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=0eV , while for all values of U_eff>0eV an inverse spinel structure is predicted, in agreement with experiment. For low values of U_eff (0<U_eff<4eV) 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≥4eV 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≥2eV , 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.