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 Kp 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+.