Polar Layered Magnet Ba₂Cu₃(SeO₃)₄F₂ Composed of Bitriangular Chains: Observation of 1/3-Magnetization Plateau

We prepared a new polar layered quantum magnet Ba₂Cu₃(SeO₃)₄F₂, by a combined use of F^- and pyramidal SeO₃^2- anions, determined its crystal structure by X-ray diffraction, and characterized its magnetic properties by magnetization, electron spin resonance (ESR) and specific heat measurements, and by density functional theory calculations. The title compound has first experimentally reported bitriangular chains of Cu²⁺ ions aligned along the b direction to form layers parallel to the ab plane, and these layers are separated by Ba²⁺ ions. The magnetic susceptibility data reveal that, despite strong predominant antiferromagnetic intrachain interactions indicated by the large negative Weiss temperature θ of −143.9 K within bitriangular chains, no long-range order occurs down to 2 K. The latter, further confirmed by the specific heat measurements, is attributed to the extremely weak interlayer interaction. The spins in each bitriangular chain become ferrimagnetically ordered to exhibit a 1/3-magnetization plateau, which persists at least up to 30 T. This reveals that each bitriangular chain acts as an S = 1/2 entity at low temperatures, as observed from the decrease of the effective magnetic moment P_eff from 3.67 μ_B in the high temperature range to 1.89 μ_B in the low temperature range, equivalently, from three free Cu²⁺ ions to only one effective Cu²⁺ ion per formula unit. In each layer of Ba₂Cu₃(SeO₃)₄F₂, the interaction between adjacent ferrimagnetic chains is ferromagnetic rather than antiferromagnetic, contrary to the observations in other reported cases.