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.