The isotypic hydrogen phosphate and arsenate dihydrates M2HXO4·2H2O (M = Rb, Cs; X = P, As).

The four isotypic alkaline metal monohydrogen arsenate(V) and phosphate(V) dihydrates M2HXO4·2H2O (M = Rb, Cs; X = P, As) [namely dicaesium monohydrogen arsenate(V) dihydrate, Cs2HAsO4·2H2O, dicaesium monohydrogen phosphate(V) dihydrate, Cs2HPO4·2H2O, dirubidium monohydrogen arsenate(V) dihydrate, Rb2HAsO4·2H2O, and dirubidium monohydrogen phosphate(V) dihydrate, Rb2HPO4·2H2O] were synthesized by reaction of an aqueous H3XO4 solution with one equivalent of aqueous M2CO3. Their crystal structures are made up of undulating chains extending along [001] of tetrahedral XO3(OH) anions connected via strong O-H...O hydrogen bonds. These chains are in turn connected into a three-dimensional network via medium-strength hydrogen bonding involving the water molecules. Two crystallographically different M(+) cations are located in channels running along [001] or in the free space of the XO3(OH) chains, respectively. They are coordinated by eight and twelve O atoms forming irregular polyhedra. The structures possess pseudosymmetry. Due to the ordering of the protons in the XO3(OH) chains in the actual structures, the symmetry is reduced from C2/c to P21/c. Nevertheless, the deviation from C2/c symmetry is minute.