1. Raman spectroscopic study of frustrated ferroelectric phase in hydroxyl salts Co2(OH)3Br/Co2(OD)3Br
Xing-Liang Xu, Dong-Dong Meng, Xu-Guang Zheng, Qi-Xin Guo J Phys Condens Matter. 2021 May 28;33(27). doi: 10.1088/1361-648X/abfe22.
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/Tc2βwhereinTc= 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.
2. Hydrogen/Deuterium Dynamics in Hydroxyl Salts Co2(OH)3Br/Co2(OD)3Br Revealed by Muon Spin Relaxation
Xing-Liang Xu, Xu-Guang Zheng, Isao Watanabe Materials (Basel). 2019 Jul 3;12(13):2135. doi: 10.3390/ma12132135.
The temperature-dependent dynamics of the hydrogen/deuterium atoms in geometrically frustrated magnets Co2(OH)3Br and its deuterated form Co2(OD)3Br were investigated by muon spin relaxation (μSR). The deuterium atoms in Co2(OD)3Br were found to be rapidly fluctuating at high temperatures, which should be arising as a quantum atomic effect due to the small mass of deuterium, then they drastically slowed down toward Tc = 250 K where a broad anomaly appeared in the dielectric response, and finally became quasi-static at around 180 K. Meanwhile, the hydrogen atoms in Co2(OH)3Br also exhibited a two-step slowing at ~240 K and ~180 K, respectively. The revealed properties in Co2(OH)3Br/Co2(OD)3Br are reminiscent of relaxor-type ferroelectrics. The present study suggested the effectiveness of the μSR technique on revealing the hydrogen/deuterium (H/D) dynamics in Co2(OH)3Br/Co2(OD)3Br. Furthermore, magnetic coupling was found to be existing at high temperatures in this system. This work provides clear evidence to the mechanism of ferroelectric responses in the hydroxyl salts, i.e., the slowing of protons (deuterium ions) is directly related to the newly revealed ferroelectricity.
3. (H3O)3Sb2Br9: the first member of the M3E2X9 structure family with oxonium cations
Jen Hui Chang, Thomas Doert, Michael Ruck Acta Crystallogr C Struct Chem. 2016 Dec 1;72(Pt 12):966-970. doi: 10.1107/S2053229616017198. Epub 2016 Nov 7.
(H3O)3Sb2Br9 [trihydroxonium enneabromidodiantimonate(III)] is the first representative of the M3E2X9 family (M = cation, E = Sb and Bi, and X = Br and I) with oxonium cations. The metastable compound was obtained in trace amounts from a solution of CsBr and SbBr3 in concentrated aqueous HBr. Single crystals were isolated from the mother liquor and investigated by single-crystal X-ray diffraction at 100 K. (H3O)3Sb2Br9 crystallizes with the Tl3Bi2I9 structure type, which is a distorted defect variant of cubic perovskite. The crystal structure comprises characteristic 2∞[SbBr3Br3/2] double layers of corner-sharing SbBr6 octahedra with a [001] stacking direction. Due to the small size of the H3O+ cation and O-H...Br hydrogen bonding, the octahedra are tilted.
