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

We report on an investigation of the temperature-dependent ordering of the hydrogen/deuterium atoms in geometrically frustrated magnets Co(OH)Br and its deuterated Co(OD)Br, 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 Co(OD)Br and ∼240 K in Co(OH)Br, respectively, the analysis of which revealed strong spin-phonon couplings in this system. Further, for Co(OD)Br, six new phonon bands appeared below around 260 K, with the corresponding intensities obeying a power-law equationI∝1-T/Tc2βwherein= 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 Co(OH)Br, 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.