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

¹ Dipartimento Geomineralogico, Università di Bari, I-70125 Bari, Italy
² Dipartimento di Scienze della Terra, Università di Modena e Reggio Emilia, I-41100 Modena, Italy
³ Dipartimento di Fisica "E.Amaldi", Università di RomaTre, I-00146 Roma, Italy

Correspondence: ^* E-mail: f.scordari{at}geomin.uniba.it

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 a_J = 7.33, b_J = 6.42, and c_J = 7.14 Å (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 = 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 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 Å. 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 P2₁/c, a_M = 7.33, b_M = 7.14, c_M = 7.39 Å, and ß = 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.