CRYSTAL-STRUCTURE AND POLARIZED ELECTRONIC ABSORPTION-SPECTRA OF NACO2(SEO3)(2)(OH)

The crystal structure of hydrothermally synthesized NaCo2 (SeO3)(2)(OH ) has been determined by direct and Fourier methods using single cryst al X-ray diffraction data up to sin theta/lambda = 0.8 Angstrom(-1) [o rthorhombic, space group Pbnm, a = 8.338(1) Angstrom, b = 13.268(3) An gstrom, c = 6.042(1) Angstrom, Z = 4; R(w) = 0.023 for 1407 unique ref lections with F-o > 2 sigma(F-o)]. The structure of NaCo2(SeO3)(2)(OH) consists of [4 + 2]-coordinated Na atoms, distorted CoO6 octahedra, p yramidal SeO3 groups, and hydroxyl groups. The CoO6 polyhedra form (1) (infinity)[Co4O14](20-) zigzag chains, analogous to the respective Me( 2+) chains in the olivine structure type. These chains are connected v ia the selenite groups and sodium atoms. The O atom of the hydroxyl gr oup belongs to three CoO6 polyhedra and forms a bifurcated hydrogen bo nd (3.312 Angstrom, 2x). Furthermore, polarized electronic absorption spectra of NaCo2(SeO3)(2)(OH) have been measured in the range from 400 0 to 40,000 cm(-1) using microscope spectrometric techniques. The spec tra are characterized by electric dipole transitions at the acentric C o2+ site [Co(2)]. An interpretation of the band splitting and polariza tion behavior is attempted in terms of a pseudotetragonal field compon ent and the imposed actual monoclinic site perturbation, yielding (4)A (2)(F) and (4)A''(F) as respective ground states. Tetragonal crystal h eld calculations give Dq = 630, Dt = -135, Ds = -111, B = 843, and C = 3520 cm(-1), which corresponds to a ''cubic'' Dq of 709 cm(-1) The si gn and magnitude of the tetragonal parameter Dt is in good agreement w ith the pseudotetragonal compression of the Co(2)O-6 polyhedron. (C) 1 995 Academic Press, Inc.