High-pressure phases of MnCO3 by random structure search

Within the framework of density functional theory (DFT), we carry out a random structure search for an MnCO3 system under high pressures. The results identify a number of candidate structures under pressure ranged from 50 to 110 GPa, and we found several stable structures including the P1 symmetry structure, which was suggested by previous experiments; the structural properties are highly consistent with measured structural data. Furthermore, our results also indicate that the P1 symmetry structure becomes more energetically stable than the R3c symmetry structure above 55 GPa, which corresponds to the measured structural discontinuity in the experiment. Our results indicate that there are some candidate structures to be energetically comparable to the observed P1 symmetry structure, which may explain the inconsistency between the previous experimental results. Above 65 GPa, a phase transition into a high-pressure phase with P21/c symmetry is also predicted. As the pressure increases, the coordinated number of Mn atoms is increased, from sixfold-coordinated octahedral geometry (MnO6, below 53.5 GPa) to sevenfold- and eightfold-coordinated polyhedron mixture phases (MnO7 and MnO8, 53.5-65 GPa), and finally to the eightfold-coordinated polyhedron (MnO8, above 65 GPa). For C-O bondings, the trigonal planar carbonate CO32- ion transits into the fourfold coordinated CO44- ion with distorted tetrahedron geometry and forms a one-dimensional chain structure above 65 GPa, which has also been predicted and observed in analogous systems.