The isotypic hydrogen phosphate and arsenate dihydratesM2HXO4·2H2O (M= Rb, Cs;X= P, As)
The four isotypic alkaline metal monohydrogen arsenate(V) and phosphate(V) dihydratesM2HXO4·2H2O (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 H3XO4solution with one equivalent of aqueousM2CO3. Their crystal structures are made up of undulating chains extending along [001] of tetrahedral [XO3(OH)]−anions connectedviastrong O—H...O hydrogen bonds. These chains are in turn connected into a three-dimensional networkviamedium-strength hydrogen bonding involving the water molecules. Two crystallographically differentM+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 fromC2/ctoP21/c. Nevertheless, the deviation fromC2/csymmetry is minute.