Crystal Structure, Physical Properties, and Electronic and Magnetic Structure of the Spin S = ⁵/₂ Zigzag Chain Compound Bi₂Fe(SeO₃)₂OCl₃

We report the synthesis and characterization of the new bismuth iron selenite oxochloride Bi₂Fe­(SeO₃)₂OCl₃. The main feature of its crystal structure is the presence of a reasonably isolated set of spin S = ⁵/₂ zigzag chains of corner-sharing FeO₆ octahedra decorated with BiO₄Cl₃, BiO₃Cl₃, and SeO₃ groups. When the temperature is lowered, the magnetization passes through a broad maximum at T_max ≈ 130 K, which indicates the formation of a magnetic short-range correlation regime. The same behavior is demonstrated by the integral electron spin resonance intensity. The absorption is characterized by the isotropic effective factor g ≈ 2 typical for high-spin Fe³⁺ ions. The broadening of ESR absorption lines at low temperatures with the critical exponent β = ⁷/₄ is consistent with the divergence of the temperature-dependent correlation length expected for the quasi-one-dimensional antiferromagnetic spin chain upon approaching the long-range ordering transition from above. At T_N = 13 K, Bi₂Fe­(SeO₃)₂OCl₃ exhibits a transition into an antiferromagnetically ordered state, evidenced in the magnetization, specific heat, and Mössbauer spectra. At T < T_N, the ⁵⁷Fe Mössbauer spectra reveal a low saturated value of the hyperfine field H_hf ≈ 44 T, which indicates a quantum spin reduction of spin-only magnetic moment ΔS/S ≈ 20%. The determination of exchange interaction parameters using first-principles calculations validates the quasi-one-dimensional nature of magnetism in this compound.