  X-ray absorption spectroscopy (XAS) Fe K-edge spectra of the FeSex (x=1-0.8)
single crystals cleaved in situ in vacuum reveal characteristic Fe 4sp states,
a lattice distortion and 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 were evaluated from these spectra in the FeSex
(x=0.88). The charge balance between Fe and Se is assigned 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).
