High-Pressure Synthesis of an Iron Carbonate, Fe₂[CO₃]₃

We synthesized an iron carbonate, Fe₂³⁺[CO₃]₃, by reacting Fe₂O₃ with CO₂ at high temperatures and pressures of approximately 33(3) GPa. The structure was solved by single-crystal X-ray diffraction. Full geometry optimizations based on density functional theory reproduced the crystal structure. This compound crystallizes in the monoclinic space group P2₁/n. The characteristic feature of the Fe₂[CO₃]₃-structure is the presence of 7- and 8-fold coordinated trivalent cations and noncoplanar [CO₃]^2- groups. The normals of the [CO₃]^2- groups point in four different directions. The bulk modulus of Fe₂[CO₃]₃ is K₀ = 138(34)GPa (when K_p is fixed to 4). While previous studies have shown that siderite, Fe²⁺CO₃ decomposes at lower mantle conditions (pressures between 20 and 50 GPa and high temperatures), Fe₂³⁺[CO₃]₃ may be stable around 33(3) GPa and up to 2600(300) K. Iron carbonates with Fe³⁺ are therefore more likely present at lower mantle conditions than carbonates containing Fe²⁺.