M + M 3+ 2As(HAsO4)6 (M + M 3+ = TlGa, CsGa, CsAl): three new metal arsenates containing AsO6 octahedra

Abstract

The crystal structures of hydrothermally synthesized (T = 493 K, 7 d) thallium(I) digallium arsenic(V) hexakis[hydrogenarsenate(V)], TlGa2As(HAsO4)6, caesium digallium arsenic(V) hexakis[hydrogenarsenate(V)], CsGa2As(HAsO4)6, and caesium dialuminium arsenic(V) hexakis[hydrogenarsenate(V)], CsAl2As(HAsO4)6, were solved by single-crystal X-ray diffraction. The three compounds are isotypic and adopt the structure type of RbAl2As(HAsO4)6 (R\overline{3}c), which itself represents a modification of the RbFe(HPO4)2 structure type and consists of a tetrahedral–octahedral framework in which the slightly disordered M + cations are located in channels. The three new compounds contain AsO6 octahedra assuming the topological role of M 3+O6 octahedra. The As—O bond lengths are among the shortest As—O bond lengths known so far in AsO6 octahedra.