Valence state of Tm in TmX (X = S, Se, Te) investigated by resonant inelastic x-ray scattering

The electronic structure of thulium monochalcogenides $\mathrm{Tm}X$ $(X=\mathrm{S},\mathrm{Se},\mathrm{Te})$ is investigated by x-ray absorption spectroscopy (XAS) in the partial fluorescence yield mode (PFY-XAS) and resonant x-ray emission spectroscopy around and far below the edge. These three resonant inelastic x-ray scattering (RIXS)-derived techniques yield consistent values of the thulium valency of 2.67, 2.65, and 2.70, respectively, in the mixed-valent compound TmSe. These values are in closer agreement with the valency derived from the lattice parameter measurement compared to previous x-ray photoemission spectroscopy and XAS studies. The study demonstrates that RIXS is a powerful and accurate probe of mixed-valent states in $f$-electron systems.