Three new acid M + arsenates and phosphates with multiply protonated As/PO 4 groups

The crystal structures of caesium dihydrogen arsenate(V) bis[trihydrogen arsenate(V)], Cs(H 2 AsO 4 )(H 3 AsO 4 ) 2 , ammonium dihydrogen arsenate(V) trihydrogen arsenate(V), NH 4 (H 2 AsO 4 )(H 3 AsO 4 ), and dilithium bis(dihydrogen phosphate), Li 2 (H 2 PO 4 ) 2 , were solved from single‐crystal X‐ray diffraction data. NH 4 (H 2 AsO 4 )(H 3 AsO 4 ), which was hydrothermally synthesized ( T = 493 K), is homeotypic with Rb(H 2 AsO 4 )(H 3 AsO 4 ), while Cs(H 2 AsO 4 )(H 3 AsO 4 ) 2 crystallizes in a novel structure type and Li 2 (H 2 PO 4 ) 2 represents a new polymorph of this composition. The Cs and Li compounds grew at room temperature from highly acidic aqueous solutions. Li 2 (H 2 PO 4 ) 2 forms a three‐dimensional (3D) framework of PO 4 tetrahedra sharing corners with Li 2 O 6 dimers built of edge‐sharing LiO 4 groups, which is reinforced by hydrogen bonds. The two arsenate compounds are characterized by a 3D network of AsO 4 groups that are connected solely via multiple strong hydrogen bonds. A statistical evaluation of the As—O bond lengths in singly, doubly and triply protonated AsO 4 groups gave average values of 1.70 (2) Å for 199 As—OH bonds, 1.728 (19) Å for As—OH bonds in HAsO 4 groups, 1.714 (12) Å for As—OH bonds in H 2 AsO 4 groups and 1.694 (16) Å for As—OH bonds in H 3 AsO 4 groups, and a grand mean value of 1.667 (18) Å for As—O bonds to nonprotonated O atoms.