Electronic and magnetic structure of Fe3S4: GGA+U investigation

The electronic and magnetic behavior of the iron sulphide mineral greigite (Fe3S4) is studied using ab initio density-functional theory in the generalized gradient approximation (GGA) with the on-site Hubbard U-eff parameter (GGA+U). The effect of the Hubbard correction is investigated and is found to be a necessary requirement for the accurate description of both the unit cell structure and the magnetic moment. A ferrimagnetic normal-spinel structure is found when U-eff=0 eV, while for all values of U-eff>0 eV an inverse spinel structure is predicted, in agreement with experiment. For low values of U-eff (0 < U-eff < 4 eV) the predicted electronic structure corresponds to that of a semimetal, with semimetallicity arising from electron hopping between ferric and ferrous Fe on octahedral sites. For values of U-eff >= 4 eV the S atoms are found to oxidize the ferrous octahedral sites Fe to the ferric state. To determine whether GGA+U predicts a stable monoclinic form of greigite arising from a Verwey-type low-temperature transition, analogous to that seen in magnetite, a monoclinic form of greigite is postulated. It is found that such a phase is stable, with an electronic band-gap opening up for values of U-eff >= 2 eV, but is energetically unfavorable when compared with the spinel phase for all U-eff values tested. It is argued that an accurate description of all the properties of greigite requires a U-eff value of approximately 1 eV.