The 2.5- and 6-hydrates of dipotassium hydrogen arsenate, K2HAsO4: complex hydrogen bonding networks, one with an “ambiguous” order-disorder structure

Abstract

Abstract Crystals of the 2.5- and 6-hydrates of K2HAsO4 were grown from H3AsO4 solutions which were neutralized with two equivalents of aqueous KOH. The crystal structures were determined by X-ray diffraction at 100 K. K2HAsO4 · 2.5H2O [P21, Z = 8, a = 7.1607(3) Å, b = 18.1352(8) Å, c = 13.8156(5) Å, β = 125.153(3)°, R[F 2 > 3σ(F 2)] = 0.040, 10500 F data, 226 parameters] is a polytypic structure. The major polytype is of a maximum degree of order (MDO) and makes up ≈87% of the investigated crystals. Reflections pertaining to the second MDO polytype and disordered domains resulting in diffuse scattering were also observed. Two equivalent descriptions according to order/disorder (OD) theory are given in terms of layers of two kinds and of layers of one kind. Whereas the former emphasizes a local symmetry increase compared to the symmetry of the polytypes, the latter is useful for the straightforward determination of the MDO polytypes. The layer choice is ambiguous because parts of the structure belong to more than two maximum equivalence regions (MERs). The 2.5-hydrate as well as the higher hydrated K2HAsO4 · 6 H2O [Cc, Z = 4, a = 9.1788(2) Å, b = 9.4179(3) Å, c = 12.5935(5) Å, β = 93.7840(10)°, R[F 2 > 3σ(F 2)] = 0.013, 4812 F data, 169 parameters] are composed of K+ cations with highly irregular coordination environments defined by O atoms from [AsO3(OH)]2– anions and water molecules. A complex hydrogen-bonded network between the constituents is observed for both structures.