ZEMANNITE-TYPE SELENITES - CRYSTAL-STRUCTURES OF K2[CO2(SEO3)3].2H2O AND K2[NI2(SEO3)3].2H2O

Crystals of K2[Co2(SeO3)3].2H2O and K2[Ni2(SeO3)3].2H2O were synthesiz ed under low-hydrothermal conditions. Their structures were determined using single crystal X-ray data up to sin THETA/lambda = 0.7angstrom- 1. [Space group P6(3)/m; a = 9.091(3), 9.016(2)angstrom; c = 7.562(2), 7.476(2)angstrom; Z = 2; R(w) = 1.6, 2.5%]. The investigations confir med that K2[Co2(SeO3)3].2H2O and K2[Ni2(SeO3)3].2H2O represent the fir st selenites belonging to the zemannite structure type, a framework st ructure with wide channels running parallel [0001]. In both compounds four maxima were clearly located in the channel by Fourier summations and attributed to two K atoms and two H2O molecules, each with an occu pancy factor of 1/6; a possible ordering scheme (full occupancy) with local symmetry 1BAR and [6]-coordinated K atoms could be derived for t he channel atoms.