Elastic properties of CaCO₃high pressure phases from first principles

Abstract

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 -h-CaCO structure, respectively. By fitting the third-order Birch–Murnaghan equation of state (EOS), the values of bulk modulus and pressure derivative 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 -h-CaCO, 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 CaCO phases are obtained. Post-aragonite exhibits 25.90%–32.10% anisotropy and 74.34%–104.30% splitting anisotropy, and -h-CaCO shows 22.30%–25.40% anisotropy and 42.81%–48.00% splitting anisotropy in the calculated pressure range. Compared with major minerals of the lower mantle, CaCO 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.