CsAl(H₂AsO₄)₂(HAsO₄): a new monoclinic protonated arsenate with decorated kröhnkite-like chains

The new structure type of CsAl(H₂AsO₄)₂(HAsO₄) is characterized by decorated kröhnkite-like chains of corner-sharing octa­hedra and tetra­hedra. Cs atoms lie between these polyhedra and hydrogen bonds provide re-enforcement. The compound is closely related to CsGa(H_1.5AsO₄)₂(H₂AsO₄), but crystallizes with lower space-group symmetry.