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

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