Polymorphism in mercury(I) selenite(IV): preparation, crystal structures of α-, β-and γ-Hg 2SeO 3, and thermal behavior of the α- and β-modification

Abstract

Mercury(I) selenite(IV) is polymorphic and crystallizes at least in three modifications, named α-, β-and γ-Hg 2SeO 3. Polycrystalline β-Hg 2SeO 3 was prepared by precipitation of a concentrated mercurous nitrate solution with selenous acid. Hydrothermal treatment of the colorless β-Hg 2SeO 3 powder in demineralized water at 250°C (10 days) yields light-yellow single crystals of α-Hg 2SeO 3 which show the highest density of the three modifications. Colorless needle-shaped single crystals of β-Hg 2SeO 3 and very few single crystals of γ-Hg 2SeO 3 co-crystallize from strongly diluted Hg 2(NO 3) 2 and H 2SeO 3 solutions and were grown by a diffusion technique. All crystal structures were solved and refined from single crystal diffractometer data sets and are based on Hg 2 2+ dumbbells and trigonal pyramidal SeO 3 2− anions as the main building units. A common structural feature of all modifications is the formation of open channels extending parallel to the shortest crystallographic axis. The non-bonding orbitals of the Se IV atoms are stereochemically active and protrude into the channels. Upon heating in an open system under N 2 atmosphere, both α- and β-Hg 2SeO 3 decompose in a well-separated three-step mechanism. The first step ( T > 250°C) involves disproportionation into elementary mercury and α-HgSeO 3 which at ca. 400°C subsequently transforms into β-HgSeO 3. The second step between T = 400 and 500°C is accompanied by a loss of Hg and SeO 2 and the formation of the basic salt Hg 3SeO 6. In the third step, at temperatures between T = 500° and 600°C, this material decomposes completely. Upon heating in a closed system (sealed silica capillaries), β-Hg 2SeO 3 transforms between 320-340°C into the more dense α-Hg 2SeO 3 which on further heating likewise converts into elementary mercury and β-HgSeO 3.