High-Pressure Synthesis of an Iron Carbonate, Fe2CO33

We synthesized an iron carbonate, Fe 2 3+ CO 3 3, by reacting Fe 2 O 3 with CO 2 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 P 2 1 / n . The characteristic feature of the Fe 2 CO 3 3 -structure is the presence of 7- and 8-fold coordinated trivalent cations and noncoplanar CO 3 2– groups. The normals of the CO 3 2– groups point in four different directions. The bulk modulus of Fe 2 CO 3 3 is K 0 = 138(34)GPa (when K p is fixed to 4). While previous studies have shown that siderite, Fe 2+ CO 3 decomposes at lower mantle conditions (pressures between 20 and 50 GPa and high temperatures), Fe 2 3+ CO 3 3 may be stable around 33(3) GPa and up to 2600(300) K. Iron carbonates with Fe 3+ are therefore more likely present at lower mantle conditions than carbonates containing Fe 2+ .