A case for resonant X-ray Bragg diffraction by a collinear antiferromagnet Li ₂ Ni ₃ P ₄ O ₁₄

Magnetic axial and polar (Dirac) nickel multipoles contribute to resonant X-ray Bragg amplitudes in a symmetry-informed analysis of monoclinic Li 2 Ni 3 P 4 O 14 presented for future diffraction experiments. Magnetic long-range order below a temperature of ≃ 14.5 K can be viewed as a two-dimensional trimerized antiferromagnet with Ni ions in two Wyckoff positions in the centrosymmetric ( 1 ) magnetic space group P 2 1 / c . It permits the coupling to circular polarization in the primary X-ray beam, unlike the corresponding diffraction by an antiferromagnet characterized by anti-inversion ( 1 ′) and a linear magnetoelectric effect, e.g. historically significant chromium sesquioxide (Cr 2 O 3 ) and Cu 2 (MoO 4 )(SeO 3 ) [Lovesey & van der Laan (2024). Phys. Rev. B 110 , 174442]. The space group is inferred from neutron Bragg diffraction patterns, without an allowance for permitted Dirac dipoles (anapoles) and quadrupoles [Chikara et al. (2025). Phys. Rev. B 112 , 014438].