M 3+ (H 2 AsO 4 )(H 2 As 2 O 7 ) ( M 3+ = Al, Ga) and In 2 (H 2 AsO 4 ) 2 (H 2 As 2 O 7 ) 2 : a new layer structure type and a new framework structure type containing the rare H 2 As 2 O 7 2− group

The crystal structures of hydrothermally synthesized aluminium dihydrogen arsenate(V) dihydrogen diarsenate(V), Al(H 2 AsO 4 )(H 2 As 2 O 7 ), gallium dihydrogen arsenate(V) dihydrogen diarsenate(V), Ga(H 2 AsO 4 )(H 2 As 2 O 7 ), and diindium bis[dihydrogen arsenate(V)] bis[dihydrogen diarsenate(V)], In 2 (H 2 AsO 4 ) 2 (H 2 As 2 O 7 ) 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 M 3+ O 6 octahedra ( M = Al, Ga, In) that are connected via one H 2 AsO 4 − and two H 2 As 2 O 7 2− groups into chains, and further via H 2 As 2 O 7 2− groups into layers. In Al/Ga(H 2 AsO 4 )(H 2 As 2 O 7 ), these layers are interconnected by weak‐to‐medium–strong hydrogen bonds. In In 2 (H 2 AsO 4 ) 2 (H 2 As 2 O 7 ) 2 , the H 2 As 2 O 7 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 H 2 AsO 4 − and H 2 As 2 O 7 2− groups.