The iron sulfate FeOHSO4 studied was obtained as a dehydration product of metahohmannite Fe2(H2O)4[O(SO4)2] during a synchrotron real-time powder diffraction experiment. As quoted in the literature, FeOHSO4 has iron atoms octahedrally coordinated with two hydroxyl groups and four sulfate O atoms, while each hydroxyl group is bonded to two iron atoms. This compound is commonly described in the orthorhombic system with space group Pnma, lattice parameters aJ = 7.33, bJ = 6.42, and cJ = 7.14 Å (aJ, bJ, and cJ are the Johansson lattice parameters), and Z = 4. However a preliminary Rietveld refinement of the pattern at about 220 °C using the structural model from the literature yielded a poor fit of the observed data and a final Rp value of about 23%. A careful analysis of the calculated powder diffraction pattern showed unexpected peaks, not observed in the experimental trace, for h = 2n + 1, while sharp reflections for h = 2n seemed to point to different lattice constants and space group. The recognition of the order-disorder character of the FeOHSO4 compound was the key to successfully interpreting the unexpected features of the experimental powder pattern and the misfit with respect to the calculated pattern. In fact, FeOHSO4 belongs to a family of OD structures formed by equivalent layers of symmetry Pbmm. Only two MDO (Maximum Degree of Order) polytypes are possible. MDO1 results from a regular alternation of stacking operators 21/2 and 2−1/2, and yields an orthorhombic structure with space group Pnma and lattice parameters aJ = 7.33, bJ = 6.42, and cJ = 7.14 Å. MDO2 results from the 21/2|21/2 |21/2... sequence of symmetry operators and yields a monoclinic structure with space group P21/c, aM = 7.33, bM = 7.14, cM = 7.39 Å, and β = 119.7°.

The analysis of one-dimensional stacking disorder was performed by fitting the observed XRPD pattern with a calculated intensity curve generated by DIFFaX. The disorder model was investigated by taking into account a probability matrix for the occurrence of OD layer sequences. The best fit (Rp = 0.009) to the observed powder pattern was obtained with a 61:39 ratio of monoclinic and orthorhombic polytypes for a fully disordered OD layers sequence.

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