Localization of Hydrogen and Content of Oxonium Cations in Alunite via Neutron Diffraction

Polycrystalline alunite‐d₆ KAl₃(OD)₆(SO₄)₂, prepared by hydrothermal reaction of Al₂(SO₄)₃, K₂SO₄ and D₂SO₄, was studied by neutron powder diffraction performed on the diffractometer E2 (HMI‐BENSC, Berlin). Rietveld refinement of the data set for T = 2 K yielded the crystallographic data: space group R3m, Z = 3, trigonal setting, a = 694.3(1) pm, c = 1722.7(2) pm, N(I/σ(I) > 1) = 172, N(Var.) = 19, R_p = 0.036, wR_p = 0.046, R_B(I/σ(I) > 1) = 0.020. The deuterium nuclei could be located precisely. 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 refinement procedure. In good agreement with results of other methods a site occupation fraction n(D₃O⁺) = 0.0104 was obtained.