Mössbauer Resonance Studies of Ferrous Ions in Ice

Abstract

Mössbauer resonance experiments have been carried out over a wide temperature range on frozen aqueous solutions of ferrous chloride and ferrous sulfate. Well-resolved spectra were obtained which show the two-line absorption pattern characteristic of a nuclear quadrupole hyperfine interaction in 57Fe. Measurements of quadrupole splitting, resonance linewidths, and anisotropic intensities were made as a function of temperature and for samples with different temperature histories. Upon quenching from the liquid state to −196°C, hexaquo ferrous ions are trapped in the ice lattice and induce the formation of cubic ice, rather than the stable hexagonal form. Warming of the quenched sample to −80°C causes a nonreversible transformation to hexagonal ice and associated pronounced changes in the Mössbauer spectrum. Analysis of the experimental data, and comparison with results obtained for the crystal FeSiF6·6H2O which contains hexaquo ferrous ions, has permitted a detailed description of the symmetry properties of ferrous ion in ice. In hexagonal ice, the ferrous ion is located at the center of an axially ``squashed'' octahedron of water molecules, the ground state wavefunction being the orbital singlet | xy 〉. The temperature dependence of the quadrupole splitting in hexagonal ice is consistent with a crystal-field axial splitting δ∼700 cm−1. In cubic ice, the symmetry contains a rhombohedral field component which can be fitted with splitting parameters Δ1∼450 cm−1 and Δ2∼950 cm−1. A model is suggested for the structure of ferrous ion in cubic and hexagonal ice. The model satisfies the necessary symmetry requirements and is consistent with the observed anisotropic Mössbauer intensities in hexagonal ice and isotropic intensities in cubic ice. Several interesting phase changes in the ice—salt systems have been observed. In ferrous chloride frozen solutions, the sharp disappearance of the Mössbauer effect above −40°C is associated with eutectic formation. Also, by careful temperature treatment, the hydrate FeCl2·6H2O has been produced (it is unstable above −10°C) and its Mössbauer spectrum in ice was obtained. In the ferrous sulfate experiments, complex precipitation phenomena were observed and are not well understood.