POLARIZED ELECTRONIC ABSORPTION-SPECTRA OF TETRAHEDRALLY COORDINATED CO2+ IONS IN THE NEW COMPOUND CA3CO(SEO3)(4) AND ITS STRUCTURAL CHARACTERIZATION

The crystal structure of the new compound Ca3Co(SeO3)(4) was investiga ted using single crystal X-ray diffraction data up to sin theta/lambda = 0.86 [tetragonal, space group I4(1)/a, a = 7.571(1), c = 22.493(3) Angstrom, Z = 4; R1 = 0.024 for 1125F(0) > 4 sigma(F-0) and 47 variabl es]. The structure is composed of nearly perfectly regular CoO4 tetrah edra, pyramidal SeO3 groups, and two types of CaO8 polyhedra. Ca3Co(Se O3)(4) represents the first example for a selenite compound containing CoO4 tetrahedra. Furthermore, polarized electronic absorption spectra of Ca3Co(SeO3)(4) in the range from 2000 to 35,000 cm(-1) were obtain ed by means of UV/VIS microscope- and FTIR-spectrometry. The observed strong polarization of the spectra is in contradiction to the regular shape of the CoO4 tetrahedron and indicates a significant tetragonal f ield perturbation imposed by the further coordination of the CoO4 grou p. The band positions, band widths, and splittings are interpreted bas ed on crystal field calculations employing the crystal field parameter s Dq, Dt, Ds, the Racah parameters B and C, and the spin-orbit couplin g constant zeta. A rather good agreement of observed and calculated en ergies is obtained for a parameter set with Dq = 395, Dt = 25, Ds = 25 0, B = 770, C = 3400, and zeta = 500 cm(-1). (C) 1996 Academic Press, Inc.