Thermodynamic properties, low-temperature heat-capacity anomalies, and single-crystal X-ray refinement of hydronium jarosite, (H₃O)Fe₃(SO₄)₂(OH)₆

Abstract

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 (|F_o| > 4σ_F) and wR₂=0.0451 for all data gave a=7.3559(8) Å, c=17.019(3) Å, V^o=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)_0.91Fe_2.91(SO₄)₂[(OH)_5.64(H₂O)_0.18]. The enthalpy of formation (ΔH^o _f ) 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 (S^o) is 438.9±0.7 J mol⁻¹ K⁻¹, calculated from adiabatic and semi-adiabatic calorimetry data. The heat capacity (C _p ) data between 273 and 400 K were fitted to a Maier-Kelley polynomial C _p (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 C _p data, and as a broad maximum in the zero-field-cooled magnetic susceptibility data at 16.5 K. Another anomaly in C _p , 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: ΔG^o _f = −3226.4 ± 4.6 kJ mol⁻¹, ΔH^o _f =−3770.2 ± 4.6 kJ mol⁻¹, S^o=448.2 ± 0.7 J mol⁻¹ K⁻¹, C _p (T in K)=287.2 + 0.6281T–3286000T⁻² (between 273 and 400 K).