On the impact of zero-point vibrations in calcium carbonate.

By performing calculations based on density functional theory (DFT), while adopting different exchange and correlations functionals, we have investigated pressure-induced phase transitions in calcium carbonate (CaCO₃) up to 30 GPa. We have focused on the effect of zero-point vibrations on the stability sequence of three CaCO₃ polymorphs, i.e. calcite-III, calcite-IIIb and calcite-VI, as well as on the evolution of their elastic properties. Our results show that the zero-point vibrations have a minor effect on the bulk modulus of the three CaCO₃ polymorphs, in the inspected pressure range. However, we demonstrate that the zero-point vibrations have a major impact on phase transitions leading to the following polymorphic sequence: calcite-IIIb→calcite-III→calcite-VI, where transitions of the first order occur at 4 and 18 GPa. The obtained sequence shows that the boundary phase of calcite-II is calcite-IIIb; and the latter doesn't reappear at higher pressure, as previously predicted by theory. [ABSTRACT FROM AUTHOR]