The struvite‐type compounds M [Mg(H2O)6](X O4), where M = Rb, Tl and X = P, As

Phases with composition M [Mg(H2O)6](X O4), where M = Rb, Tl and X = P, As, were obtained by means of the gelatine‐gel diffusion technique from Mg‐EDTA and corresponding metal phosphate/arsenate solutions at pH > 9. The crystal structures were determined from single crystal X‐ray diffractometer data sets. All compounds crystallize isotypically with two formula units in the orthorhombic struvite [NH4[Mg(H2O)6](PO4)] structure type in space group Pmn 21 and with lattice parameters of a ≈ 6.88, b ≈ 6.16, c ≈ 11.34 Å. The structures consist of slightly distorted [Mg(OH2)6] octahedra (m symmetry), X O4 tetrahedra (m symmetry) and [M O10] polyhedra (m symmetry) as single building units, held together by an intricate hydrogen bonding network. The structure can also be described as made up of closed packed pseudo ‐hexagonal layers of [Mg(OH2)6] units stacked along [010] with the X O4 tetrahedra and the M atoms in the voids of this arrangement. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)