Neutron diffraction studies on the Pr₃HfBi₅ magnetic structure

Members of the Ln3MBi5 family (Ln=Pr, Nd, Sm; M=Zr, Hf) crystallize in the P6₃/mcm space group, hosting the anti-Hf5Sn3Cu structure type. [1] Hypervalent Bi²⁻ chains and face-sharing MBi₆ octahedra form one-dimensional chains along the c-axis, and a framework of Ln³⁺ cations charge balances and separates the two motifs. The local symmetry of the Ln ion is orthorhombic, which for Pr³⁺ splits the J = 4 multiplet into 9 singlets and would typically result in a non-magnetic Pr³⁺ ion. However, Pr₃HfBi₅ orders antiferromagnetically at 4 K with a wavevector k = (1/2, 1/2, 1/10). Bulk magnetization measurements further show strong magnetic anisotropy, with a broad ordering temperature in susceptibility along the c-axis, and a sharp transition within the ab-plane. Here, we present neutron diffraction results on the magnetic structure of Pr₃HfBi₅ and compare to the magnetic structures of isostructural materials. For example, Pr₃TiSb₅ is non-magnetic [2] despite the Pr-Pr bonds being closer together than in Pr₃HfBi₅, where an increase in the exchange coupling strength would be expected, suggesting that subleties in the CEF splitting lead to the long range magnetic order here.

[1] J. F. Khoury et al. Z. Anorg. Allg. Chem., 648, e202200123 (2022).

[2] C. Ritter et al. J. Phys.: Condens. Matter, 33, 245801 (2021).