First-principles study on the high-pressure physical properties of orthocarbonate Ca 2 CO 4

Orthorhombic Ca 2 CO 4 is a recently discovered orthocarbonate whose high-pressure physical properties are critical for understanding the deep carbon cycle. Here, we study the structure, elastic and seismic properties of Ca 2 CO 4 - Pnma at 20–140 GPa using first-principles calculations, and compare them with the results of CaCO 3 polymorphs. The results show that the structural parameters of Ca 2 CO 4 - Pnma are in good agreement with the experimental results. It could be the potential host of carbon in the Earth's mantle subduction slab, and its low wave velocity and small anisotropy may be the reason why it cannot be detected in seismic observation. The thermodynamic properties of Ca 2 CO 4 - Pnma at high temperature and high pressure are obtained using the quasi-harmonic approximation method. This study is helpful in understanding the behavior of Ca-carbonate in the Earth’s lower mantle conditions.