Evolution of Eu valence and superconductivity in layered Eu0.5La0.5FBiS2−xSex system

We have studied the effect of Se substitution on Eu valence in a layered ${\mathrm{Eu}}_0.5{\mathrm{La}}_0.5{\mathrm{FBiS}}_{2\ensuremath{-}x}{\mathrm{Se}}_x$ superconductor using a combined analysis of x-ray absorption near-edge structure (XANES) and x-ray photoelectron spectroscopy (XPS) measurements. Eu ${L}₃$-edge XANES spectra reveal that Eu is in the mixed valence state with coexisting ${\mathrm{Eu}}²⁺$ and ${\mathrm{Eu}}³⁺$. The average Eu valence decreases sharply from $\ensuremath{\sim}2.3$ for $x=0.0$ to $\ensuremath{\sim}2.1$ for $x=0.4$. Consistently, Eu $3d$ XPS shows a clear decrease in the average valence by Se substitution. Bi $4f$ XPS indicates that effective charge carriers in the ${\mathrm{BiCh}}₂$ (Ch = S, Se) layers are slightly increased by Se substitution. On the basis of the present results it has been discussed that the metallic character induced by Se substitution in ${\mathrm{Eu}}_0.5{\mathrm{La}}_0.5{\mathrm{FBiS}}_{2\ensuremath{-}x}{\mathrm{Se}}_x$ is likely to be due to increased in-plane orbital overlap driven by reduced in-plane disorder that affects the carrier mobility.