Magnetic Multipole Order and Symmetry Considerations on the Ground State of Np4+ Ions in NpO2

The ground-state symmetries of Np ions in the low temperature phase of NpO2 are discussed with reference to the results of resonant X-ray scattering experiments suggesting the occurrence of long-range multipolar order with electric quadrupole secondary order parameter. Two models have been proposed in the literature: one based on a 4 doublet (in D3d point group), the other on a 5 (6) singlet.We show, through a theoretical group analysis, that the 4 doublet is not compatible with the available experimental data. Our analysis supports the hypothesis of a singlet ground state.JRC.E.6-Actinides researc