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

The four isotypic arsenate(V) and phosphate(V) dihydrates M₂HXO₄·2H₂O (M = Rb, Cs; X = P, As) crystallize in the space group P2₁/c. A hypothetical proton-disordered C2/c structure is discussed.