Density functional theory study of the high- and low-temperature phases of cubic iron sulfide

The iron sulfide mineral cubic FeS is studied using generalized gradient approximation (GGA) and GGA + U methods in both its high-temperature cubic phase and low-temperature orthorhombic form. It is found that pure GGA is unable to predict the experimentally determined structure of either cubic or orthorhombic FeS. Similarly, application of a FeS interatomic potential which does not consider magnetic effects also fails to describe these structures properly. Upon introduction of a nonzero U-eff Hubbard-type parameter into the GGA, significant improvement is observed in the description of the lattice parameters and Fe magnetic moment for the cubic and orthorhombic structures, using ferromagnetic and antiferromagnetic arrangements, respectively. Very little change in the predicted properties is seen in the GGA and GGA+U nonmagnetic calculations for either the cubic or orthorhombic structures, indicating the importance of magnetostructural effects in this sulfide. The structure of high-temperature cubic FeS is accurately reproduced using a U-eff value of 2 eV and a ferromagnetic arrangement of moments, which would describe a paramagnetic nature once thermal excitations at room temperature are taken into account. GGA+U calculations with an applied U-eff value of 2 eV, in conjunction with an antiferromagnetic arrangement, predict both the low-temperature orthorhombic lattice parameters and Fe magnetic moments to within experimental error. This orthorhombic phase is found to be metallic in nature, with a band gap of 0.7 eV residing around 0.1 eV above the Fermi level. Further calculations on the orthorhombic phase have determined the magnetic exchange constants for this low-temperature phase as J(12)=-57.833 K, J(13)=-68.025 K, and J(14)=-57.893 K with an applied U-eff parameter of 2 eV.