The order-disorder character of FeOHSO4 obtained from the thermal decomposition of metahohmannite, Fe3+2(H2O)4[O(SO4)2]

The iron sulfate FeOHSO 4 studied was obtained as a dehydration product of metahohmannite Fe 2 (H 2 O) 4 [O(SO 4 ) 2 ] during a synchrotron real-time powder diffraction experiment. As quoted in the literature, FeOHSO 4 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 a J = 7.33, b J = 6.42, and c J = 7.14 A ( a J , b J , and c J 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 R p value of about 23%. A careful analysis of the calculated powder diffraction pattern showed unexpected peaks, not observed in the experimental trace, for h = 2 n + 1, while sharp reflections for h = 2 n seemed to point to different lattice constants and space group. The recognition of the order-disorder character of the FeOHSO 4 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, FeOHSO 4 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 2 1/2 and 2 −1/2 , and yields an orthorhombic structure with space group Pnma and lattice parameters a J = 7.33, b J = 6.42, and c J = 7.14 A. MDO2 results from the 2 1/2 |2 1/2 |2 1/2 ... sequence of symmetry operators and yields a monoclinic structure with space group P 2 1 / c , a M = 7.33, b M = 7.14, c M = 7.39 A, 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 ( R p = 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.