Neptunium Octupole and Hexadecapole Motif in NpO(sub 2) Directly from Electric-Dipole (E1) Enhanced x-ray Bragg Diffraction

摘要

The phase transition in NpO2 at T(sub o) approximately 25.5K is accompanied by the onset of superlattice reflections in the x-ray Bragg diffraction pattern, with intensity enhanced by an electric-dipole (E1) event. Additional experiments using other techniques indicate no ordering at T(sub o) of Np magnetic moments. Absence of long-range magnetic order below T(sub o) fits with the outcome of a polarization analysis of superlattice intensities at 12K; signals are observed in both the unrotated and rotated channels of scattering while magnetic (dipole) moments would contribute only in the rotated channel. The authors demonstrate that these empirical findings, together with a narrow energy profile of the Braff intensity at the Np M(sub 4) edge, are consistent with magnetic and charge contributions to the E1 Bragg amplitude described by Np 5f multipoles of ranks 3 (octupole) and 4 (hexadecapole).