Structural properties. Figure 1 shows the crystal structures of Ca2CO4-Pnma in the unit cell. The lattice parameters and equations of state for Ca2CO4-Pnma are presented in Fig. 2. It is found that the calculated results are in good agreement with the available experimental24 and previous theoretical results21, 24, indicating the validity of the structure. As the pressure increases, the lattice parameter c decreases the fastest, followed by a and b, indicating that the c-axis is the easiest to compress, while the a-axis is the most difficult to compress. The sensitivity of the axis to compression is c > b > a. The unit-cell volume at 0 GPa is 303.38 Å3 and the bulk modulus and its first pressure derivative are K0 = 113.40 GPa and \(K_{0}^{\prime }\) =4.00 by fitting the third-order Birch–Murnaghan equation, respectively, which are consistent with the results(V0 = 302.0(3) Å3, K0 = 108(1) GPa, and \(K_{0}^{\prime }\)= 4.43(3)) of Binck et al.24.
In order to better understand the elastic and seismic properties of Ca2CO4-Pnma, the candidate CaCO3 structures(aragonite, P21/c-l, post-aragonite, P21/c-h, C2221, ’-l = low pressure’, ’-h = high pressure’) in the Earth's mantle are considered. The relative stabilities of the CaCO3 polymorphs considered in this work are evaluated from their enthalpies. According to Fig. S1(see Supplementary Material), P21/c-l stabilizes above 30 GPa and retains its stability up to 46 GPa, while P21/c-h stabilizes above 75 GPa and retains its stability up to at least 140 GPa, which are consistent with the experimental and previous theoretical results3, 5. CaCO3-C2221 above 137 GPa is stable relative to post-aragonite, but this does not make any sense5, 57. Because in this interval, the modification P21/c-h is more favorable. For comparison with calcium orthocarbonate, four modifications of CaCO3 must be considered, namely aragonite (20–35 GPa), P21/c-l (35–45 GPa), post-aragonite (45–75 GPa) and P21/c-h (75–140 GPa).
Elastic properties. The calculated elastic constants of Ca2CO4-Pnma are shown in Fig. 3. Within the studied pressure range, \({c_{11}}>{c_{22}}>{c_{33}}\), indicating that compression is easier along the c-axis than along the a- and b-axes. These results are consistent with those of Fig. 2, where the lattice parameter c decreases faster than the lattice parameters a and b with increasing pressure. The calculated elastic constants of CaCO3 polymorphs are shown in Fig. 4, Fig. 5, Fig. 6, and Fig. 7, respectively. Therefore, we believe that the calculated elastic constants are correct, but experimental verification is required.
The bulk modulus (B) and shear modulus (G) of Ca2CO4-Pnma can be obtained by the Voigt58-Reuss59-Hill60 scheme. As can be seen from Fig. 8, B is greater than G, indicating that with the change of volume, Ca2CO4-Pnma is more and more difficult to be compressed, and G is the main factor for the deformation of Ca2CO4-Pnma. The B and G of Ca2CO4-Pnma at < 75 GPa are larger than those of CaCO3 polymorphs. The B of Ca2CO4-Pnma at 75–140 GPa is equal to that of P21/c-h, and the G is slightly larger and almost parallel.
In order to evaluate the elastic anisotropy of Ca2CO4-Pnma, we adopt the scheme of Ravindran et al.61. The shear anisotropic factors of A100 in (100) plane, A010 in (010) plane, and A001 in (001) plane can be obtained from the following expression:
$${A_{100}}=\frac{{4{c_{44}}}}{{{c_{11}}+{c_{33}} - 2{c_{13}}}}$$
1
$${A_{010}}=\frac{{4{c_{55}}}}{{{c_{22}}+{c_{33}} - 2{c_{23}}}}$$
2
$${A_{001}}=\frac{{4{c_{66}}}}{{{c_{11}}+{c_{22}} - 2{c_{12}}}}$$
3
The variation of shear anisotropic factors A100, A010 and A001 of Ca2CO4-Pnma with pressure is displayed in Fig. 9. A010 and A001 gradually decrease with increasing pressure, A100 first increases with the increase of pressure, and then gradually decreases at > 40 GPa. It can also be found that the elastic anisotropy of Ca2CO4-Pnma in the lower mantle is very small, and the anisotropy of the (010) plane between [101] and [001] directions is the smallest.
Seismic properties. The compressional and shear wave velocities of minerals can be calculated from the elastic constants and densities. The compressional (VP) and shear (VS) wave velocities of Ca2CO4-Pnma and CaCO3 polymorphs can be obtained from the Navier's equations62:
$${V_P}=\sqrt {\frac{{3B+4G}}{{3\rho }}} ,\begin{array}{*{20}{c}} {}&{} \end{array}{V_S}=\sqrt {\frac{G}{\rho }}$$
4
The densities and wave velocities of Ca2CO4-Pnma, CaCO3 polymorphs and the Preliminary Reference Earth Model (PREM)63 are displayed in Fig. 10. From Fig. 10 (a), it is found that the densities of Ca2CO4-Pnma in the lower mantle is less than those of PREM, and greater than those of CaCO3 polymorphs. As shown in Fig. 10(b), the VP and VS of CaCO4-Pnma and CaCO3 polymorphs are lower than those of PREM, and the VP and VS of Ca2CO4-Pnma are greater than those of P21/c-l and post-aragonite, which are almost the same as those of P21/c-h. The wave velocities in various crystallographic directions can be obtained by solving the Christoffel equation \(\left| {{C_{ijkl}}{n_j}{n_l} - \rho {V^2}{\delta _{ik}}} \right|=0\)64. Figure 11 shows the wave velocities of Ca2CO4-Pnma along different crystallization directions at various pressures. The VP of Ca2CO4-Pnma propagates the fastest in the [100] direction. The shear fast-wave velocity propagates the slowest in the [001] direction. With the increase of pressure, the propagation in the [100] and [010] directions become slower. The shear slow-wave velocity in [100] direction propagates more and more slowly as pressure increases.
The anisotropy AP of the compressional waves and the polarization anisotropy AS of the shear waves are defined as66:
$${A_P}=\frac{{{V_{P,\hbox{max} }} - {V_{P,\hbox{min} }}}}{{{V_{P,aggregate}}}} \times 100\%$$
5
$${A_S}=\frac{{{{\left| {{V_{S1}} - {V_{S2}}} \right|}_{\hbox{max} }}}}{{{V_{S,aggregate}}}} \times 100\%$$
6
Figure 12 shows the AP and AS of Ca2CO4-Pnma and CaCO3 polymorphs. It can be seen that the seismic anisotropy AP and AS of Ca2CO4-Pnma are less than those of CaCO3 polymorphs, and decrease with the increase of pressure, and gradually increase at > 45 GPa.
The seismic properties of Ca2CO4-Pnma indicate that it is a potential host of carbon in the subduction slab and coexists with CaCO3 polymorphs, as suggested by Sagatova et al.21, 23. It was also verified by Binck et al.24. The low wave velocity and small anisotropy of Ca2CO4-Pnma may be one of the reasons why it is impossible to detect the presence of carbonate in the lower mantle during the seismic observation of the subduction slab.
Thermodynamic properties. The thermodynamic parameters of minerals are a prerequisite for deriving the thermal state of the Earth's interior. In order to obtain the variation of thermodynamic parameters of Ca2CO4-Pnma with temperature and pressure, we first verify the constant pressure heat capacity \({C_P}\) of aragonite at 0 GPa, and find that the calculated results are in good agreement with the experimental results44(Fig. 13). On this basis, the predicted heat capacity and thermal expansion coefficient \(\alpha\) of Ca2CO4-Pnma are shown in Figs. 14 and 15, respectively.