Puzzling Lack of Temperature Dependence of the PuO₂ Magnetic Susceptibility Explained According to Ab Initio Wave Function Calculations

The electronic structure and the magnetic properties of solid PuO₂ 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 PuO₈^12- cluster model. The calculations indicate that the surprising lack of temperature dependence of the magnetic susceptibility of solid PuO₂ can be rationalized based on the properties of a single Pu⁴⁺ 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 that depends delicately on the mixing of ion levels in the electronic states, their relative energies, and the magnetic coupling between them.