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

Abstract

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 formulaM+M3+(HAsO4)2adopt the RbFe(HPO4)2structure type (R\overline{3}c) and a closely related new structure type, which is characterized by a different stacking order of the building units, leading to noncentrosymmetric space-group symmetryR32. The second new structure type, with the general formulaM+M3+2As(HAsO4)6(R\overline{3}c), is also a modification of the RbFe(HPO4)2structure type, in which one third of theM3+O6octahedra are replaced by AsO6octahedra, and two thirds of the voids in the structure, which are usually filled byM+cations, remain empty to achieve charge balance.