CsGa(H1.5AsO4)2(H2AsO4) and isotypic CsCr(H1.5AsO4)2(H2AsO4): decorated kröhnkite-like chains in two unusual hydrogen arsenates.

Hydrothermally synthesized caesium gallium(III) hydrogen arsenate(V), CsGa(H1.5AsO4)2(H2AsO4), (I), and isotypic caesium chromium(III) hydrogen arsenate(V), CsCr(H1.5AsO4)2(H2AsO4), (II), represent a new structure type and stoichiometry among MI-MIII hydrogen arsenates. The crystal structure, determined from single-crystal X-ray diffraction data, is based on an infinite octahedral-tetrahedral chain and can be described as a decorated kröhnkite-like chain. The chains extend parallel to [100] and are separated by ten-coordinated Cs atoms. The hydrogen-bonding scheme comprises one very short symmetry-restricted hydrogen bond, with O...O distances of 2.519 (4) and 2.508 (4) A in (I) and (II), respectively, and two further medium-strong hydrogen bonds, all of which reinforce the connections between adjacent chains. The average Ga-O and Cr-O bond lengths are 1.973 (15) and 1.980 (13) A, respectively, and the average As-O bond lengths in the two protonated arsenate groups lie within a very narrow range [1.690 (18)-1.69 (3) A]. The Cs atom is located on a centre of inversion, while the MIII and As2 atoms lie on twofold axes. Relationships to CaBa2(HPO4)2(H2PO4)2 and other compounds containing decorated kröhnkite-type or kröhnkite-like chains are discussed.