Neptunium octupole and hexadecapole motifs in NpO2 directly from electric dipole (E1) enhanced x-ray Bragg diffraction

摘要

The phase transition in NpO2 at T-0 approximate to 25.5 K is accompanied by the onset of superlattice reflections in the x-ray Bragg diffraction pattern, with intensity enhanced by an electric dipole (El) event. Additional experiments using other techniques indicate no ordering at T-0 of Np magnetic moments. Absence of long-range magnetic order below T-0 fits with the outcome of a polarization analysis of superlattice intensities at 12 K; signals are observed in both the unrotated (sigma'sigma) and rotated (pi'sigma) channels of scattering while magnetic (dipole) moments would contribute only in the rotated channel. We demonstrate that these empirical findings, together with a narrow energy profile of the Bragg intensity at the Np M-4 edge, are consistent with magnetic and charge contributions to the El Bragg amplitude described by Np 5f multipoles of ranks 3 (octupole) and 4 (hexadecapole). Key to our understanding of the x-ray diffraction data gathered in the vicinity of the Np M-4 edge is recognition of an exchange field creating 3d(3/2) core-level structure. The particular importance of the exchange field at the Np M-4 edge is emphatically revealed in calculations of the corresponding x-ray absorption spectrum with and without the core-valence interaction. From the experimental information about NpO2 we can infer a ground-state wavefunction for the Np 5f(3) valence shell and estimate saturation values for the octupole and hexadecapole. We are led to null values for Np multipoles of ranks 2 (quadrupole) and 5 (triakontadipole). [References: 12]