Comparative study on high-pressure physical properties of monoclinic MgCO₃ and Mg₂CO₄

The physical properties of Mg-carbonate at high temperature and pressure are crucial for understanding the deep carbon cycle. Here, we use first-principles calculations to study the physical properties of MgCO₃-C2/m and Mg₂CO₄-P2₁/c under high pressure. The research shows that the structure and equation of state of MgCO₃-C2/m are in good agreement with the experimental results, and the phase transition pressure of Mg₂CO₄ from pnma to P2₁/c structure is 44.66 GPa. By comparing the elastic properties, seismic properties and anisotropy of MgCO₃-C2/m and Mg₂CO₄-P2₁/c, it is found that the elastic modulus and sound velocity of Mg₂CO₄-P2₁/c are smaller than those of MgCO₃-C2/m, while the anisotropy is larger than that of MgCO₃-C2/m. These results indicate that Mg₂CO₄-P2₁/c exists in the deep mantle and may be the main reason why carbonate cannot be detected. The minimum thermal conductivity of MgCO₃-C2/m and Mg₂CO₄-P2₁/c is the largest in the [010] direction and the smallest in the [001] direction. The thermodynamic properties of MgCO₃-C2/m and Mg₂CO₄-P2₁/c are predicted using the quasi-harmonic approximation (QHA) method.