Preparation and crystal structures of two new modifications of mercury(II) selenite(IV), HgSeO_3, and the mixed-valent mercury(II) selenite(IV) selenide(-II), (HgSeO_3)_3HgSe

摘要

Two new modifications of mercury(II0 selenite(IV), β- and γ-HgSeO_3, as well as the hitherto unknown mixed-valent compound (HgSeO_3)_3HgSe with Se in oxidation states IV and -II have beem prepared. Both β- and γ-HgSeO_3 were grown as light-yellow needles by a diffusion technique; light-yellow sword-like crystals of (HgSeO_3)_3HgSe were obtained after hydrothermal treatment of a mixture of different polycrystalline HgSeO_3 polymorphs. All crystal structures were determined from single crystal diffractometer data sets. The structures of both β-HgSeO_3 (P2_1/c, a = 4.3610(9) A, b = 10.220(2) A, c = 7.1190(14) A, β = 102.70(3)°, 912 structure factors, 47 parameters, R[F~2 > 2σ(F~2)] = 0.0241) and γ-HgSeO_3 (P2_1/c, a = 4.7380(10) A, b = 9.0530(18) A, c = 7.1460(14) A, β = 92.06(2)°, 892 structure factors, 47 parameters, R[F~2 > 2σ(F~2)] = 0.0405) comprise trigonal Se~(IV)O_3 pyramids and [Hg~(II)O_7] polyhedra. (HgSeO_3)_3HgSe (P2_1, a = 9.6309(3)A, b = 10.2257(3)A, c = 11.7515(4) A, β = 93.569(1)°, 3429 structure factors, 155 parameters, R[F~2 > 2σ(F~2)] = 0.0252) is composed of four crystallographically independent Hg~(II) atoms, three Se~(IV) O_3 groups and one Se~(-II) atom forming a complex framework structure with channels extending along [010]. The [Hg_3Se~(-II)] pyramid as the most remarkable structure has a mean distance of d-bar(Hg-Se) = 2.489 A and a mean angle <(Hg-Se-Hg) = 93.85°. Upon heating no phase transition between the different HgSeO_3 modifications has been observed by thermoanalytical methods. Above 400 ℃ all modifications transform into Hg_3SeO_6 which subsequently decomposes on further heating.