XPS and UPS studies of pyrite compounds (CoX2 → CuX2, X: S; Se): Investigation of the insulator-metal transition in the NiS2−xSexsystem

We present in this paper a photoemission study (XPS and UPS) of several transition metal disulphides and diselenides (CoX₂ → CuX₂) with the pyrite structure. Our results show that all these compounds have narrow conduction bands and that the spectra for the disulphides and the diselenides are very similar. Nevertheless some changes occur in the electronic structure when going in the same series from CoX₂ → CuX₂, which are qualitatively well understood from simple theoretical considerations. Our main purpose is to discuss the influence of correlations between electrons in these narrow band systems and particularly in the case of the half-filled band (Mott-Hubbard insulator NiS₂). Considering the energy shift of the core levels deduced by XPS experiments we show that the effects of these correlations become smaller when going from CoX₂ to CuX₂, whereas they seem to remain unchanged in NiS_2?xSe_x, where an insulatormetal transition takes place. We present an attempt to quantitatively analyse our UPS results on the conduction bands. According to our results the particular behaviour corresponding to the Mott-Hubbard insulating state of NiS₂ and NiS_2?xSe_x (x < 0.5) mixed systems can be revealed by photoemission studies.