Hydrogen/Deuterium Dynamics in Hydroxyl Salts Co2(OH)3Br/Co2(OD)3Br Revealed by Muon Spin Relaxation

Abstract

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.

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

“…Co 2 (OH) 3 Br/Co 2 (OD) 3 Br is an antiferromagnetic system with strong magnetic couplings between Co 2+ ions up to high temperatures, as demonstrated by previous μSR study [18]. In view of its crystal structure, the antiferromagnetic couplings should be dominated by the super-exchange…”

“…To investigate the unconfirmed mechanism of ferroelectric response in antiferromagnets Co 2 (OH) 3 Br/Co 2 (OD) 3 Br, with identical structure to previously reported ferromagnetic Co 2 (OH) 3 Cl/Co 2 (OD) 3 Cl [6], the muon spin relaxation (μSR) measurement was carried out and it provided preliminary evidence to the nature of the ferroelectric response which is directly related to the hydrogen/deuterium dynamics. In addition, uncovered magnetic coupling between Co 2+ ions was found to be existing at high temperatures in this system [18].…”

“…Among them, the modes H10/D10, H11/D11 and H12/D12, which are associated with the Co-OH/OD and Co-Br, respectively, as denoted in table 1, exhibited prominent deviations in the frequencies and line-widths. Since hydrogen/deuterium ordering has been suggested by previous muon spin relaxation study [18], one would expect the largest deviation in the modes directly associated to the OH/OD, i.e., the modes H3-5/D3-5, other than the H10-12/D10-12 of Co-OH/OD and Co-Br. The experimentally observed results apparently deviated from this expectation.…”