The ideal Weyl "hydrogen-atom" semimetal exhibits only a single pair of Weyl nodes and no other trivial states at the Fermi energy. Such a material would be a panacea in the study of Weyl quasi-particles, allowing direct unambiguous observation of their topological properties. The alluaudite-like ${\mathrm{K}}_2{\mathrm{Mn}}_3{\left({\mathrm{AsO}}_4\right)}_3$ compound was recently proposed as such a material. Here, we use comprehensive experimental work and first-principle calculations to assess this prediction. We find ${\mathrm{K}}_2{\mathrm{Mn}}_3{\left({\mathrm{AsO}}_4\right)}_3$ crystallizes in the $C2/c$ symmetry with a quasi-one-dimensional Mn sublattice, growing as small needle-like crystals. Bulk property measurements reveal magnetic transitions at $\approx 8$ and $\approx 4$ K, which neutron scattering experiments show correspond to two distinct magnetic orders, first a partially ordered ferrimagnetic ${\mathbf{k}}_{\mathbf{1}}=(0,0,0)$ structure at 8 K and a second transition of ${\mathbf{k}}_{\mathbf{2}}=(1,0,0)$ at 4 K to a fully ordered state. Below the second transition, both ordering vectors are necessary to describe the complex magnetic structure with modulated spin magnitudes. Both of the best-fit magnetic structures in this work are found to break the symmetry necessary for the generation of Weyl nodes, though one of the magnetic structures allowed by ${\mathbf{k}}_{\mathbf{1}}$ does preserve this symmetry. However, the crystals are optically transparent and ellipsometry measurements reveal a large band gap, undermining expectations of semimetallic behavior. Density functional theory calculations predict an insulating antiferromagnetic ground state, in contrast to previous reports, and suggest potential frustration on the magnetic sublattice. Given the wide tunability of the alluaudite structure, we consider ways to push the system closer to a semimetallic state.

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• Electronic structure
• Magnetism
• Topological materials
• Ferrimagnets
• Weyl semimetal
• Density functional theory
• Magnetization measurements
• Neutron diffraction