Lattice dynamics of the frustrated kagome compound Y-kapellasite

Studying the magnetic ground states of frustrated antiferromagnets provides unique insight into the stability of quantum spin liquids, even if the anticipated state is not realized toward T = 0. Particularly relevant are structural modifications setting in at temperatures where the magnetic correlations come into play. Here, we explore the lattice dynamics of Y-kapellasite $[{\mathrm{Y}}_{3}\mathrm{C}{\mathrm{u}}_{9}{(\mathrm{OH})}_{19}\mathrm{C}{\mathrm{l}}_{8}]$ single crystals by infrared spectroscopy in combination with ab initio calculations. We observe significant changes in the phonon spectra at ${T}_{s}=32$ K that gradually evolve down to low temperatures. The increase in the number of phonon modes provides evidence for a lowering of symmetry, and we discuss several possibilities of crystal structure modifications. Our analysis also reveals that the structural variation involves exclusively H and O atoms, while the other atoms remain rather unaffected. An 8% redshift of the lowest-lying phonon mode upon cooling indicates strong magnetoelastic effects upon decoupling $\mathrm{Cu}\text{\ensuremath{-}}6f$ hexagons through the lattice vibrations.