Investigations in the Systems Ag-Hg-X-O (X = AsV, SeIV, SeVI): Hydrothermal Single Crystal Growth of Ag3AsO4, AgHgI2AsO4, AgHgIIAsO4, Ag2SeO4 and the Crystal Structure of Ag2HgII(SeO3)2
2014
Weil, Matthias
Single crystals of the already known phases Ag₃AsO₄, AgHgᴵ₂AsO₄, AgHgᴵᴵAsO₄, Ag₂SeO₄ and of the hitherto unknown compound Ag₂Hgᴵᴵ(SeO₃)₂ were obtained under hydrothermal conditions (250 °C, 5d) from starting mixtures of the metal nitrates and the respective acids. Both Ag₃AsO₄ and AgHgᴵ₂AsO₄ are isotypic with the corresponding phosphates, Ag₃PO₄ and AgHgᴵ₂PO₄, whereas AgHgᴵᴵAsO₄ and Ag₂SeO₄ crystallize in the thenardite (Na₂SO₄ (V)) structure. All crystal structures were refined by means of single crystal X-ray data. The crystal structure of Ag2Hgᴵᴵ(SeO₃)₂ [Pbca, Z = 8, a = 6.8206(11), b = 11.237(3), c = 16.876(2) Å , 1677 structure factors, 101 parameters, R[F² > 2σ (F²)] = 0.0193, wR(F² all) =0.0394] consists of considerably distorted [AgO₆] and [HgO₆] octahedra, and trigonal SeᴵⱽO₃ pyramids as the main building units. The [MO₆] octahedra build a complex framework by sharing common edges and corners, and the SeᴵⱽO₃ pyramids are located in the vacancies of this arrangement. The average Hg-O distance of 2.399 Å is significantly shorter than the average Ag-O distance of 2.551Å . The geometries of the two crysta̱llographically independent SeᴵⱽO₃ pyramids are very similar and the average distance and angle (d(Se-O) = 1.709 Å , ∠(O-S-O) = 100.1°) lie in the characteristic range for a selenite(IV) group. A short comparative structural discussion between the various compounds obtained during the hydrothermal experiments is given.
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