X-ray absorption spectroscopy investigation of the electronic structure of superconducting FeSe

X-ray absorption spectroscopy (XAS) Fe K-edge spectra of the FeSex (x = 1–0.8) single crystals cleaved in situ under vacuum reveal characteristic Fe 4sp states and a lattice distortion. The Se K-edge spectra point to a strong Fe 3d-Se 4p hybridization giving rise to itinerant charge carriers. A formal charge of ∼1.8+ for Fe and ∼2.2− for Se was evaluated from these spectra in the FeSex (x = 0.88). The charge balance between Fe and Se is assigned to itinerant electrons located in the Fe-Se hybridization bond. As x decreases the 4p hole count increases and a crystal structure distortion is observed that in turn causes the Fe separation in the ab-plane change from 4p orbital to varying (modulating) coordination. Powder X-ray diffraction (XRD) measurements also show a slight increase in lattice parameters as x decreases (increasing Se deficiency)