CO2- ELECTRONIC ABSORPTION-SPECTRA OF BUETSCHLIITE-TYPE K2CO(SEO3)2, K2CO2(SEO3)3 AND ZEMANNITE-TYPE K2CO2(SEO3)3.2H2O( IN TRIGONAL FIELDS OF OXYGEN BASED STRUCTURES )

Polarized single crystal absorption spectra, in the spectral range 40 000-5 000 cm-1, were obtained on Co2+ in trigonally distorted octahedr al oxygen fields of buetschliite-type K2Co(SeO3)2 (I), K2Co2(SeO3)3 (I I) and zemannite-type K2Co2(SeO3)3.2H2O (III). Site symmetries of Co2 are 3mBAR (D3d) in I, 3m (C3v) in II, and 3 (C3) in III. The spectra can be interpreted on the basis of an electric dipole mechanism, where in transitions of Co2+ in the centrosymmetric site in I gain intensity from dynamic removal of the inversion centre by vibronic coupling. In accordance with the elongation of the CoO6 octahedra along the trigon al axis, the split component E(g) of the ground state 4T1g in octahedr al fields is the ground state in all three compounds. Trigonal field p arameters Dq(trig), Dtau, Dsigma and the Racah parameters B have been fitted to the energies of spin allowed transitions (293 K) as follows: I: 744, 94, -16, and 838 cm-1, resp.; II: 647, 227, 42, and 798 cm-1, resp.; III: 667, 181, 21, and 809 cm-1, respectively. Racah parameter s C were estimated from the energy of some observed spin-forbidden tra nsitions to be 3770 (I), 3280 (II), and 3465 cm-1 (III). Values of Dq and of the Racah parameters B and C indicate slight differences of Co2 + - 0 bonding in I as compared to II and III, with somewhat higher cov alency in compounds II and III which contain face-sharing CoO6 octahed ra with short Co - Co contacts. Also, in II and III the observed Dtau values do not agree with theoretical DT values, predicted from the mag nitude of the mean octahedral distortions.