Synthesis and crystal structure of (NH4)[Ni3(HAsO4)(AsO4)(OH)2]

The title compound, ammonium trinickel(II) hydrogen arsenate arsenate dihydroxide, was synthesized under hydrothermal conditions. Its crystal structure is isotypic with that of K[Cu3(HAsO4)(AsO4)(OH)2] and is characterized by pseudo-hexagonal (001) 2∞[Ni3As2O18/3(OH)6/3O1/1(OH)1/1]− layers formed from vertex- and edge-sharing [NiO4(OH)2] octahedra and [AsO3.5(OH)0.5] tetrahedra as the building units. The hydrogen atom of the OH group shows occupational disorder and was refined with a site occupation factor of 1/2, indicating the equal presence of [HAsO4]2– and [AsO4]3– groups. Strong asymmetric hydrogen bonds between symmetry-related (O,OH) groups of the arsenate units [O...O = 2.588 (18) Å] as well as hydrogen bonds accepted by these (O,OH) groups from OH groups bonded to the NiII atoms [O...O = 2.848 (12) Å] link adjacent layers. Additional consolidation of the packing is achieved through N—H...O hydrogen bonds from the ammonium ion, which is sandwiched between adjacent layers [N...O = 2.930 (7) Å] although the H atoms could not be located in the present study. The presence of the pseudo-hexagonal 2∞[Ni3As2O18/3(OH)6/3O1/1(OH)1/1]− layers may be the reason for the systematic threefold twinning of (NH4)[Ni3(HAsO4)(AsO4)(OH)2] crystals. Significant overlaps of the reflections of the respective twin domains complicated the structure solution and refinement.