Lattice dynamics of the frustrated kagome compound Y-kapellasite

TL;DR

This study investigates the lattice dynamics of Y-kapellasite using infrared spectroscopy and ab initio calculations, revealing structural changes at low temperatures and strong magneto-elastic effects related to magnetic frustration.

Contribution

It provides new insights into the temperature-dependent lattice dynamics and structural modifications of Y-kapellasite, highlighting the role of phonons and magneto-elastic coupling in frustrated magnets.

Findings

Significant phonon spectrum changes at 32 K indicating symmetry lowering

Structural variations involve only H and O atoms

Strong magneto-elastic effects observed with an 8% phonon mode shift

Abstract

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 towards 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 (Y3Cu9(OH)19Cl8) single crystals by infrared spectroscopy in combination with ab initio calculations. We observe significant changes in the phonon spectra at Ts = 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% red shift of the lowest-lying phonon mode upon cooling indicates strong magneto-elastic effects upon decoupling Cu-6f hexagons through the lattice vibrations.