M+M3+2As(HAsO4)6 and α- and β-M+M3+(HAsO4)2 (M+M3+ = RbAl or CsFe): six new compounds crystallizing in three closely related structure types.

The crystal structures of hydrothermally synthesized (T = 493 K, 7-9 d) rubidium aluminium bis[hydrogen arsenate(V)], RbAl(HAsO4)2, caesium iron bis[hydrogen arsenate(V)], CsFe(HAsO4)2, rubidium dialuminium arsenic(V) hexakis[hydrogen arsenate(V)], RbAl2As(HAsO4)6, and caesium diiron arsenic(V) hexakis[hydrogen arsenate(V)], CsFe2As(HAsO4)6, were solved by single-crystal X-ray diffraction. The four compounds with the general formula M+M3+(HAsO4)2 adopt the RbFe(HPO4)2 structure type (R-3c) and a closely related new structure type, which is characterized by a different stacking order of the building units, leading to noncentrosymmetric space-group symmetry R32. The second new structure type, with the general formula M+M3+2As(HAsO4)6 (R-3c), is also a modification of the RbFe(HPO4)2 structure type, in which one third of the M3+O6 octahedra are replaced by AsO6 octahedra, and two thirds of the voids in the structure, which are usually filled by M+ cations, remain empty to achieve charge balance.