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 under low-hydrothermal conditions. The structure was determined by;direct methods 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 structure composed of one type of [4 + 2] coordinated Li atoms and each two crystallographically different CoO6 octahedra and pyramidal SeO3 groups. The polyhedra of lithium and of one type of the cobalt atoms exhibit uncommonly strong irregular distortions. Polarized electronic absorption spectra of Li2Co3(SeO3)(4) from 35000 to 4000 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 superposition 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 Racah B = 805 cm(-1) and Racah C = 3630 cm(-1). As already observed for several 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.