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 Eu0.5La0.5FBiS2-x 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 X Lambda NES spectra reveal that Eu is in the mixed valence state with coexisting Eu2+ and Eu3+. The average Eu valence decreases sharply from similar to 2.3 for x = 0.0 to similar to 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 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 Eu0.5La0.5FBiS2- x Se-x is likely to be due to increased in-plane orbital overlap driven by reduced in-plane disorder that affects the carrier mobility.