Magnetic properties and helical spin structure of another compound Ca₃SbFe₃Si₂O₁₄ from the family of iron-containing langasites

A strong interest is attracted to the langasite-type compounds containing magnetic 3d ions. Coexistence of electric and magnetic order parameters in such materials can provide a new variety of multiferroics. In our work, another Fe-based langasite with a nearly stoichiometric composition Ca₃SbFe₃Si₂O₁₄ was synthesized. X-ray diffraction and X-ray fluorescence analyses, studies of magnetic susceptibility, specific heat and Mössbauer spectroscopy were performed. The crystal structure of Ca₃SbFe₃Si₂O₁₄ (CSFS) was found to be hexagonal, space group P321. From full-profile analysis by the Le Bail method, the lattice parameters a = 8.115(1) Å, c = 5.058(1) Å, V = 288.4(1) ų were determined. The temperature dependence of magnetic susceptibility χ(Т) indicates the formation of long-range magnetic order at temperatures below T_N = 34 K. The negative value of Weiss temperature θ_CW indicates the domination of antiferromagnetic exchange interaction. From the value of the Curie constant, the effective magnetic moment per Fe³⁺ ion µ_eff = 6.12 µ_B was estimated, which is slightly higher than the expected value 5.9 µ_B for the 3d⁵ - S = 5/2 state of iron. Measurements of the specific heat allowed a Debye temperature value Θ_D = 390(10) K to be obtained. Based on the assessment of the magnetic contribution to the specific heat, the value of magnetic entropy ΔS_mag ∼ 47.8 ± 5.6 J/mol K was determined. Hyperfine interaction parameters obtained from Mössbauer spectra indicate a helical magnetic ordering of iron moments in langasite Ca₃SbFe₃Si₂O₁₄. The period of a magnetic helicoid is about five periods of the crystal structure along the c axis. The calculated value of the angle (α) between the main axis of the electric field gradient Z′ at iron nuclei and the crystallographic c axis (67.2°) is significantly greater than in other langasites Ba₃SbFe₃Si₂O₁₄ (28.8°), Ba₃NbFe₃Si₂O₁₄ (36°), Ba₃TaFe₃Si₂O₁₄ (34.8°). This may be due to the more contracted FeO₄ tetrahedra in CSFS compared to those in the other compositions.