CRYSTAL-STRUCTURE AND THERMAL-DECOMPOSITION OF THE COQUIMBITE-TYPE COMPOUND FE-2(SEO4)(3) CENTER-DOT 9H(2)O

Crystals of Fe-2(SeO4)(3) . 9H(2)O were slowly grown at room condition s (T = 22 degrees C, p = 1 bar) from concentrated aqueous solutions of iron selenate and selenic acid. The coquimbite-type crystal structure was refined from single-crystal X-ray data: space group P31c, a = 11. 190(5)Angstrom, c 17.518(5)Angstrom, V = 1899.7 Angstrom(3), Z = 4, R = 0.042 for 803 unique reflections up to sin circle minus/lambda = 0.6 0 Angstrom(-1). The crystal structure features a [Fe-2(H2O)(6)(SeO4)(3 )] framework consisting of isolated [Fe-3(Ow1)(6)](3+) octahedra and [ Fe1Fe2(2)(Ow3)(6)(SeO4)(6)](3-) clusters which are connected to each o ther only by hydrogen bonds. Channels running parallel to [100] within the framework contain nonframework H2O molecules (H(2)Ow2). Based on the sites of the hydrogen atoms obtained from difference-Fourier summa tions a complete hydrogen-bonding scheme can be given. TGA experiments reveal a thermal decomposition of the title compound that is very sim ilar to that one of coquimbite. The mass loss (dm = -23.6wt.%) between 80 degrees C and 250 degrees C can be attributed to the complete dehy dration (9H(2)O p.f.u. = 23.07wt.%); a further step at 420-480 degrees C (dm = -50.1wt.%) reveals the final decomposition by the release of 3 SeO3 p.f.u. The corresponding DTG curve in the range 80-250 degrees C indicates that the dehydration actually proceeds in at least four di stinct steps at partially coinciding temperature intervals.