POLARIZED ELECTRONIC ABSORPTION-SPECTRA OF CO2-TYPE COMPOUNDS COSO4-CENTER-DOT-H2O AND COSEO4-CENTER-DOT-H2O( IONS IN THE KIESERITE)

Polarized electronic absorption spectra of the kieserite-type compound s CoSO4 . H2O and CoSeO4 . H2O have been obtained at room temperature (spectral range 35000-5000 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 cry stal field formalism for the d(7) configuration, in regard to the pseu dotetragonal elongation of the CoO4(H2O)(2) octahedra, known from prev ious X-ray structure investigations, employing the tetragonal paramete rs Dq, Dt, and Ds, and the Racah parameters B and C. The observed and calculated energy levels are in good agreement for the following param eter sets: CoSO4 . H2O: Dq=826, Dt=40, Ds=350, B=856, C=3580 cm(-1); C oSeO4 . H2O: Dq=817, Dt=44, Ds=406, B=841, C=3490 cm(-1); correspondin g 'cubic' crystal field strengths Dq(cub) are 803 and 792 cm(-1), resp ectively. The values of Dq((cub)), Racah B and C are in the common ran ge for Co2+ ions in (pseudo)octahedral fields of oxygen ligands, and t heir differences in CoSO4 . H2O compared to CoSeO4 . H2O are consisten t with somewhat different mean Co-O bond lengths and with a slightly h igher 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 fro m purely geometrical considerations, thus confirming a significantly h igher position of H2O ligands in the spectrochemical series compared t o oxygen ligands belongingto SO4 or SeO4 groups.