Lattice dynamics, phase transitions and spin relaxation in [Fe(C₅H₅)₂] PF₆

The organometallic compound ferrocenium hexafluorophosphate, [Fe(C₅H₅)₂] PF₆, has been studied by Mössbauer spectroscopy in the past, mainly to determine the crystal structure at high temperatures. Here we present studies at 95 K to 305 K and analyze the spectra in terms of spin relaxation theory which yields accurately the hyperfine interaction parameters and the spin-spin and spin-lattice relaxation rates in this paramagnetic compound. The spectral area under the resonance curve yields the recoil free fraction and thus the mean square of the vibration amplitude <x²>. One observes a large discontinuity in the slope of <x²> versus T at ˜210 K, indicative of a phase transition. The analysis of the spectra proves that the quadrupole interaction is small but certainly negative, ½e²qQ = -0.12(2) mm/s, and causes the asymmetry observed in the spectra. The detailed analysis yields also, for the first time, the fluctuating effective magnetic hyperfine field, H _eff = 180(50) kOe.