Polarized electronic absorption spectra of Co²⁺ ions in the kieserite-type compounds CoSO₄ · H₂O and CoSeO₄ · H₂O

Abstract

Polarized electronic absorption spectra of the kieserite-type compounds CoSO₄ · H₂O and CoSeO₄ · H₂O have been obtained at room temperature (spectral range 35 000-5000 cm^-1) and at liquid nitrogen temperature (visible spectral region), using microscope-spectrometric techniques. The spectra are interpreted and evaluated in terms of a tetragonal crystal field formalism for the d⁷ configuration, in regard to the pseudotetragonal elongation of the CoO₄(H₂O)₂ octahedra, known from previous X-ray structure investigations, employing the tetragonal parameters Dq, Dt, and Ds, and the Racah parameters B and C. The observed and calculated energy levels are in good agreement for the following parameter sets: CoSO₄ · H₂O: Dq=826, Dt=40, Ds=350, B=856, C=3580 cm^-1; CoSeO₄· H₂O: Dq=817, Dt=44, Ds=406, B=841, C=3490 cm^-1; corresponding ‘cubic’ crystal field strengths Dq_cub are 803 and 792 cm^-1, respectively. The values of Dq_(cub), Racah B and C are in the common range for Co²⁺ ions in (pseudo) octahedral fields of oxygen ligands, and their differences in CoSO₄· H₂O compared to CoSeO₄ · H₂O are consistent with somewhat different mean Co-O bond lengths and with a slightly higher covalent contribution to Co-O bonding in the selenate compound. The values found for the parameter Dt, which is directly correlated to the extent of tetragonal distortion, are much lower than expected from purely geometrical considerations, thus confirming a significantly higher position of H₂O ligands in the spectrochemical series compared to oxygen ligands belonging to SO₄ or SeO₄ groups.