High-pressure transformations of CaCO - a full structural trend from double [CO] triangles through the isolated group of [CO] tetrahedra to framework and layered structures.

Over the past few years, the concept of carbonates, as the salts of MCO or composition with [CO] triangles in the crystal structures, was sufficiently extended. In addition to carbonates, crystal structures with stoichiometry MCO, MCO and MCO were predicted and successfully synthesized. In the present study, based on density functional theory and crystal structure prediction algorithms, we found a novel structure of CaCO, namely Ca-pyrocarbonate with monoclinic symmetry , which is one of the possible agents of the global carbon cycle. This structure is characterized by the isolated [CO] groups consisting of two [CO] triangles connected through a common oxygen atom. The thermodynamic stability field of Ca-pyrocarbonate with respect to the decomposition reaction into calcium carbonate and carbon dioxide begins at a pressure of 10 GPa. As the pressure increases to 21 GPa, the structure of Ca-pyrocarbonate transforms into the recently synthesized tetragonal modification 4̄2, in the structure of which carbon is in the sp-hybridized state and [CO] tetrahedra form isolated pyramidal [CO] anionic groups. At 59 GPa in the temperature range of 0-2500 K, CaCO-4̄2 undergoes a phase transition to CaCO-2, with the framework structure of [CO] tetrahedra. On further compression to about 80 GPa, the framework structure transforms into layered ones, 2 and . In addition, we estimated the thermodynamic stability of CaCO with respect to the minerals of the Earth's mantle. We found that CaCO can coexist with bridgmanite up to pressures of 54 GPa at 300 K, where it reacts with the formation of a Ca-perovskite, magnesite, and solid CO-V.