Paramagnetic and Electric Quadrupole Hyperfine Interactions of Ferric Ions in Ice and FeCl3·6H2O

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Abstract
Mössbauer resonance in 57Fe has been used to study the species Fe(H2O)5Cl2+ and Fe(H2O)6 3+ in frozen aqueous solutions of ferric chloride and ferric nitrate. The spectral lines display pronounced broadening as the temperature is lowered, as a result of paramagneticrelaxation effects. In dilute samples at 4.2°K, resolved paramagnetichyperfine structure was observed for both species and permitted identification of the lowest crystal‐field states of the ferric ion. The effective magnetic fields originating from the | ± 5 2 〉 Kramers doublets were determined as 553 kG in Fe(H2O)5Cl2+ and 588 kG in Fe(H2O)6 3+. The latter ion showed no evidence of an electric quadrupole interaction, whereas the former exhibited a significant quadrupole splitting due to the coordinated Cl− ligand. The apparent electronic relaxation in the pentaquo–chloro complex was found to be substantially faster than that in the hexaquo complex, which may be indicative of a smaller zero‐field splitting in the former case or an unbalanced spin pairing in the Cl ion. Results are presented for crystalline FeCl3·6H2O, in which the ion Fe(H2O)4Cl2 + exists as a structural unit. These spectra exhibit unusual line shapes and temperature variations which can be qualitatively understood by considering the effects of paramagneticrelaxation on the hyperfine interactions. The study of FeCl3·6H2O and its frozen solutions also demonstrates that common features as well as differences between the ion complex in solution and in the salt crystal may be inferred from a comparison of the Mössbauer spectra.