Unveiling the Formation Pathway of Vaterite from Amorphous Calcium Carbonate Using Metadynamics Simulations

Calcium carbonate is a compound that is widely distributed throughout the Earth as a natural mineral and a material produced by biological activities. The crystal structure of calcium carbonate has three polymorphs: the most thermodynamically stable calcite, followed by aragonite, vaterite, and amorphous \ce{CaCO3} (ACC). Of the three crystalline phases, the most unstable vaterite remains mysterious regarding its formation process and structure. In this study, the pathway of forming the vaterite crystal structure from ACC was reproduced using well-tempered metadynamics molecular dynamics simulations. The structures sampled at multiple minima on the energy landscape were refined through first-principles calculations based on density functional theory. The sampled structures were assigned space groups (SGs) and classified as calcite- and vaterite-like structures according to the arrangement of \ce{CO3^{2-}} and \ce{Ca^{2+}} sheets. The initial crystal structure produced from ACC was a monoclinic crystal with \ce{Ca^{2+}} sheets and \ce{CO3^{2-}} lying in the interlayer; however, it did not exhibit the three-fold symmetry of calcite. Calcite structures with three-fold symmetry, or orthorhombic crystals with rotated \ce{CO3^{2-}} units as found in vaterite, could be derived from this structure. The orthorhombic structure then transitioned to the more stable monoclinic form, which is likely vaterite. The understanding of phase transitions based on the diverse crystal structures of calcium carbonate revealed in this study provides a predictive view of biomineralization and carbonation reactions of cementitious materials.