Ln₃MBi₅ (Ln=Pr, Nd, Sm; M=Zr, Hf): Intermetallics with Hypervalent Bismuth Chains

We report five new isostructural compounds in the Ln₃MBi₅ (Ln=Pr, Nd, Sm; M=Zr, Hf) family, and compare them to the recently reported Sm₃ZrBi₅ analogue. Ln₃MBi₅ crystallizes in the P6₃/mcm space group, hosting the anti-Hf₅Sn₃Cu structure type. The one-dimensional structure consists of hypervalent Bi²⁻ chains and face-sharing MBi₆ octahedra that form chains along the c axis. A framework of Ln³⁺ cations charge balances and separates the two motifs. The Bi−Bi and M−Bi bond lengths decrease as the Ln cation becomes smaller across the period, but there is almost no difference in either bond length when the identity of M changes, as expected because Zr and Hf have the exact same radii due to lanthanide contraction. We present a structural stability map showing the limits of cation stability in the structure, with La³⁺ and U³⁺ as the largest cations and Sm³⁺ as the smallest cation. X-ray photoelectron spectra suggest ambiguous valence states in the Ln₃MBi₅ structure depending on the identity of the M atom. This work further expands the Ln₃MBi₅ family and sheds new light on how their bonding behavior may vary based on chemical composition.