Carbonate orientational order and superlattice structure in vaterite

Abstract

Vaterite is considered to play an important role as a precursor phase in the formation of calcium carbonate phases, including those related to biomineralization. An accurate description of vaterite׳s structure associated with the order of carbonate groups is essential to understanding the formation, stabilization, and functionality of vaterite in organisms. Molecular dynamics simulations, synchrotron X-ray diffraction, and transmission electron microscopy have been combined in order to investigate the structure of vaterite. The electrostatic interactions between Ca and neighboring CO3 groups promote local and long-range ordering of CO3 groups, which may result in a superstructure of vaterite. Molecular dynamics simulations show that the superstructure (P6522) with ordered carbonate ions has a relatively lower energy than the disordered structure. The kinetics of the disorder-to-order transition suggests that the transition is rapid and that the superstructure is expected to form. X-ray diffraction data confirm the presence of the P6522 superstructure. The measured diffraction peaks are consistent with the calculated diffraction peaks, especially those weak peaks predicted as a result of the superstructure. Transmission electron microscopy also reveals minor satellite electron diffraction peaks with the more intense peaks of the primary pattern, suggesting a superlattice structure resulted from ordering in both crystallographic ab plan and c direction, which is consistent with the proposed superstructure.