The title compound, ammonium trinickel(II) hydrogen arsenate arsenate di-hydroxide, was synthesized under hydro-thermal conditions. Its crystal structure is isotypic with that of K[Cu₃(HAsO₄)(AsO₄)(OH)₂] and is characterized by pseudo-hexa-gonal (001) ²∞[Ni₃As₂O₁₈/₃(OH)₆/₃O₁/₁(OH)₁/₁]⁻ layers formed from vertex- and edge-sharing [NiO₄(OH)₂] octa-hedra and [AsO₃.₅(OH)₀.₅] tetra-hedra 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-hexa-gonal 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.
