FLUOROCOMPLEXES OF NIOBIUM-IV .6. MOSSBAUER STUDY AND MAGNETIC-STRUCTURE OF FENBF6

The 3D-framework of FeNbF6, related to that of LiSbF6, is built up from corner sharng NbF6 and FeF6 octahedra. Two crystallographic phase transitions occur at T=448 and 150 K: the crystal symmetry decreases from cubic to rhombohedral and triclinic. Cationic disorder between Fe2+ and Nb4-, estimated to 10%, results in the presence of three types of Fe2- site and the spreading of the magnetic ordering around TN=60 K. Belkow this temperature, the magnetic moment at the dominant site of Fe2+ (μ=4.40(0.3)μB at 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+ neighbors adopt a G-type antiferromagnetic arrangement.