XPS study of isostructural multinary thallium chalcogenides

The thallium copper chalcogenides TlCu2S2 and TlCu2Se2, as well as the solid solutions TlCu2SeS and TlCu2-xFexSe2 (x = 0.5 and 1) have been investigated in the form of powders or single crystals by means of X-ray photoelectron spectroscopy. It is concluded from the absence of Cu 2p shake-up satellites that copper is monovalent, as previously suggested on the basis of electrical transport properties. The metallic character of the copper compounds is due to the presence of valence band holes. Indications of chalcogen ordering were found in TlCu2SeS. On extensive iron substitution the compounds became semiconductors.