A case for polar uranium octupoles in cubic U2N3

Abstract

Uranium ions in sesquinitride alpha-U2N3 occupy independent acentric and centrosymmetric sites according to conventional x-ray diffraction patterns [R. Tro\'c, J. Solid State Chem. 13, 14 (1975)]. We submit that polar uranium multipoles in acentric sites are revealed in resonant x-ray diffraction data recently published by Lawrence Bright et al. [Phys. Rev. B 100, 134426 (2019)]. To this end, their diffraction data gathered with a primary x-ray energy in the vicinity of the uranium M4 absorption edge are compared to symmetry-informed diffraction amplitudes calculated for the bixbyite alpha-Mn2O3 lattice structure. Bragg spots forbidden in this lattice diffraction pattern appear to provide clear-cut evidence for high-order polar uranium multipoles.