Trapping Amorphous Intermediates of Carbonates - A Combined Total Scattering and NMR Study

Crystallization via metastable phases plays an important role in chemical manufacturing, biomineralization, and protein crystallization, but the kinetic pathways leading from metastable phases to the stable crystalline modifications are not well understood. In particular, the fast crystallization of amorphous intermediates makes a detailed characterization challenging. To circumvent this problem, we devised a system that allows trapping and stabilizing the amorphous intermediates of representative carbonates (calcium, strontium, barium, manganese, and cadmium). The long-term stabilization of these transient species enabled a detailed investigation of their composition, structure, and morphology. Total scattering experiments with high-energy synchrotron radiation revealed a short-range order of several angstroms in all amorphous intermediates. From the synchrotron data, a structural model of amorphous calcium carbonate was derived that indicates a lower coordination number of calcium compared to the crystalline polymorphs. Our study shows that a multistep crystallization pathway via amorphous intermediates is open to many carbonates. We could isolate and characterize these transient species, thereby providing new insights into their crystallization mechanism.