Thermodynamic properties, low-temperature heat-capacity anomalies, and single-crystal X-ray refinement of hydronium jarosite, (H3O)Fe3(SO4)2(OH)6
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Crystals of hydronium jarosite were synthesized by hydrothermal treatment of Fe(III)–SO4 solutions. Single-crystal XRD refinement with R1=0.0232 for the unique observed reflections (|Fo| > 4σF) and wR2=0.0451 for all data gave a=7.3559(8) Å, c=17.019(3) Å, Vo=160.11(4) cm3, and fractional positions for all atoms except the H in the H3O groups. The chemical composition of this sample is described by the formula (H3O)0.91Fe2.91(SO4)2[(OH)5.64(H2O)0.18]. The enthalpy of formation (ΔHof) is −3694.5 ± 4.6 kJ mol−1, calculated from acid (5.0 N HCl) solution calorimetry data for hydronium jarosite, γ-FeOOH, MgO, H2O, and α-MgSO4. The entropy at standard temperature and pressure (So) is 438.9±0.7 J mol−1 K−1, calculated from adiabatic and semi-adiabatic calorimetry data. The heat capacity (Cp) data between 273 and 400 K were fitted to a Maier-Kelley polynomial Cp(T in K)=280.6 + 0.6149T–3199700T−2. The Gibbs free energy of formation is −3162.2 ± 4.6 kJ mol−1. Speciation and activity calculations for Fe(III)–SO4 solutions show that these new thermodynamic data reproduce the results of solubility experiments with hydronium jarosite. A spin-glass freezing transition was manifested as a broad anomaly in the Cp data, and as a broad maximum in the zero-field-cooled magnetic susceptibility data at 16.5 K. Another anomaly in Cp, below 0.7 K, has been tentatively attributed to spin cluster tunneling. A set of thermodynamic values for an ideal composition end member (H3O)Fe3(SO4)2(OH)6 was estimated: ΔGof= −3226.4 ± 4.6 kJ mol−1, ΔHof=−3770.2 ± 4.6 kJ mol−1, So=448.2 ± 0.7 J mol−1 K−1, Cp (T in K)=287.2 + 0.6281T–3286000T−2 (between 273 and 400 K).
KeywordsHydronium jarosite Formation enthalpy Entropy Spin glass Crystal structure
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We thank D.J. Wesolowski for valuable discussion about aqueous speciation of metal–sulfate solutions. Two anonymous reviewers provided helpful comments that improved the manuscript. The synthesis and calorimetry at UC Davis was supported by the US Department of Energy (grant DE FG03 97 ER14749). R.S. thanks the BYU Office of Research and Creative Works for partial support. Portions of this work were performed at the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) Synchrotron Research Center of the Advanced Photon Source at Argonne National Laboratory. DND-CAT is supported by the E.I. Dupont de Nemours & Co., The Dow Chemical Company, the US National Science Foundation through grant no. DMR-9304725, and the State of Illinois through the Department of Commerce and the Board of Higher Education grant no. IBHE HECA NWU 96. Use of the Advanced Photon source was supported by the US Department of Energy, Basic Energy Sciences, Office of Energy Research under contract no. W-31–102-Eng-38.