Structures of fluoroarsenates KAsF6-n(OH)(n), n = 0, 1, 2: Application of the heavy-atom method for modulated structures

Structures of KAsF6-n(OH)(n), n = 0, 1, 2, have been determined from single -crystal X-ray diffraction data. Potassium hexafluoroarsenate, KAsF6, has a standard structure, potassium hydroxopentafluoroarsenate. KAsF5(OH), has a superstructure, and potassium dihydroxotetrafluoroarsenate, KAsF4(OH)(2), has an incommensurate modulated structure. The superstructure has been solv ed both in the supercell by standard methods and in the basic cell as a com mensurate modulated structure, giving the same results in both cases. The h eavy-atom method for modulated structures has been developed and applied to identify a good starting point for the refinement of strongly modulated KA sF4(OH)(2). Moreover, the influence of hydrogen bonding on a regular distur bance of simple translation symmetry of the structure is discussed.