A new compound, heptamagnesium bis(arsenate) tetrakis(hydrogenarsenate), Mg7(AsO4)2(HAsO4)4, was synthesized by a hydrothermal method. The structure is based on a three-dimensional framework of edge- and corner-sharing MgO6, MgO4(OH)2, MgO5, AsO3(OH) and AsO4 polyhedra. Average Mg-O and As-O bond lengths are in the ranges 2.056-2.154 and 1.680-1.688 A, respectively. Each of the two non-equivalent OH groups is bonded to both an Mg and an As atom. One OH group is involved in a very short hydrogen bond [O.O = 2.468 (3) A]. The formula unit is centrosymmetric, with all atoms in general positions except for one Mg atom, which has site symmetry -1. The compound is isotypic with Mn7(AsO4)2(HAsO4)4 and M7(PO4)2(HPO4)4, where M is Fe, Co or Mn.
Bond. -The new title compound is prepared by hydrothermal reaction of Mg(OH)2, Sc2O3, and H3AsO4 (pH 4.5, 493 K, autoclave, 7 d). Mg7(AsO4)2(HAsO4)4 crystallizes in the triclinic space group P1 with Z = 1 and is isotypic with Mn7(AsO4)2(HAsO4)4 or M7(PO4)2(HPO4)4 (M: Fe, Co, Ni). The structure consists of a three-dimensional framework of edge-and corner-sharing MgO 6 , MgO 4 (OH) 2 , MgO 5 , AsO 3 OH, and AsO 4 polyhedra. One of the two hydrogen sites is involved in a very short hydrogen bond (O-H···O = 2.468 Å). -(KOLITSCH*, U.; BARTU, P.; Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 60 (2004) 9, i94-i96; Inst. Mineral. Kristallogr., Univ. Wien, A-1090 Wien, Austria; Eng.) -Schramke 50-007
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