M³⁺(H₂AsO₄)(H₂As₂O₇) (M³⁺ = Al, Ga) and In₂(H₂AsO₄)₂(H₂As₂O₇)₂: a new layer structure type and a new framework structure type containing the rare H₂As₂O₇^2- group

The crystal structures of hydrothermally synthesized aluminium dihydrogen arsenate(V) dihydrogen diarsenate(V), Al(H₂AsO₄)(H₂As₂O₇), gallium dihydrogen arsenate(V) dihydrogen diarsenate(V), Ga(H₂AsO₄)(H₂As₂O₇), and diindium bis[dihydrogen arsenate(V)] bis[dihydrogen diarsenate(V)], In₂(H₂AsO₄)₂(H₂As₂O₇)₂, were determined from single-crystal X-ray diffraction data collected at room temperature. The first two compounds are representatives of a novel sheet structure type, whereas the third compound crystallizes in a novel framework structure. In all three structures, the basic building units are M³⁺O₆ octahedra (M = Al, Ga, In) that are connected via one H₂AsO₄^- and two H₂As₂O₇^2- groups into chains, and further via H₂As₂O₇^2- groups into layers. In Al/Ga(H₂AsO₄)(H₂As₂O₇), these layers are interconnected by weak-to-medium-strong hydrogen bonds. In In₂(H₂AsO₄)₂(H₂As₂O₇)₂, the H₂As₂O₇^2- groups link the chains in three dimensions, thus creating a framework topology, which is reinforced by weak-to-medium-strong hydrogen bonds. The three title arsenates represent the first compounds containing both H₂AsO₄^- and H₂As₂O₇^2- groups.