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[Cu 3 (HAsO 4 )(AsO 4 )(OH) 2 ] and is characterized by pseudo-hexagonal (001) 2 ∞ [Ni 3 As 2 O 18/3 (OH) 6/3 O 1/1 (OH) 1/1 ] − layers formed from vertex- and edge-sharing [NiO 4 (OH) 2 ] octahedra and [AsO 3.5 (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 [HAsO 4 ] 2– and [AsO 4 ] 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 Ni II 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 ∞ [Ni 3 As 2 O 18/3 (OH) 6/3 O 1/1 (OH) 1/1 ] − layers may be the reason for the systematic threefold twinning of (NH 4 )[Ni 3 (HAsO 4 )(AsO 4 )(OH) 2 ] crystals. Significant overlaps of the reflections of the respective twin domains complicated the structure solution and refinement.