerned. A discussion of ultraviolet valence band photoemission (UPS) and bremsstrahlung isochromat spectroscopy (BIS) is also made. Other theories involving configuration interaction (Cl) in the modelization are then shown to be necessary for an understanding of the core-level photoemis sion spectra and the observed satellite features. The electronic structure of a wide range of early TMCs, from Sc to Cr, is discussed by means of the Cl cluster model analysis of the metal 2s-, 2p-, 3s- and 3p-level X-ray pho toemission spectra (XPS). Early TMCs, like Ti, V, Cr oxides and halides (e. g. CrF3, CrCl3) have been originally regarded as typical Mott-Hubbard (MH) systems. The MH model results from the electron correlations which dominate the inter-atomic overlaps that lead to bands. The concept of 3d-ligand orb ital hybridization leads to the Zaanen-Sawatsky-Allen (ZSA) theory and to t he charge transfer (CT) systems. Moreover, we discuss how the analysis of 3 s XPS spectra can predict or not the formation of localized magnetic moment s. The values of the charge transfer energy Delta and d-d Coulomb repulsion energy U point to systematic trends for the early TM compounds as found in the case of late TM compounds. Simple and competing mechanisms for the exc itation of photoemission satellites are presented and the systematic trends for the compounds of the early TM series are discussed. Finally, in additi on to the study of the above stoichiometric compounds, we review recent res ults on the electronic properties of substoichiometric binary alloy (TiNx, TiCx,) by means of core and valence XPS spectra. Self-consistent ab initio calculations with empty spheres at the empty lattice ligand sites performed on these alloys provide the total densities of the occupied states to be c ompared with the observed valence XPS spectra. An extension of calculations to full potential methods is necessary for interpreting the elastic proper ties, e.g. the bulk modulus. (C) 2001 Elsevier Science B.V. All rights rese rved.