Single-site mixed valence of thulium: A comparative photoemission study of TmxY1-xSe and TmxY1-xS

The dilution of Tm by Y in mixed-valent TmSe and trivalent TmS has been studied by means of photoelectron spectroscopy with the use of synchrotron radiation. TmSe is found to stay mixed-valent down to at least 5 at.% Tm, while Tm appears to be trivalent in all the TmxY1-xS compounds studied. The results point towards a single-site description of mixed valence. We find that both the selenides and the sulfides have a divalent surface layer due to a surface shift of the 4f level. In TmSe this shift is resolved and found to be 0.3 eV. © 1982 The American Physical Society.