MIn(HAsO(4))(2) (M = K, Rb, Cs): three new hydrogen­arsenates adopting two different structure types

Potassium indium bis­[hydrogen arsenate(V)], KIn(HAsO(4))(2), rubidium indium bis­[hydrogen arsenate(V)], RbIn(HAsO(4))(2), and caesium indium bis­[hydrogen arsenate(V)], CsIn(HAsO(4))(2), were grown under mild hydro­thermal conditions (T = 493 K, 7–8 d). KIn(HAsO(4))(2) adopts the KSc(HAsO(4))(2) structure type (space group C2/c), while RbIn(HAsO(4))(2) and CsIn(HAsO(4))(2) crystallize in the space group R-3c and are the first arsenate representatives of the RbFe(HPO(4))(2) structure type. All three compounds have tetra­hedral–octa­hedral framework topologies. The M (+) cations, located in voids of the respective framework, are slightly disordered in RbIn(HAsO(4))(2). In KIn(HAsO(4))(2), there is a second K-atom position with a very low occupancy, which may suggest that the K atom can easily move in the channels extending along [101].