High-pressure transformations of CaC 2 O 5 - a full structural trend from double [CO 3 ] triangles through the isolated group of [CO 4 ] tetrahedra to framework and layered structures.

Over the past few years, the concept of carbonates, as the salts of MCO 3 or composition with [CO 3 ] triangles in the crystal structures, was sufficiently extended. In addition to carbonates, crystal structures with stoichiometry M 3 CO 5 , M 2 CO 4 and MC 2 O 5 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 CaC 2 O 5 , namely Ca-pyrocarbonate with monoclinic symmetry Cc , which is one of the possible agents of the global carbon cycle. This structure is characterized by the isolated [C 2 O 5 ] groups consisting of two [CO 3 ] 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 I 4̄2 d , in the structure of which carbon is in the sp 3 -hybridized state and [CO 4 ] tetrahedra form isolated pyramidal [C 4 O 10 ] anionic groups. At 59 GPa in the temperature range of 0-2500 K, CaC 2 O 5 - I 4̄2 d undergoes a phase transition to CaC 2 O 5 - Fdd 2, with the framework structure of [CO 4 ] tetrahedra. On further compression to about 80 GPa, the framework structure transforms into layered ones, C 2 and Pc . In addition, we estimated the thermodynamic stability of CaC 2 O 5 with respect to the minerals of the Earth's mantle. We found that CaC 2 O 5 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 2 -V.