Localization of hydrogen and content of oxonium cations in alunite via neutron diffraction

Polycrystalline alunite-d(6) KAl3(OD)(6)(SO4)(2), prepared by hydrothermal reaction of Al-2(SO4)(3), K2SO4 and D2SO4, was studied by neutron powder di ffraction performed on the diffractometer E2 (HMI-BENSC, Berlin). Rietveld refinement of the data set for T = 2 K yielded the crystallographic data: s pace group R (3) over bar m. Z = 3, trigonal setting, a = 694.3(1) pm, c = 1722.7(2) pm, N(I/sigma(I) > 1) = 172, N(Var.) = 19, R-p = 0.036, wR(p) = 0 .046, R-B(I/sigma(I) > 1) = 0.020. The deuterium nuclei could be located pr ecisely. Three equivalent O-D bonds with nuclear distances r(O(4)-D) = 96.6 (3) pm directed to each of the terminal oxygen atoms of the SO4 groups are found. Partial substitution of K+ by D3O+ was also considered in the refine ment procedure. In good agreement with results of other methods a site occu pation fraction n(D3O+) = 0.0104 was obtained.