Electronic, spectroscopic and elastic properties of early transition metal compounds

Photoemission experiments on early 3d transition metal compounds (TMC), involving 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 the emphasis is put on understanding the results of experiments more than on the experimental details and methods. Compounds of the TMs are first analyzed in terms of ab initio band structure calculations, which are shown to be usually sufficient as far as the interpretation of valence photoemission spectra is concerned. A discussion of ultraviolet valence band photoemission (UPS) and bremsstrahlung isochromat spectroscopy (BIS) is also made. Other theories involving configuration interaction (CI) in the modelization are then shown to be necessary for an understanding of the core-level photoemission 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 CI cluster model analysis of the metal 2s-, 2p-, 3s- and 3p-level X-ray photoemission 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 orbital hybridization leads to the Zaanen–Sawatsky–Allen (ZSA) theory and to the charge transfer (CT) systems. Moreover, we discuss how the analysis of 3s XPS spectra can predict or not the formation of localized magnetic moments. The values of the charge transfer energy Δ 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 excitation of photoemission satellites are presented and the systematic trends for the compounds of the early TM series are discussed. Finally, in addition to the study of the above stoichiometric compounds, we review recent results 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 compared with the observed valence XPS spectra. An extension of calculations to full potential methods is necessary for interpreting the elastic properties, e.g. the bulk modulus.