Mössbauer study of transferred hyperfine interactions and magnetic ordering in Fe(III) bis(dithiocarbamato) iodides

Mössbauer spectra of ⁵⁷Fe and ¹²⁹I have been measured in three members of the group of Fe(III) bis(dithiocarbamato) iodides in the temperature region 1.4 to 4.2°K. All three complexes order magnetically below 4.2°K. Transition temperatures have been determined from the abrupt appearance of magnetic hyperfine splitting. The values of isomer shift, quadrupole coupling, and the magnitude and direction of the effective field in the EFG principal axis system were determined by computer fits of the spectra. The temperature dependence of the hyperfine magnetic field is essentially the same for all three compounds and differs considerably from the calculated dependence from a molecular field model for a spin S = 3/2 system. Spin and charge transfer coefficients for the iron-iodine bond have been determined by an analysis of the hyperfine parameters for ¹²⁹I. It has been found that about 30% of one electronic charge is transferred to iron 3d orbitals and that the bond has a predominant σ character. The ⁵⁷Fe data have been analyzed in terms of an electronic spin Hamiltonian of rhombic symmetry. It has been concluded that the expectation value of the spin and the molecular exchange magnetic field are essentially perpendicular to the direction of the iron-iodine bond.