Puzzling Lack of Temperature Dependence of the PuO Magnetic Susceptibility Explained According to Ab Initio Wave Function Calculations.

The electronic structure and the magnetic properties of solid PuO are investigated by wave function theory calculations, using a relativistic complete active space (CAS) approach including spin-orbit coupling. The experimental magnetic susceptibility is well reproduced by calculations for an embedded PuO cluster model. The calculations indicate that the surprising lack of temperature dependence of the magnetic susceptibility χ of solid PuO can be rationalized based on the properties of a single Pu ion in the cubic ligand field of the surrounding oxygen ions. Below ∼300 K, the only populated state is the nonmagnetic ground state, leading to standard temperature-independent paramagnetism (TIP). Above 300 K, there is an almost perfect cancellation of temperature-dependent contributions to χ that depends delicately on the mixing of ion levels in the electronic states, their relative energies, and the magnetic coupling between them.