First-principles study on the high-pressure physical properties of orthocarbonate Ca2CO4

Abstract

Orthorhombic Ca2CO4 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 Ca2CO4-Pnma at 20–140 GPa using first-principles calculations, and compare them with the results of CaCO3 polymorphs. The results show that the structural parameters of Ca2CO4-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 Ca2CO4-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.

Phase diagram and physical properties of pyrocarbonate CaC2O5: first-principles calculations

“…The calculated volume and energy are fitted using the third-order Birch−Murnaghan equation of state, V 0 = 619.44 Å 3 , K 0 = 93.55 GPa, and K 0 ¢ = 4.86, which agree with the results (V 0 = 616.8(1) Å 3 , K 0 = 93.4(3) GPa, and K 0 ¢ = 4.83(2)) of König et al [17]. In addition, in our previous work [39], the calculated structural parameters of Ca 2 CO 4 -Pnma are in good agreement with the available experimental [14] and theoretical results [12,14]. It shows that the calculation method used in this work is reliable and the obtained structure parameters of CaC 2 O 5 polymorphs are accurate.…”

“…In the previous work [39], to ensure the accuracy of the elastic constants of orthocarbonate Ca 2 CO 4 -Pnma, we calculated the elastic constants of calcite and aragonite, which are in good agreement with the available experimental results [27,28]. In this work, the elastic constants of CaC 2 O 5 polymorphs are calculated by the same method, and it is believed that the calculation results are reliable, but further verification is needed.…”

“…,max ,min , ( ) = -Figure 13 shows the compressional waves anisotropy A P and the shear waves anisotropy A S of CaC 2 O 5 polymorphs and Ca 2 CO 4 -Pnma [39]. It can be seen that the A P and A S of Ca 2 CO 4 -Pnma in the Earth's lower mantle are smaller than those of CaC 2 O 5 polymorphs and CaCO 3 polymorphs [39].…”

“…In figure 11, we compare the densities and wave velocities of CaC 2 O 5 polymorphs with those of Ca 2 CO 4 -Pnma [39]. The densities of CaC 2 O 5 polymorphs increase with increasing pressure, but is less than those of The compressional waves anisotropy A P and shear waves anisotropy A S are defined as [57]:…”

“…In this work, the phase diagram of CaC 2 O 5 is redrawn, and the elastic and seismic properties of its various phases are calculated using first-principles, and compared with those of orthocarbonate Ca 2 CO 4 -Pnma [39]. The thermodynamic properties of CaC 2 O 5 polymorphs are predicted by quasi-harmonic approximation.…”