Raman spectroscopic study of frustrated ferroelectric phase in hydroxyl salts Co₂(OH)₃Br/Co₂(OD)₃Br

We report on an investigation of the temperature-dependent ordering of the hydrogen/deuterium atoms in geometrically frustrated magnets Co₂(OH)₃Br and its deuterated Co₂(OD)₃Br, to shed light on the origin of the newly-identified ferroelectricity using Raman spectroscopy. Significant changes in the Raman frequencies and line-widths of the Raman-active modes were observed below ∼260 K in Co₂(OD)₃Br and ∼240 K in Co₂(OH)₃Br, respectively, the analysis of which revealed strong spin-phonon couplings in this system. Further, for Co₂(OD)₃Br, six new phonon bands appeared below around 260 K, with the corresponding intensities obeying a power-law equationI∝1-T/Tc2βwhereinT_c= 260 K, suggesting that an ordering process occurred below ∼260 K. The ordering process subsequently affected the local structure and brought out the reported ferroelectric phase, which is considered as frustrated. Meanwhile, in Co₂(OH)₃Br, only one new band was observed below ∼240 K, followed by two 'softened' modes correlated to the [OH] sub-lattice below ∼185 K, wherein an incomplete ordering was suggested. The present work reveals a new multiferroic system combining geometrically frustrated magnetism and deuterium ordering-type ferroelectricity.