Magnetic susceptibilities between 80 and 300 K and 57Fe Mössbauer parameters between 4.2 and 295 K are reported for Fe(SO3F)2. These data have been analysed via a crystal field model including spin–orbit and spin–spin coupling. The compound is trigonally distorted by an elongation along the [111] axis of the FeO6 octahedron, and the ground state is the orbital doublet [Formula: see text]. The quadrupole coupling constant e2qQ is positive, and no rhombic distortion could be detected. The electronic spectrum shows a splitting of the 5Eg excited level, presumably by a dynamic Jahn–Teller effect, and [Formula: see text] Attempts to fit a low-temperature magnetic perturbation Mössbauer spectrum using a pseudo-spin Hamiltonian were only partially successful, but suggest that the g tensor is highly anisotropic with [Formula: see text] and that the internal hyperfine field is small. Spin relaxation in Fe(SO3F)2 is fast at all temperatures down to 4.2 K and in applied magnetic fields of up to 5.0 T.
