Iron orbital occupancies upon valence mixing of charge-ordered GdBaFe^IIFe^IIIO₅

Mössbauer spectra of charge-ordered/valence-mixed GdBaFe₂O₅ have been analyzed in order to determine the orbital occupancies of the minority-spin d electron as a function of temperature. In the charge-ordered state below the Verwey temperature T_V, it occupies the Fe²⁺ d_xz orbital, which is ordered. Above T_V, the electron partially occurs also at the trivalent iron and spreads over two additional orbitals, d_x²-y² and d_z². In the valence-mixing state (above T_p) also orbital d_yz participates. The study illustrates how the Mössbauer electric-field gradient, center shift and internal field jointly reflect the distribution of the minority-spin electron culprit behind the Verwey transition.