Crystal Structure and Polarized Electronic Absorption Spectra of NaCo 2(SeO 3) 2(OH)

Abstract

The crystal structure of hydrothermally synthesized NaCo 2 (SeO 3) 2(OH) has been determined by direct and Fourier methods using single crystal X-ray diffraction data up to sin θ/λ = 0.8 Å -1 [orthorhombic, space group Pbnm, a = 8.338(1) Å, b = 13.268(3) Å c = 6.042(1) Å Z = 4; R w = 0.023 for 1407 unique reflections with F o > 2σ(F o)|. The structure of NaCo 2(SeO 3) 2(OH) consists of [4 + 2]-coordinated Na atoms, distorted CoO 6 octahedra, pyramidal SeO 3 groups, and hydroxyl groups. The CoO 6 polyhedra form 1 ∞ [Co 4O 14] 20- zigzag chains, analogous to the respective Me 2+ chains in the olivine structure type. These chains are connected via the selenite groups and sodium atoms. The O atom of the hydroxyl group belongs to three CoO 6 polyhedra and forms a bifurcated hydrogen bond (3.312 Å, 2×). Furthermore, polarized electronic absorption spectra of NaCo 2(SeO 3) 2(OH) have been measured in the range from 4000 to 40,000 cm -1 using microscope spectrometric techniques. The spectra are characterized by electric dipole transitions at the acentric Co 2+ site [Co(2)]. An interpretation of the band splitting and polarization behavior is attempted in terms of a pseudotetragonal field component and the imposed actual monoclinic site pertubation, yielding 4 A 2 (F) and 4 A″( F) as respective ground states. Tetragonal crystal field 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 sign and magnitude of the tetragonal parameter Dt is in good agreement with the pseudotetragonal compression of the Co(2)O 6 polyhedron.