Reevaluating CaCO$_3$ behavior at high pressure: formation of Ca$_2$CO$_4$ under reduced conditions

Due to its importance for the global carbon cycle, the behavior of CaCO$_3$ under high pressures and temperatures has been extensively studied. Previous research has suggested the possibility of a transition to a disordered phase and/or amorphization of CaCO$_3$. In our experiments, conducted using a multi-anvil apparatus with a boron nitride (BN) capsule, we also observed the disappearance of almost all diffraction peaks at pressures of $\sim27$ GPa within a specific temperature range. However, at $1773$ K, numerous diffraction peaks reappeared. The observed diffraction pattern could be perfectly indexed by more than $20$ diffraction peaks, matching the structure of Ca$_2$CO$_4$-$Pnma$ corresponding the recently discovered compound, calcium orthocarbonate. Upon decreasing the pressure, the Ca$_2$CO$_4$-$Pnma$ phase persisted through three heating cycles in the pressure range of $20$–$27$ GPa. At a pressure of $\sim20$ GPa and a temperature of $1573$ K, it transformed into a new phase or a mixture of phases, different from the expected CaCO$_3$ + CaO. Our results challenge the previously proposed amorphization and transition to disordered state of CaCO$_3$, suggesting instead that the observed phase transformation can be the result of interaction with the BN capsule.