Structural transitions of calcium carbonate by molecular dynamics simulation

Calcium carbonate plays a crucial role in the global carbon cycle, and its phase diagram has always been of significant scientific interest. In this study, we used molecular dynamics (MD) to investigate several structural phase transitions of calcium carbonate. Using the Raiteri potential model, we explored the structural transitions occurring at a constant pressure of 1 bar with temperatures ranging from 300 K to 2500 K, and at a constant temperature of 1600 K with pressures ranging from 0 to 13 GPa. At increasing temperatures, the transitions calcite, CaCO$_3$-IV, and CaCO$_3$-V are observed and characterized. Within the calcite structure, CO$_3^{2-}$ ions are ordered between layers. As temperature increases, the calcite to CaCO$_3$-IV transition occurs, determining the partial disordering of CO$_3^{2-}$ ions. At a higher temperature, CaCO$_3$-IV transforms into CaCO$_3$-V. By applying free energy analysis, we have classified the last transition as a continuous order-disorder phase transition. At a temperature of 2000 K, it appears a `disordered CaCO$_3$' structure, characterized by low order within the calcium and carbonate sublattices and the free rotation of CO$_3^{2-}$ ions. At increasing pressures, two calcium carbonate transformations were observed. At $P=$ 2 GPa, the CaCO$_3$-IV phase undergoes a phase transition into CaCO$_3$-V, demonstrating that the model can describe the transition between these two phases as pressure and temperature-driven. Another phase transition was detected at $P=$ 4.25 GPa -- CaCO$_3$-V transits into the recently discovered CaCO$_3$-Vb phase. This transition is classified as a first-order phase transition by structural analysis and free energy-based arguments.