  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.
