(H₃O)Fe(SO₄)₂ is a compound that precipitates from the most acidic solutions in the system Fe₂O₃-SO₃-H₂O. We have solved the structure of this phase from synchrotron powder X-ray diffraction (XRD) data in the space group P-1. The lattice parameters are a = 4.8087(1) Å, b = 8.3180(1) Å, c = 8.3034(1) Å, α = 70.181(1)º, β = 90.276(1)º, and γ = 89.993(1)º. The unit cell volume is 312.44(1) ų. (H₃O)Fe(SO₄)₂ has layered structure in which the layers are built by FeO6 octahedra and SO4 tetrahedra. Three oxygen ions in the SO₄ unit are bridging and provide bonding to the Fe³⁺ ions within the layers. The fourth oxygen ion points into the interlayer space where the H₃O⁺ ions are located. The structure of (H₃O)Fe(SO₄)₂ belongs to the yavapaiite (KFe(SO₄)₂) family of layered structures with octahedral-tetrahedral layers and interlayer cations and can be interpreted as a triclinic distortion of the structures of CsFe(SO₄)₂ or RbFe(SO₄)₂. The partial refinement (excluding H atoms) was augmented and supported by static ab-initio calculations. The orientation of the hydronium ions and the hydrogen bond scheme was obtained from ab-initio molecular dynamics simulations. These calculations revealed that all hydrogen atoms participate in hydrogen bonding and the hydrogens of the alternating H₃O⁺ groups are oriented “up” or “down” toward the Fe-SO₄ layers. The ab-initio molecular dynamic simulations were performed at 300 K for 4.5 ps. The H₃O⁺ groups re-arrange their orientation in time.

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1. Synthesis and Magnetic Properties of the La_0.50Ba_0.50MnO₃ Nanomanganites