Electronic structure of plutonium monochalcogenides

The anomalous properties of the Pu monochalcogenides are investigated on the basis of electronic structure calculations. The Pu monochalcogenides are calculated to be semimetallic because, first the large spin-orbit interaction of the Pu $5f$ states splits the 5f5/2 and 5f7/2 subbands away from the Fermi energy, and second the hybridization between Pu $6d$ and chalcogenide p and $5f$ bands leads to a hybridization gap. The anomalous lattice constants, which correspond neither to Pu²⁺ nor to Pu³⁺ are consistent with the energy band approach, as is the lattice constant where the transition to Pu²⁺ is expected. Our calculations suggest that Pu has a 5f^6-x6dˣ configuration, where x depends on the lattice parameter, but the sum of $5f$ and $6d$ occupancy is constant. Calculations of the optical conductivity spectra show that there are two optical pseudogaps, one of about 20 meV and one of 0.2 eV. A magnetic phase transition is predicted to occur in the NaCl structure under pressure. When this phase transition is enforced in a magnetic field and takes place before the martensitic transition to the CsCl structure occurs, it is predicted to lead to a giant magnetoresistance of about -85%.