Electronic structure of plutonium monochalcogenides

The anomalous properties of the Pu monochalcogenides are investigated on th e basis of electronic structure calculations. The Pu monochalcogenides are calculated to be semimetallic because, first the large spin-orbit interacti on of the Pu 5f states splits the 5f(5/2) and 5f(7/2) subbands away from th e Fermi energy, and second the hybridization between Pu 6d and chalcogenide p and 5f bands leads to a hybridization gap. The anomalous lattice constan ts, which correspond neither to Pu2+ nor to Pu3+ are consistent with the en ergy band approach, as is the lattice constant where the transition to Pu2 is expected. Our calculations suggest that Pu has a 5f(6-x)6d(x) configura tion, where x depends on the lattice parameter, but the sum of 5f and 6d oc cupancy 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 e V. 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 occ urs, it is predicted to lead to a giant magnetoresistance of about -85%.