Mg7(AsO4)2(HAsO4)4: a new magnesium arsenate with a very strong hydrogen bond.

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.