OD character and structure disorder of FeOHSO4 obtained from the thermal decomposition of methahomannite

The iron sulfate FeOHSO4 studied here has been obtained as a dehydration product of the thermal decomposition of metahohmannite Fe2(H2O)4[O(SO4)2]1 during a synchrotron real time powder diffraction experiment. This compound is commonly described in the orthorhombic system [ Pnma, a = 7.33, b = 6.42, c = 7.14 Å, and Z = 4] with Fe3+ cations coordinating two hydroxyl groups and four sulfate O atoms2. Each hydroxyl group is bonded to two iron atoms. However a careful analysis of the experimental powder diffraction pattern exhibited unexpected missing peaks for h = 2n +1, whereas sharp reflections for h = 2n seemed to point to different lattice constants and space group. The recognition of the OD character of the FeOHSO4 compound was the key element leading to the successful interpretation of the features of the observed powder pattern and to the understanding of the misfit with respect to the calculated pattern. In fact, FeOHSO4 seems to belong to a family of OD structures formed by equivalent layers with symmetry Pbmm. Only two maximum-degree order (MDO) polytypes are possible. The form MDO1 results from a regular alternation of stacking operators 21/2 and 2-1/2, and yields the known orthorhombic structure; the form MDO2 results from the sequence 21/