Molecular motion in solids: Mössbauer spectra of [Fe(η-C6H6)(η-C5H5)][AsF6]

The 57Fe Mossbauer spectrum of the compound [Fe(η-C6H6)(η-C5H5)][AsF6] has been recorded in the temperature range 77–315 K. At low temperatures the spectrum is the expected quadrupole doublet but as the temperature is raised a new doublet gradually develops in the centre of the absorption. This feature is fully developed at 312 K. A relaxation model based on anisotropic reorientation of the sandwich cations within a monoclinic lattice is put forward.