Structural models of FeSex

Two different structural models for non-stoichiometric FeSex are examined and compared with soft x-ray spectroscopy findings for FeSex (x = 0.8,0.50). A structural model of tetragonal FeSe with excess interstitial Fe gives better agreement with experiment than a structural model of tetragonal FeSe with Se vacancies. This interstitial Fe increases the number of 3d states at the Fermi level. We find evidence that large non-stoichiometric ratios of Fe:Se, such as that of FeSe0.50, yield clusters of pure Fe in the crystal structure.