Fluorocomplexes of niobium IV: VI. Mössbauer study and magnetic structure of FeNbF6

The 3D-framework of FeNbF6, related to that of LiSbF6, is built up from corner sharing NbF6 and FeF6 octahedra. Two crystallographic phase transitions occur at T = 448 and 150 K: the crystal symmetry decreases from cubic Fm3m, a = 7.954(4) Å, Z = 4, X-ray, 293 K) to rhombohedral (R3, aHex = 5.4201(8) Å, CHex = 14.072(2) Å, Z = 3, X-ray, 293 K) and triclinic (P1, a = 7.7930(10) Å, b = 7.7398(12) Å, c = 7.7435(12) Å, α = 86.585(12)°, β = 87.046(8)°, γ = 86.138(9)°, Z = 4, neutron, 108 K), respectively. Cationic disorder between Fe2+ and Nb4+, estimated to ≈ 10%, results in the presence of three types of Fe2+ sites and the spreading of the magnetic ordering around TN = 60 K. Below this temperature, the magnetic moment at the dominant site of Fe2+ (μ = 4.4(0.3)μBat 4 K) lies mainly along the a axis of the triclinic cell but no significant magnetization can be evidenced at the dominant site of Nb4+. The magnetic moments of next nearest Fe2+ neighbours adopt a G-type antiferromagnetic arrangement.