Stability Frontiers in the AM6X6 Kagome Metals: The LnNb6Sn6 (Ln:Ce–Lu,Y) Family and Density-Wave Transition in LuNb6Sn6

The kagome motif is a versatile platform for condensed matter physics, hosting rich interactions between magnetic, electronic, and structural degrees of freedom. In recent years, the discovery of a charge density wave (CDW) in the AV3Sb5 superconductors and structurally-derived bond density waves (BDW) in FeGe and ScV6Sn6 have stoked the search for new kagome platforms broadly exhibiting density wave (DW) transitions. In this work, we evaluate the known AM6X6 chemistries and construct a stability diagram that summarizes the structural relationships among the >125 member family. Subsequently, we introduce our discovery of the broader LnNb6Sn6 (Ln:Ce-Nd,Sm,Gd-Tm,Lu,Y) family of kagome metals and an analogous DW transition in LuNb6Sn6. Our X-ray scattering measurements clearly indicate a (1/3, 1/3, 1/3) ordering wave vector (3×3×3 superlattice) and diffuse scattering on half-integer L-planes. Our analysis of the structural data supports the "rattling mode" DW model proposed for ScV6Sn6 and paints a detailed picture of the steric interactions between the rare-earth filler element and the host Nb-Sn kagome scaffolding. We also provide a broad survey of the magnetic properties within the HfFe6Ge6-type LnNb6Sn6 members, revealing a number of complex antiferromagnetic and metamagnetic transitions throughout the family. This work integrates our new LnNb6Sn6 series of compounds into the broader AM6X6 family, providing new material platforms and forging a new route forward at the frontier of kagome metal research.