Structures of Fluoroarsenates KAsF_6n(OH)_n, n = 0, 1, 2: Application of the Heavy-Atom Method for Modulated Structures

Structures of KAsF_6-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 solved both in the supercell by standard methods and in the basic cell as a commensurate modulated structure, giving the same results in both cases. The heavy-atom method for modulated structures has been developed and applied to identify a good starting point for the refinement of strongly modulated KAsF₄(OH)₂. Moreover, the influence of hydrogen bonding on a regular disturbance of simple translation symmetry of the structure is discussed.

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