A first principles Hartree-Fock interpretation of the X-ray oxygen K-edge spectrum of haematite (a-Fe₂O₃)

This study presents a comparison of the experimental O K-edge absorption spectrum of haematite (alpha-Fe2O3) with a theoretical spectrum obtained from all electron ab initio periodic Hartree-Fock calculations of the antiferromagnetic R (3) over bar structure. There is good overall accord between the two spectra and agreement to within about 0.2eV for the major peak-to-peak separations. From a consideration of the empty p density of states (DOS), calculations predict the first 20 eV of the absorption to result almost entirely from excitations of the type \ O I]--> \ O np] with negligible participation of the Fe p states. The lower part of the spectrum is attributed to O p states hybridized with Fe d states while, at energies greater than 5eV above the absorption edge, in the region of the broad absorption at about 11 eV, calculations suggest that the predominant hybridization is with the Fe s states. Important differences are found between the near-edge empty p DOS of the antiferromagnetic R (3) over bar and ferromagnetic R (3) over bar c spin orderings which suggests that spin-selective O K-edge spectra might be sensitive to the weak canting below the Morin temperature. The satisfactory agreement between experiment and theory, which is based on the calculated ground-state conduction band, confirms the view that core-hole states in charge-transfer insulators such as alpha-Fe2O3 are screened effectively by the valence electrons, which in these systems, are predominantly O in character.