High-pressure transformations of CaC₂O₅ - a full structural trend from double [CO₃] triangles through the isolated...

Over the past few years, the concept of carbonates, as the salts of MCO₃ or composition with [CO₃] triangles in the crystal structures, was sufficiently extended. In addition to carbonates, crystal structures with stoichiometry M₃CO₅, M₂CO₄ and MC₂O₅ 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 CaC₂O₅, 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 [C₂O₅] groups consisting of two [CO₃] 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 I4̄2d, in the structure of which carbon is in the sp³-hybridized state and [CO₄] tetrahedra form isolated pyramidal [C₄O₁₀] anionic groups. At 59 GPa in the temperature range of 0-2500 K, CaC₂O₅-I4̄2d undergoes a phase transition to CaC₂O₅-Fdd2, with the framework structure of [CO₄] tetrahedra. On further compression to about 80 GPa, the framework structure transforms into layered ones, C2 and Pc. In addition, we estimated the thermodynamic stability of CaC₂O₅ with respect to the minerals of the Earth's mantle. We found that CaC₂O₅ 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 CO₂-V.