Tiny Sc Allows the Chains to Rattle: Impact of Lu and Y Doping on the Charge-Density Wave in ScV₆Sn₆

The kagome metals display an intriguing variety of electronic and magnetic phases arising from the connectivity of atoms on a kagome lattice. A growing number of these materials with vanadium-kagome nets host charge-density waves (CDWs) at low temperatures, including ScV₆Sn₆, CsV₃Sb₅, and V₃Sb₂. Curiously, only the Sc version of the RV₆Sn₆ materials with a HfFe₆Ge₆-type structure hosts a CDW (R = Gd-Lu, Y, Sc). In this study, we investigate the role of rare earth size in CDW formation in the RV₆Sn₆ compounds. Magnetization measurements on our single crystals of (Sc,Lu)V₆Sn₆ and (Sc,Y)V₆Sn₆ establish that the CDW is suppressed by substituting Sc by larger Lu or Y. Single-crystal X-ray diffraction reveals that compressible Sn-Sn bonds accommodate the larger rare earth atoms within loosely packed R-Sn-Sn chains without significantly expanding the lattice. We propose that Sc provides extra room in these chains crucial to CDW formation in ScV₆Sn₆. Our rattling chain model explains why both physical pressure and substitution by larger rare earth atoms hinder CDW formation despite opposite impacts on lattice size. We emphasize the cooperative effect of pressure and rare earth size by demonstrating that pressure further suppresses the CDW in a Lu-doped ScV₆Sn₆ crystal. Our model not only addresses why a CDW only forms in the RV₆Sn₆ materials with tiny Sc but also advances our understanding of why unusual CDWs form in the kagome metals.