High-Pressure Synthesis of an Iron Carbonate, Fe[CO].

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