The crystal structures of Fe-bearing MgCO(3) sp (2)- and sp (3)-carbonates at 98 GPa from single-crystal X-ray diffraction using synchrotron radiation

The crystal structure of MgCO(3)-II has long been discussed in the literature where DFT-based model calculations predict a pressure-induced transition of the carbon atom from the sp (2) to the sp (3) type of bonding. We have now determined the crystal structure of iron-bearing MgCO(3)-II based on single-crystal X-ray diffraction measurements using synchrotron radiation. We laser-heated a synthetic (Mg(0.85)Fe(0.15))CO(3) single crystal at 2500 K and 98 GPa and observed the formation of a monoclinic phase with composition (Mg(2.53)Fe(0.47))C(3)O(9) in the space group C2/m that contains tetra­hedrally coordinated carbon, where CO(4) (4−) tetra­hedra are linked by corner-sharing oxygen atoms to form three-membered C(3)O(9) (6−) ring anions. The crystal structure of (Mg(0.85)Fe(0.15))CO(3) (magnesium iron carbonate) at 98 GPa and 300 K is reported here as well. In comparison with previous structure-prediction calculations and powder X-ray diffraction data, our structural data provide reliable information from experiments regarding atomic positions, bond lengths, and bond angles.