Dimorphism in mercury(II) iodate(V):: Preparation and thermal behaviour of α- and β-Hg(IO3)2, and single crystal structure analysis of β-Hg(IO3)2

Mercury(II) iodate(V) is dimorphous and crystallizes in two polymorphs named beta- and beta-Hg(IO3)(2). The alpha-modification was prepared by precipitation of a slightly acidified Hg(NO3)(2) solution with an excess of an aqueous HIO3 Solution; for the polycrystalline material unindexed powder data is given. Colourless to light-yellow single crystals of the beta-modification, with an edge-length of up to 0.3 inm and a plate-like habit, were grown during hydrothermal treatment (220 degreesC, 10 d) of the polycrystalline precipitate. The crystal structure of beta-Hg(IO3)(2) was determined from single crystal X-ray data (space group P2(1) (no. 4), Z = 2, a = 5.7818(9), b = 5.6077(10), c = 8.9849(12) Angstrom, beta = 102.890(2)degrees, 1668 structure factors, 83 parameter, R[F-2 > 2sigma(F-2)] = 0.0175, wR(F-2 all) = 0.0382) and is made up from trigonal pyramidal IO3 groups with an average I-O distance of 1.825 Angstrom and distorted [HgO8] polyhedra with a mean Hg-O bond length of 2.537 Angstrom as the main building units. Infinite zig-zag chains of cis-corner-sharing [HgO4/2O4/1] polyhedra extend parallel to [010]. The chains are connected by IO3 groups along the [100] direction to form layers parallel to the (001) plane. The three-dimensional framework is held together by weak intermolecular I-O interactions > 2.58 Angstrom along the [001] direction. No a phase transformation was detected upon heating. Both polymorphs decompose completely in an one-step mechanism around 500 degreesC.