Post-aragonite phases of CaCO$_{3}$ at lower mantle pressures

The stability, structure and properties of carbonate minerals at lower mantle conditions has significant impact on our understanding of the global carbon cycle and the composition of the interior of the Earth. In recent years, there has been significant interest in the behavior of carbonates at lower mantle conditions, specifically in their carbon hybridization, which has relevance for the storage of carbon within the deep mantle. Using high-pressure synchrotron X-ray diffraction in a diamond anvil cell coupled with direct laser heating of CaCO$_{3}$ using a CO$_{2}$ laser, we identify a crystalline phase of the material above 40 GPa $-$ corresponding to a lower mantle depth of around 1,000 km $-$ which has first been predicted by \textit{ab initio} structure predictions. The observed $sp^{2}$ carbon hybridized species at 40 GPa is monoclinic with $P2_{1}/c$ symmetry and is stable up to 50 GPa, above which it transforms into a structure which cannot be indexed by existing known phases. A combination of \textit{ab initio} random structure search (AIRSS) and quasi-harmonic approximation (QHA) calculations are used to re-explore the relative phase stabilities of the rich phase diagram of CaCO$_{3}$. Nudged elastic band (NEB) calculations are used to investigate the reaction mechanisms between relevant crystal phases of CaCO$_{3}$ and we postulate that the mineral is capable of undergoing $sp^{2}$-$sp^{3}$ hybridization change purely in the $P2_{1}/c$ structure $-$ forgoing the accepted post-aragonite $Pmmn$ structure.