Mechanochemical Access to Defect-Stabilized Amorphous Calcium Carbonate

Amorphous calcium carbonate (ACC) is an important precursor in the biomineralization of crystalline CaCO3. The lifetime of transient ACC in nature is regulated by an organic matrix, to use it as an intermediate storage buffer or as a permanent structural element. The relevance of ACC in material science is related to our understanding of CaCO3 crystallization pathways. ACC can be obtained by liquid-liquid phase separation, and it is typically stabilized with the help of macromolecules. We have prepared ACC by milling calcite in a planetary ball mill. The ball-milled amorphous calcium carbonate (BM-ACC) was stabilized with small amounts of Na2CO3. The addition of foreign ions in form of Na2CO3 is crucial to achieve complete amorphization. Their incorporation generates defects that hinder recrystallization kinetically. In contrast to wet-chemically prepared ACC, the solvent-free approach makes BM-ACC an anhydrous modification. The amorphization process was monitored by quantitative Fourier transform infrared (FTIR) spectroscopy and solid-state Na-23 magic angle spinning nuclear magnetic resonance (Na-23 MAS NMR) spectroscopy, which is highly sensitive to changes in the symmetry of the local sodium environment. The structure of BM-ACC was probed by vibrational spectroscopy (FTIR, Raman) and solid-state MAS NMR (Na-23, C-13) spectroscopy. A structural model revealing the partly unsaturated coordination sphere for the Ca2+ ions was derived from the analysis of total scattering data with high-energy synchrotron radiation. Our findings aid in the understanding of mechanochemical amorphization of calcium carbonate and emphasize the effect of impurities on the stabilization of the amorphous phase, which allowed the synthesis of a so far unknown defect variant of ACC with new properties. This may also represent a general approach to obtain new amorphous phases in a variety of different systems.