Formation of sp3 hybridized bonds and stability of CaCO3 at very high pressure.

By performing ab initio electronic structure calculations, we observed a new high-pressure phase transition within the Pmcn structure of CaCO3. This transition is characterized by the change of the carbon's sp hybridization state and is driven by the intrinsic property of the carbon atom to form tetrahedral covalent bonds at high pressure. The formation of sp(3) hybridized bonds explains the stability of MgCO3 and CaCO3 at Earth's lower mantle pressure conditions and may serve as a criterion for searching new possible high-pressure phases of carbon bearing minerals.