Crystal structure, electronic absorption spectra, and crystal field superposition model analysis of Li2Co3(SeO3)(4)

Single crystals of the new compound Li2Co3SeO3)(4) have been synthesized un der low-hydrothermal conditions. The structure was determined by;direct met hods from single crystal X-ray diffraction data (monoclinic, space group P2 (1)/c, a = 8.095(2) Angstrom, b = 9.236(3) Angstrom, c = 7.781(2) Angstrom, beta = 115.76(1)degrees. V = 523.9 Angstrom(3), Z = 2; R1 = 0.034 for 1613 F-o > 4 sigma(F-o) and 98 variables). Li2Co3(SeO3)(4) builds a framework s tructure composed of one type of [4 + 2] coordinated Li atoms and each two crystallographically different CoO6 octahedra and pyramidal SeO3 groups. Th e polyhedra of lithium and of one type of the cobalt atoms exhibit uncommon ly strong irregular distortions. Polarized electronic absorption spectra of Li2Co3(SeO3)(4) from 35000 to 40 00 cm(-1) were obtained by microscope-spectrometric techniques. The spectra were evaluated in terms of crystal field superposition model calculations for both triclinic Co2+ sites giving the first complete set of superpositio n model parameters for Co2+, i.e. the intrinsic parameters (B) over bar(2) = 7000 cm(-1) and (B) over bar(4) = 4740 cm(-1) and the power-law exponents t(2) = 5.5 and t(4) = 3.1. The interelectronic repulsion parameters are Ra cah B = 805 cm(-1) and Racah C = 3630 cm(-1). As already observed for sever al other selenite compounds, the derived crystal field strength parameters Dq(cub) again indicate a rather low position of the [SeO3](2-) anion within the range of oxygen-based ligands in the spectrochemical series.