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

Abstract

Abstract 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.