Calcium orthocarbonate, Ca2CO4-Pnma: A potential host for subducting carbon in the transition zone and lower mantle

A novel structure of calcium orthocarbonate Ca₂CO₄-Pnma has been discovered using ab-initio crystal structure prediction methods (AIRSS and USPEX) based on the density functional theory. This phase appears above 13 GPa and remains stable to at least 50 GPa and 2000 K according to the calculations within quasi-harmonic approximation. Thus, the discovered phase can be stable at Earth's transition zone and lower mantle P-T conditions. The carbon atoms in the discovered phase are presented in 4-fold coordination, and its structure is similar to the high-pressure and high-temperature α'_H-Ca₂SiO₄ phase.