Elastic properties of CaCO_3 high pressure phases from first principles

Elastic properties of three high pressure polymorphs of CaCO₃ are investigated based on first principles calculations. The calculations are conducted at 0 GPa-40 GPa for aragonite, 40 GPa-65 GPa for post-aragonite, and 65 GPa-150 GPa for the P2₁/c-h-CaCO₃ structure, respectively. By fitting the third-order Birch-Murnaghan equation of state (EOS), the values of bulk modulus K₀ and pressure derivative K₀' are 66.09 GPa and 4.64 for aragonite, 81.93 GPa and 4.49 for post-aragonite, and 56.55 GPa and 5.40 for P2₁/c-h-CaCO₃, respectively, which are in good agreement with previous experimental and theoretical data. Elastic constants, wave velocities, and wave velocity anisotropies of the three high-pressure CaCO₃ phases are obtained. Post-aragonite exhibits 25.90%-32.10% V_P anisotropy and 74.34%-104.30% V_S splitting anisotropy, and P2₁/c-h-CaCO₃ shows 22.30%-25.40% V_P anisotropy and 42.81%-48.00% V_S splitting anisotropy in the calculated pressure range. Compared with major minerals of the lower mantle, CaCO₃ high pressure polymorphs have low isotropic wave velocity and high wave velocity anisotropies. These results are important for understanding the deep carbon cycle and seismic wave velocity structure in the lower mantle.