First-principles calculations of high-pressure physical properties anisotropy for magnesite

Abstract

The first-principles calculations based on density functional theory with projector-augmented wave are used to study the anisotropy of elastic modulus, mechanical hardness, minimum thermal conductivity, acoustic velocity and thermal expansion of magnesite (MgCO3) under deep mantle pressure. The calculation results of the phase transition pressure, equation of state, elastic constants, elastic moduli, elastic wave velocities and thermal expansion coefficient are consistent with those determined experimentally. The research results show that the elastic moduli have strong anisotropy, the mechanical hardness gradually softens with increasing pressure, the conduction velocity of heat in the [100] direction is faster than that in the [001] direction, the plane wave velocity anisotropy first increases and then gradually decreases with increasing pressure, and the shear wave velocity anisotropy increases with the increase of pressure, the thermal expansion in the [100] direction is greater than that in the [001] direction. The research results are of great significance to people’s understanding of the high-pressure physical properties of carbonates in the deep mantle.

First-principles study on the physical properties of monoclinic MgCO 3 under high pressure

“…Lattice parameters, phase transition and equation of state. In our previous work 21 , the structural parameters and equation of state for magnesite were investigated, the calculated results were in good agreement with the available experimental and theoretical results, indicating the feasibility and reliability of the computational method. The crystal structures of monoclinic MgCO 3 in the primitive cell and unit cell are shown in Fig.…”

“…It is also noted that the calculated wave velocities are consistent with the theoretical results of Li and Stackhouse 12 , while the results of Marcondes et al 10 are obviously larger. This result indicates that magnesite is transformed from R3c to C2/m phase in the lower mantle, resulting in its wave velocities 12,21 lower than those of monoclinic MgCO 3 . It also means that monoclinic MgCO 3 may be present in the deep mantle, and also provides strong evidence for the storage of carbon as a carbonate mineral in the deep mantle.…”

“…The convergence threshold for electronic self-consistent field and forces acting on the atoms are 1.0×10 −6 eV and 0.02 eV/Å , respectively. The parameters of magnesite use those in our previous work 21 . The elastic constant is obtained using the stress-strain method 22,23 .…”

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

“…This error may be caused by the use of different exchange correction functions, PBEsol is used in present work, while PBE was used by Gavryushkin et al 14 . The accuracy of using PBEsol to calculate the properties of Mg-carbonate has been examined in previous studies 13 . In the previous study, MgCO 3 -C2/m was stable in the lower mantle above 80 GPa 3,[5][6][7][8][9][10][11][12][13]35 .…”

“…MgCO 3 -C2/m has 30 atoms, in which adjacent oxygen atoms form tetrahedra around carbon atoms, and Mg atoms are in octet and tenfold coordination. Subsequently, the structure of MgCO 3 -C2/m was verified experimentally [4][5][6] and theoretically [7][8][9][10][11][12][13] . Recently, Gavryushkin et al used USPEX and AIRSS methods to find that MgCO 3 reacts with MgO to form Mg 2 CO 4 , which has two structures, tetragonal (space group Pnma) and monoclinic (space group P2 1 /c), and its structure is P2 1 /c when the pressure is higher than 50 GPa 14 .…”