Single Crystal Growth and Crystal Structure of Mercurous Diarsenate(V), (Hg₂)₂(As₂O₇)

Abstract

Polycrystalline mercurous diarsenate(V), (Hg 2 ) 2 (As 2 O 7 ), was prepared by a redox-reaction between stoichiometric amounts of HgO and As 2 O 3 . Canary yellow single crystals were obtained by subsequent chemical transport reactions using HgCl 2 as transport agent [550 Ǟ 500°C, 5 d, sealed and evacuated silica ampoules]. The crystal structure (orthorhombic, Pnma, Z ϭ 4, a ϭ 9.9803 (8), b ϭ 12.2039(10), c ϭ 7.2374(6) Å ) is composed of two crystallographically independent Hg 2 2ϩ dumbbells (d(HgϪHg) ϭ 2.5133 Å ) During recent years many phases within the ternary system Hg/As/O have been prepared and crystallographically full characterized by single crystal X-ray diffraction. So far, three orthoarsenates(V) with tetrahedral AsO 4 anions and Hg cations in different oxidation states are known: Mercurous (Hg 2 ) 3 (AsO 4 ) 2 [1], also known as the mineral chursinite [2], the graphtonite-type mercuric Hg 3 (AsO 4 ) 2 [3], and the fascinating (Hg 3 ) 3 (AsO 4 ) 4 with triangular Hg 3 4ϩ cluster cations [4]. For As in octahedral coordination both mercurous and mercuric metaarsenates(V), (Hg 2 )As 2 O 6 [5] and HgAs 2 O 6 [5,6], are specified. Up to now, no crystallographic information is given on mercury diarsenates(V) and on mercury arsenites(III), although several phases are described in the Gmelin textbook on mercury and its compounds [7]. In an attempt to prepare mercuric arsenites(III), starting from HgO and As 2 O 3 , single crystals of the mercurous diarsenate(V), (Hg 2 ) 2 As 2 O 7 , were obtained.