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 Li₂Ni₃P₄O₁₄ 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 P2₁/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 (Cr₂O₃) and Cu₂(MoO₄)(SeO₃) [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].